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In depth (chilled out!) model discussion


Paul

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Afternoon all :)

Apologies for the late running of Thursday - another pleasant day but it seems the warmth of Tuesday is already a memory. I note the coldest place in France at the moment is Cap Gris Nez at just 8c followed by Cherbourg and Le Havre at 9c whereas around Paris it's 20c so the line between late winter and early summer isn't far away.

Moving on and it's that time of the day when the 12Z output rolls out so this may be a bit of mix and match but it takes us to Sunday March 26th or thereabouts:

Starting with GEM 00Z at T+240:

gem-0-240.png?00

GEM builds a ridge of HP through the British Isles early next week and basically it's a protracted period of settled conditions but with a temperature inversion from the norm given cool NE'ly winds across the south and SW'ly over Scotland. The Atlantic looks set to trundle in before too long.

ECM 00Z at the same time:

ECM1-240.GIF?16-12

A similar evolution in some respects. The trough disruption leads to the formation of a cut-off LP over Iberia which supports the migration of the Azores HP to the NE over and to the north of the British Isles. The Atlantic is moribund so a settled spell but again a chilly E'ly flow across the south so spring slightly on hold.

GFS 00Z OP at T+240:

gfs-0-240.png

Much less support for lower European heights and while the Azores HP is ridging NE, the jet remains strong to the north so consecutive LP systems are passing east toward Scandinavia. The last hints of a NE'ly to the south but basically a benign W'ly flow setting in.

The 06Z OP at T+234 is very different:

gfs-0-234.png?6

Recognisably similar to both GEM and ECM. More unsettled to the south with a chill E'ly flow bringing rain and showers especially to the SW but fine in the far north.

I'll have more time for a better update tomorrow but spring not quite fully on the horizon.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Back to normal after yesterday's late show. A much cooler morning here in London and a sense of the weather not having quite decided whether it's spring or winter.

Perhaps the models will help - unlikely but you never know.

This morning's output takes us to Monday March 27th: 

Starting with GEM 00Z:

gem-0-240.png?00

The evolution is similar to yesterday with a strong ridge of HP (the Azores HP cell itself in fact) moving across the middle of the British Isles in the wake of the coming trough disruption and the cut off LP to the south. The HP is a tad further south this morning so less of a continental flow over the south and generally fine and cool conditions. As the T+240 chart shows, however, some sign of the HP receding and perhaps splitting with more than a hint of heights building into Scandinavia even as the core slips SE. With the Atlantic moribund we could see quite a warm SE'ly flow develop.

ECM 00Z at the same time:

ECM1-240.GIF?17-12

Dominated by HP albeit a weak and flabby cell and note the new HP developing in mid Atlantic. This is one of those ECM charts that's neither fish nor fowl though settled and quite warm conditions over the British Isles albeit with issues of morning fog perhaps.

GFS 00Z OP at the same time:

gfs-0-240.png

Quite a cold evolution for southern parts in particular, the migrating Azores HP cell moves from the south west around and over northern parts of Britain before heading into the North Sea. At times there's a noticeably chill NE'ly flow over southern areas and even by T+240, there's a stiff E'ly breeze. The rest of FI remains broadly anticyclonic but a hint of something chillier from the NW as we go into April.

Looking at the NH perspective, the T+240 as follows:

gfsnh-0-240.png

The PV is over the Pole and the OP FI take sit over to Siberia but that goes in the "I'll believe it when I see it" file for now.

Back to T+240 and the GEFS has a majority of members building heights to the north - most over Scandinavia but a couple building Greenland blocks as well. Plenty of cool not to say cold air on offer so definitely a swing away from the spring-like conditions of earlier this week back to something near if not below normal as the month closes.

In conclusion, the evolution to something cooler if not colder has strengthened this morning with GFS in particular and GEM to some extent. ECM remains stubbornly anticyclonic and that can't be ignored but the likelihood of our air coming from the north or east rather than the south or west from the middle of next week onward remains strong.

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Afternoon all :)

After the pre-spring warmth of last week, a reminder here in London today it is still only March with a cold strong wind and a hint of rain in the air. Those thinking spring had sprung (so to speak) have thankfully got a sharp reminder of winter and the week ahead looks set to be in complete contrast to last with colder upper air feeding down from the NW leading to rain, showers, strong winds and yes, perhaps some of the white stuff over hills.

Anyway, is this going to be a profound pattern change leading to a cold, wet spring or a mere blip soon forgotten ?

Today's analysis of output takes us to March 30th.

Starting with the GEM 00Z:

gem-0-240.png?00

The HP has drifted NE and the opportunity for heights to the north of the British Isles looks quite strong with the Atlantic messy and going nowhere slowly. The PV has headed cleanly to Siberia with a strong N'ly over western Russia. All that said, conditions over the British Isles would be pretty benign under a light ESE'ly.

ECM 00Z at the same time:

ECM1-240.GIF

Very different and much less amplified. Pleasant conditions for much of the British Isles it has to be said but a stronger northern arm to the jet keeping LP systems passing well to the north and no sign of Greenland heights.

Now a tale of two (very different) runs - GFS 00Z at T+240:

gfs-0-240.png

Closer to GEM than ECM.

06Z OP at T+234:

gfs-0-234.png?6

Very different and much closer to the morning ECM so significant inter-run variation at this stage as you might expect.

Looking at the Ensembles for the 06Z and at T+240, there are plenty still arguing for Greenland blocking (I count 7 of 20) and a smaller cluster arguing for blocking close to the east of the British Isles. I would argue the OP is atypical of the bunch but we'll see:

gens-22-1-240.png

The divergence remains the same through the end of the run.

In conclusion, the evolution from the end of the week remains far from clear - one option (and probably the form horse) would be the return of the Atlantic in one form or another but a significant minority of GFS members and the GEM OP suggest a chance for a more northern-based and therefore colder blocked regime to end the month. The evolutions vary, I think, on the development of the PV. Most models take the core of the PV to Siberia by this time next week allowing heights to rise across Greenland but the 06Z GFS and 00Z ECM soon begin a process of transferring energy back across the Pole (a pattern which dominated the winter it has to be said) and re-generating the PV back over Canada/Greenland shutting off the Greenland heights and firing up the Atlantic. Those models which prolong the blocking either have no or a much reduced energy transfer back over the Pole.

As ever, more runs are needed.

 

 

 

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

First, apologies for a lack of report over the past few days. I've had some health problems relating to my shoulder which has made typing at a keyboard quite unpleasant. I'm on the mend and the weather has been of considerable interest. Some have called the weekend "glorious" in the south and if by that is meant sunshine and fairly clear skies, it's a fair observation. The appearance has often been deceiving with a strong, cool if not cold NE or E'ly breeze which has meant that if you can be out of the wind in the sun it's been wonderful but in the shade and in the wind more like winter.

Daytime maxima have improved from last midweek to reach 15c in lowland East London on Saturday but the nights have been much cooler and I wouldn't be surprised if there had been a nip of frost in more rural areas.

As a man from Sheffield once said "that was then but this is now" and with April fast approaching how do the models look this morning.

Taking my medium term look out to early on Thursday April 6th and starting with the GEM 00Z OP chart:

gem-0-240.png?00

It's a benign and largely anticyclonic work from the Canadian model this morning. Two deep Atlantic LPs (one in existence now and another forming later in the week) have two significant and (for many) beneficial effects - one is to pull the Jet well to the north and the other is to allow HP to ridge up from the Azores. It's more typical of July than April and while the T+240 chart hints at more Atlantic influence and keeps some heights over Greenland it all looks pretty fair especially the further south and east you are.

ECM 00Z at the same time:

ECM1-240.GIF?27-12

This run also ends anticyclonic and while the positioning of the HP might not be the optimum for spring warmth, there's little to complain about. The PV looks to be just east of Greenland with a strong jet moving far to the north.

GFS 00Z OP at the same time:

gfs-0-240.png

If you're looking for something different, you've come to the wrong place. The evolution is slightly different to both GEM and ECM but it gets to a similar place. Perhaps the jet isn't as far north but HP is very much in charge of our weather and while the far NW may face Atlantic drizzle at times, for the bulk of us it looks set fair.

The NH overview of the same chart:

gfsnh-0-240.png 

The weakening PV remains poorly placed to help the UK achieve any kind of northern blocking with a new lobe setting up to the north of Greenland after a further displacement back from Siberia. However, more interesting developments into furthest FI:

gfsnh-0-384.png

The PV displaces again back to Siberia but cleanly and that allows a block to develop between the Eurasian HP, weak heights over Greenland and strong cross-Polar heights. This forces a re-alignment of the jet and sends the trough SE over the British Isles and down into Europe. For us, this would mean a change to much colder and more unsettled conditions with rain or showers becoming more prevalent. It's a very long way off but it's not unusual for mid-April to have a colder spell after a fine start to the month.

Looking at the GEFS and at T+240 there's fairly broad agreement though it has to be said some members are interested in raising heights to the NE as well so we pick up a warmer SE'ly flow but that is a minority while at the end of FI there are, as you would expect, a wide range of options.

In summary, while GFS offers a more unsettled interlude over the coming weekend, it's a generally fairly unremarkable and benign spell of weather in the offing especially for the south. Hard to say it will be warm or very warm but certainly average or just above at times. Hints in the outlook of a change to a fairly different regime towards the middle of April but far too early to be confident.

As a final observation, looking at the 10 HPA charts and again we see this relentless low-level wave 1 activity which I think has been so crucial this winter. Any attempt to displace the PV to Siberia is countered by this wave 1 warming which pushes it back over to NE Canada/Greenland. Occasionally, for brief periods, the displacement is successful and we get blocked conditions near the UK but the warming sends the PV energy back. In the absence of wave 2, we don't get the full split of the PV just an on going displacement cycle. FI charts which show complete PV displacement never come to fruition because the low-level warming prevents a clean displacement and maintains PV energy back over Greenland/NE Canada.

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

A pleasant week in southern England for the most part and another very good day looks in prospect as our spell of anticyclonic dominated weather continues.

Will it continue well into April or are signs of a change on the horizon. A glance at the extended output yesterday suggested a continuation of anticyclonic weather next week but where could we be after that ?

The output this morning takes us to Sunday April 9th:

Starting with GEM 00Z OP:

gem-0-240.png?00

The anticyclone continues to dominate having moved east and developing a ridge NE into Scandinavia with the trough further east still. Could the Atlantic trough head SE into Iberia - it's possible but for the British Isles, benign conditions with light winds and perhaps the best conditions to the NW.

ECM 00Z at the same time:

ECM1-240.GIF?30-12

The weather would be much the same for the British Isles as on the GEM but the synoptics are quite different. A strong northern jet persists but cut off features around the Azores and Iberia complicate matters slightly with the Azores HP significantly displaced and ridging NE. Perhaps more of a risk of a shower or two in the south but overall not bad. Another one of those ECM charts where it's hard to see the evolution.

GFS 00Z OP:

gfs-0-240.png

Nearer GEM than ECM I would argue but a messy set up. Note the Azores LP displacing the normal HP feature but the British Isles ends up under a weak HP cell and in fairly calm benign conditions. From there and further into FI, it goes badly pear-shaped:

gfs-0-348.png

The HP is held far out to the west and the door is opened for the trough to sink SE and introduce a much more unsettled and colder regime from the N or NW. This has been posited for a few days in FI on a few runs and I do wonder if GFS is picking up on a signal for a breakdown to more unsettled conditions in the second week of April.

Looking at the GEFS, the T+240 OP solution isn't without support but the exact position of the HP is crucial. Some members keep it to the west allowing a N;ly flow while others have the HP a notch further east introducing a warmer SE'ly so nothing decided as yet.

Picking up on a point from my previous, IF one of the factors supporting the ongoing pattern is the prevailing weak warming that looks set to disappear in 10-14 days as the overall temperature of the stratosphere rises. This might explain pattern changes and more blocked scenarios in April as the PV weakens, the winds fade and the overall temperature profile and contrasts flatten as well.  One to watch. 

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On 3/30/2017 at 09:58, stodge said:

Picking up on a point from my previous, IF one of the factors supporting the ongoing pattern is the prevailing weak warming that looks set to disappear in 10-14 days as the overall temperature of the stratosphere rises. This might explain pattern changes and more blocked scenarios in April as the PV weakens, the winds fade and the overall temperature profile and contrasts flatten as well.  One to watch. 

For a while the GFS has been forecasting an 'active' final warming - active in the sense that it there is a dynamic wave-1 forced baroclinic vortex to produce above average temperatures / wind reversal, rather than purely 'passive' warming as the vortex dissipates through radiative relaxation from increasing insolation. Indeed, some ensemble members forecast wind reversal greater than summer values and the stronger this warming the less likely a weak westerly will return this spring.

Generally an effect on the troposphere is unclear, though a couple of runs have suggested a S-T coupling as shown in the downward progression of zonal wind anomaly in the right hand chart below, from 29/03 06z

umedel60.thumb.png.de0db731a19b55737e0b3dfc6d5b0b8d.png

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Posted
  • Location: Camborne
  • Location: Camborne

From a tweet by MV just now

This really looks like the last breath of the Winter time stratospheric polar vortex; Ridge replaces vortex o/North Pole w/sister vorticies

58de77b14aa85_GFS1.thumb.JPG.d47ed54f2113db46caed3ab389fc11e9.JPG58de77b908bfb_GFS2.thumb.JPG.78e0cff38f28b3e1515d6ebd94472efb.JPG

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Apologies again for a few days break - busy time of the year for me I'm afraid.

The week coming looks very reasonable apart from a small hiccup tomorrow as HP builds strongly from the SW and sets up shop over southern Britain. The models did indicate however this would slip away at the weekend and the evolution into next week looked far from clear.

Has a new week brought new clarity ?

Today's output takes us to the cusp of Easter - Maundy Thursday, April 13th:

Starting with GEM 00Z OP:

gem-0-240.png?00

About as far away from spring as you can get but the story of the evolution is clear. The HP doesn't fade east but first west, inviting the trough to the NW to move down and eventually we see a full retrogression as heights build over Greenland. Cool if not cold and unsettled with rain and showers for most and perhaps even snow to higher ground in the north.

ECM 00Z at the same time:

ECM1-240.GIF?03-12

About as different as you can imagine. For ECM the HP declines east and pressure falls to the west and disrupts to the south - a ridge fills the gap as Easter approaches so a benign if not terribly warm outlook but certainly no Easter washout.

GFS 00Z OP at the same time:

gfs-0-240.png

Something different again - for GFS the HP declines SW and the trough moves in over the British Isles. It's not so much the Arctic trough as a re-alignment of the jet to a more NW-SE axis bringing the Atlantic :LP systems down across us. Yes, it would be unsettled with showers or longer outbreaks of rain but not as cold as GEM. Further into FI, the trough disrupts to the west and the south leaving the south of Britain in a cool E or NE'ly flow while further north it looks settled and warming up.

As others have said, the PV is becoming less of a player as we move further into April and this might be a factor in the very different evolutions on offer.

The GEFS at T+240 has a number of cold solutions but the message is a lack of consistency with a number of evolutions on offer and nothing decided. There is a minority cluster which builds the Greenland heights but others which keep heights more to the SW. Oddly enough I see more overall agreement right at the end of FI with the OP solution of northern blocking well supported.

To conclude, a fine few days from midweek onward but as the HP declines at the end of the weekend the evolution is far from clear. Easter could be cold and wet or less cold and wet or possibly dryish but I don't see an Easter heatwave at all at the moment. Beyond the middle of the month and northern blocking becomes a stronger theme with the likelihood of winds from the north or east quite high after Easter.

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2 hours ago, stodge said:

As others have said, the PV is becoming less of a player as we move further into April and this might be a factor in the very different evolutions on offer.

The GEFS at T+240 has a number of cold solutions but the message is a lack of consistency with a number of evolutions on offer and nothing decided. There is a minority cluster which builds the Greenland heights but others which keep heights more to the SW. Oddly enough I see more overall agreement right at the end of FI with the OP solution of northern blocking well supported.

To conclude, a fine few days from midweek onward but as the HP declines at the end of the weekend the evolution is far from clear. Easter could be cold and wet or less cold and wet or possibly dryish but I don't see an Easter heatwave at all at the moment. Beyond the middle of the month and northern blocking becomes a stronger theme with the likelihood of winds from the north or east quite high after Easter.

Tied in to possibility of active strat final warming coupling with troposphere; here is this morning's GFS (from weatheriscool) showing potential downward progression of zonal wind anomalies -

umedel60-17040306.thumb.png.72ab2c79294f64cc1bb2ef69c987360e.png

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Afternoon all :)

Another very pleasant day in the south and the start of a 4-5 day of fine conditions with some potentially very warm weather over the weekend as the HP migrates from west to east. Into next week and the run in to Easter wasn't looking so clever.

Where are we now with the output for the early hours of Easter Saturday, April 15th:

Starting with the GEM 00Z OP for this time:

gem-0-240.png?00

After the first HP weakens, a new HP cell builds in from the west and crosses the country through Good Friday setting up just to the E and NE the following day. Warm conditions with a SE'ly flow but perhaps increasingly unsettled from the west. No sign of Greenland heights but the Azores HP has been displaced so the evolution from here could be to a more E'ly flow.

ECM 00Z at the same time:

ECM1-240.GIF?05-12

Not a million miles away from GEM overall with HP in charge and centred to the East with a North-South orientation and the Atlantic trough digging S or SE down to the Azores. Decent conditions for the British Isles especially to the east and south.

GFS 00Z OP at the same time:

gfs-0-240.png

A more conventional west-east pattern, flatter and less amplified than GEM and ECM - fair to the south, more unsettled to the north, temperatures unremarkable.

The 06Z OP at T+234:

gfs-0-234.png?6

The flow is more amplified with the trough to the west more defined and while that throws warmer air over the British Isles, the threat of showers to the west is present.

06Z, further into FI, keeps a lobe of PV energy over Greenland and develops strong LP features which pass well to the north of the British Isles but quite an active flow for the time of year. It's not too far removed from the 00Z FI and it seems the signal to re-locate the PV over Siberia which was present earlier in the week has gone.

The GEFS, however, hasn't completely closed the door on northern blocking even at T+240 on the 06Z. I count at least five members with significant northern blocking so this isn't dead yet. I'd go further and say the scenario at the end of the run in FI on the OP is atypical of much of the suite many of which build heights over Greenland through the week after Easter.

So, it's a fine spell for now and nothing too unpleasant in the run up to Easter. The OP runs suggest a benign Easter break for now but far from a done deal and more runs are and certainly will be needed.

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Another glorious day here in lowland East London and a wonderful weekend to come but more than a hint on the midweek models of a significant change into next week and perhaps an evolution into something more synoptically interesting.

So, with Easter approaching at the same speed as every other time, what can be expect ?

The models this morning take us to the early hours of Easter Monday, April 17th:

Starting with GEM:

gem-0-240.png?00

Quite a complex evolution to get to this point, Early next week, a brief N or NW'ly gives away to a new build of HP from the SW and that gives quite a benign run-in and start to Easter though with the winds from the NW or W, not as warm as it will be this weekend. The Holiday weekend sees a deterioration as the HP collapses and the new build of HP is further north and west opening the door to pressure falling over the British Isles and NW Europe. It's more an offshoot of the Scandinavian trough which is over Finland but nonetheless an evolution to a cooler, more unsettled phase.

ECM 00Z at the same time:

ECM1-240.GIF?07-12

A very different evolution. After this weekend's HP, a longer lasting NW flow until midweek would keep it cooler and more showery but pressure builds again on Good Friday and the Easter weekend is fairly benign with a lot of dry weather - not as warm as the coming weekend perhaps - and no sign of a GEM-type evolution of heights to the NW.

GFS 00Z OP at the same time:

gfs-0-240.png

Something different again. Similar to ECM in predicting two brief plunges of Arctic air, one on Monday and one midweek before HP builds back in for Easter and GFS offers a decent Easter for most but with the HP drifting NE with time and introducing a light E'ly flow to southern areas by Easter Monday. Note the Azores HP is well out of position indicating a weak Atlantic and quite an amplified flow.

The rest of FI on the 00Z OP is strongly anticyclonic so another long dry spell with the HP initially over Scotland before declining slowly SW.

The 06Z OP at T+234:

gfs-0-234.png?6

Quite similar to the 00Z but the HP is further east and this allows a warmer SE'ly flow over the British Isles so perhaps some showers for Eire and the far west but otherwise very pleasant.

Further into FI and the LP in the Atlantic fills and disrupts south allowing pressure to build over and to the north of the British Isles with the centre over Scandinavia and a very long ridge.

Looking at the 00Z GEFS at T+240 it's an anticyclonic dominated picture but the positioning of the HP is critical and it's far from clear how this will resolve. 7-8 members keep blocking to the N or NW which would enable a colder airflow but others keep the HP just to the west or right over the British Isles. The 00Z OP looks a little out of line.

To conclude, a glorious weekend to come but a sharp reversion to a more typical (for early April) weather pattern next week with much cooler conditions and the chance of some rain or showers. It looks certain HP will try to build in again just before Easter and it may be we get away with a largely benign holiday albeit not probably too warm. The risk of a breakdown to something colder and unsettled remains high and not to be discounted but the odds have shifted in favour of a more anticyclonic outlook albeit with much uncertainty over the position ing and orientation of the HP.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Afternoon all :)

A rare foray into the weekend models for me and it's been a  wonderful day in southern Britain with 21c recorded in my part of London and something similar tomorrow before a sharp return to more average values early next week. Easter seemed very much open to question yesterday so let's see where we are now.

The 12Z output takes us to the middle of Tuesday, April 18th:

Starting with GEM 12Z OP:

gem-0-240.png?12

The GEM evolution hasn't changed too much in the short term. The weekend's heat is rapidly displaced by two thrusts of colder polar maritime air but both are brief and more as glancing blows with HP never far from the SW and building back over the British Isles to start Easter weekend but by Monday it's a weak and declining feature to the SW with a new build of heights to the NW and cooler air from a very light NE'ly flow with pressure falling to the south and east suggesting rain or showers a growing threat for SE areas but fine and dry to the NW.

ECM 00Z 12 hours earlier - so that's the early hours of the 18th:

ECM1-240.GIF?08-12 

A much less settled evolution with Good Friday the best of the days as the HP recedes west and the trough develops and moves down over the British Isles. The south might get a half-reasonable Easter Day but otherwise the accent is on rain or showers and fairly cool conditions with Easter Monday looking a right washout. Heights to the NW suggest no early break and if the trough pulls further east, we'll pull in a very chilly NNE airflow for the post-Easter period.

GFS 12Z OP at the same time:

gfs-0-240.png?12

The most progressive of the evolutions (which is no real surprise) but the most disappointing for Easter especially for the east and south with plenty of rain opr showers and a cool if not cold N'ly airflow as heights build down from the NW but by the afternoon of the 18th the heights have split from Greenland and the HP is sinking south cutting off the N'ly airflow and introducing warmer air back in from the west. The SE remains coolest for longest with a lingering NE flow. The rest of FI is dominated by the new anticyclone but it remains to the west of the British isles so while there will be plenty of dry weather, there will also be plenty of cloud and it's not going to be a heatwave. Benign and pleasant enough but unremarkable.

Looking at the T+240 GEFS it's fair to say there are a lot (a majority) of cool and cold options among the Members and many have the winds coming from a N or E position. The question seems to be the positioning of the HP - the OP keeps it close to the British Isles but a number of options keep the HP over Greenland with a ridge extending south allowing the trough to come south and leaving the British isles in an unsettled N or E airflow.

The HP alignment and orientation remains as difficult to pin down as it did a couple of days ago.

To conclude, after this weekend's exceptional weather, it's back to something more akin to April next week as pulses of colder PM air alternate with briefer HP periods but nothing too warm. Easter remains very unclear - GFS would provide a pretty poor outcome but other models delay the decline and retrogression of the HP until the latter part of the Easter break.

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

As expected, a glorious weekend. My part of London reached 23c yesterday afternoon and it's no exaggeration it was more like July than April. However, that's on the way out for now and although it's a fine morning again the maximum here today is likely to be 13-14c which is pretty close to where it should be and would still be pleasant under sunny skies (currently).

Easter beckons and it's fair to say there was plenty of uncertainty over the coming long weekend and points thereafter with more than a hint of something more akin to late winter on some of the output so where are we this morning ?

This morning's look takes us to Thursday April 20th:

Starting with GEM:

gem-0-240.png?00

GEM was perhaps the most bullish for an Arctic outbreak but has watered down the evolution somewhat this morning. During this week, HP remains close to the SW but that doesn't prevent a couple of brief NW'ly incursions before the HP declines on Good Friday and we get a more notable N'ly on Easter Saturday. Pressure tries to build back to the north and there's a brief E or ESE'ly flow before the HP migrates back SW and we finish with the above. The Atlantic looks set to roar back with a lobe of PV energy dropping south. I'm not convinced but we'll see.

ECM 00Z at the same time:

ECM1-240.GIF?10-12

A very different evolution. A messy Easter weekend but not too bad though LP never too far away and from Easter Monday a strong build of heights to the NW though signs of the PV setting up again over NE Canada and the heights looking to collapse but a few days of cool NE'ly winds especially for the SE corner with rain or showers but not too bad for the far NW.

GFS 00Z OP at the same time:

gfs-0-240.png

Not a million miles from the ECM evolution this morning. As with ECM, we just about get away with Easter but it'll be nothing like as warm as yesterday but useable for most though likely to be some rain for all at some time that will be outweighed by dry weather especially in the south.  It then turns very cool and unsettled immediately after Easter.

Into FI and the HP sinks rapidly south but the evolution remains messy with heights rebuilding strongly in Greenland while the trough is shunted south.

Looking at the 00Z GEFS and the Control isn't too far from the OP though the N'ly is more intense. The Panel shows a lot of uncertainty into next week - a number of members follow the OP and keep heights over Greenland or to the NW but a significant minority are breaking away from that and there is a mix of solutions on offer.

As we've seen repeatedly since mid February, the modelling of Greenland height rises is wonderful in low-res but falls apart in high-res. The problem remains the continuing reluctance of the PV to move or die away from NE Canada even in the throes of final warming. A pool of very cold air remains preventing the push of warmer air north (those Members which successfully build Greenland heights either dissipate the cold pool or send it further west into Canada).

In summary, yesterday's summer-like warmth isn't likely to be repeated until, presumably, summer. We're not looking at a chilly or cold washout for Easter at the moment and most will, I think, have dry and useable weather. A spell of cooler, more unsettled conditions at some point (Saturday seems the most likely) in the long weekend is probable but heights rebuild and most will get away with it but after Easter it's more uncertain with a strong likelihood of a few fays of cool wet conditions for the SE from a NE'ly or N'ly set up though GEM offers the possibility of a return to HP conditions.

In truth, it's still very uncertain and FI is really T+120 or even sooner at present.  

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

The change to something more seasonal duly occurred and indeed temperatures are now pretty much where they should be moving into mid April at around 13-15c in London.

It did look on Monday as though Easter would be okay for most - nothing terribly warm and certainly far removed from last weekend's values but no washout either. Quiet generally and especially the further south and west, but with LP closer to the NE, more risk of rain or showers and a cool or indeed cold NW wind.

The possibility of a pattern change was still present - it's often the case with these changes that there is an initial teaser before the change actually happens as though the atmosphere tries to move to a new solution, fails, resets and then tries again. That's a poor choice of words admittedly.

The evolution of LP digging SE, lowering heights over Europe and raising them to the NW as warmer air moves North into Greenland is one the models have frequently shown in low-res but which has yet to come into high-res. It looked as though it would take three "tries" to make the change happen (if it happened at all). The first LP was due today, the second on Saturday and the third on Tuesday and it was the last one that would initiate the change or so the models suggested on Monday but where are we now ?

Today's output takes us to Saturday April 22nd.

Starting then with GEM 00Z OP at that time:

gem-0-240.png?00

GEM keeps a slack cool NW'ly over the British isles through the Easter weekend until the post-Easter LP drops through the country introducing a brief N or NE'ly flow before pressure builds in again at the end of next week. This would be a decent outlook with pklenty of fine and dry weather and a slow warming with the Jet kept well to the north.

ECM 00Z at the same time:

ECM1-240.GIF?12-12

A very different evolution. The Easter NW'ly becomes a SW'ly by Easter Day as pressure builds from the SW but LP systems are never far away and brush NE Scotland on both Monday and Wednesday eventually sinking south to allow a brief E'ly across southern parts. The remnant of the LP is over Iberia with pressure higher to the west. Generally decent weather on offer with perhaps a residual shower in the south but again not very warm.

GFS 00Z at the same time:

gfs-0-240.png

As we can see, another evolution and it's all about the orientation of the HP which is different on all three models and that has huge impacts on the weather over the British Isles. GFS keeps winds in the N'ly quarter so while there would be a lot of dry weather, it wouldn't be warm with a risk of ground frost under clear skies.

00Z remains cool through much of FI with strong height development over Greenland (yes, I know) and the trough digging into Scandinavia keeping the N or NW'ly theme going.

06Z OP at T+234:

gfs-0-234.png?6

Once again something different - the HP is weak and declining and a new area of LP is forming just north of Iceland and you'd think it would move East then SE into Europe with heights rising to the NW behind it.

Well, not quite - more E than I expected into Scandinavia and in fact the trough swings south and a new LP forms just to the north of Scotland and moves down the east coast so a bleak few days of eastern Britain with rain, showers and a cold wind but not too bad further west. In furthest FI, it all goes a bit slack as the N'ly flow subsides.

Looking at the 06Z GEFS at T+240 I find the OP atypical of the suite. A majority of members have significant blocking to the NW either via ridging from Greenland or from an HP cell centred to the NW of the British Isles and nearer Iceland.  Further into FI and there's a clear distinction between those keeping the theme of Greenland heights going and those collapsing the heights and re-introducing LP systems passing well to the north of the British Isles and building ridging over the British Isles.

To conclude, Easter looks uninspiring with cool winds to start and possibly continuing though there is a chance the winds will shift SW on Easter Day as the core of HP moves across the country. Into next week and there's general agreement a new LP will form north of Iceland and move E or ESE dragging down a N'ly as heights rise behind it. By the end of next week, the core of HP is to the west but the orientation is far from clear - it seems unlikely to be warm (certainly compared to last weekend) but it should be largely dry.

There's little sign of an Easter heat wave or an Easter wash out at this time but into next week the more unsettled them looks set to dominate with the possibility of a longer or shorter spell of N or NE'ly winds.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Happy Maundy Thursday to all :)

Another fine and chilly start in London - a notable part of the current evolution has been the cold nights and clear starts - but the cloud has been building steadily.

Easter down here looks dry but uninspiring with a chill wind and temperatures barely at average values in a weak NW'ly flow.

Moving on toward the following weekend, the only certainty was uncertainty yesterday but has to day brought any change ?

The morning's output takes us to Sunday April 23rd:

Starting with GEM 00Z OP:

gem-0-240.png?00

A very anticyclonic run this morning. After the weekend, the anticyclone develops strongly over the British Isles but never gets into a position or orientation to favour the advection of warm air. Instead, it flirts with retrogression to the NW so the wind direction is either N or E - yes, it would be fine for many especially in the NW but cool with night frosts likely.  The mid-Atlantic LP starts to edge north hinting at a return of a more mobile pattern toward month end but we'll see.

ECM 00Z OP at the same time:

ECM1-240.GIF?13-12

Another anticyclonic run but again a frustrating one for those wanting spring warmth. The HP sets up to the west and is orientated to ridge across the south of the British Isles but the wind is from a N or NW direction and by T+240 retrogression is well under way and LP areas to the east of Iceland look set to advance SE toward the British Isles with the trough set to drop into Scandinavia. Still mainly dry for most at this time but rain or showers threatening from the NW. Cool and once again rural ground frost under clear skies.

GFS 00Z OP at the same time:

gfs-0-240.png

Again, very anticyclonic and in many ways similar to GEM and ECM at his time but there are some subtle differences which are worth discussion. The mid-Atlantic LP is interesting - GEM has modelled it very differently to both ECM and GFS - ECM has kept it as a minor feature but GFS has done something different and I'll put up the T+228 chart to illustrate:

gfs-0-228.png

The mid-Atlantic LP extends the trough north just as the GEM model suggests and that triggers the creation of  shortwave LP on the east coast of Greenland which has two key impacts on the evolution - one, the heights build back into the Atlantic returning the mid-Atlantic to a cut off feature - two, the new LP cuts off the retrogression of the HP nearer the British Isles (or rather delays it) as it passes east, to the north of Iceland, around the top of the HP cell.

Further into FI and the LP heads to Scandinavia while the HP rotates to the SW of Iceland with the ridge orientated south into the Atlantic before migrating NW to Greenland by which time the trough has extended SW from Scandinavia and the British Isles is in a cool if not cold N or NE'ly flow so spring very much put on hold with some cold air from Scandinavia heading down the east coast.

Looking at the 00Z GEFS at T+240 and as you'd expect there are plenty of options on the table though I don't see much promising warmth. The key is, as it has been for several days, where the HP ends up. A significant cluster have the HP more or less over the British Isles but the majority follow the OP with a centre to the west or north west. Another group go for Greenland based heights and these have the trough much closer (p10 and the extraordinary p15).

To conclude, an uninspiring but useable Easter outbreak looks to be followed by a return to anticyclonic domination for much of next week but with the HP centred to the west or north west, the accent will be on dry rather than warm conditions for most. There is a growing signal for retrogression from the end of next week and the likelihood of an even cooler regime with N or NE winds dominating and these bringing more unsettled conditions especially to the east and the south while the far north and west (as often happens in late April and early May) doing pretty well,

 

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Posted
  • Location: Cleeve, North Somerset
  • Weather Preferences: Continental winters & summers.
  • Location: Cleeve, North Somerset

Great udates as always Stodge. This April is reminding me very much of April 2010 - mainly dry and cool with occasional warmer interludes.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London
On 4/14/2017 at 00:03, MP-R said:

Great udates as always Stodge. This April is reminding me very much of April 2010 - mainly dry and cool with occasional warmer interludes.

Thank you for the kind word, my friend. Yes, that's an interesting parallel.

Onwards and sideways on this Bank Holiday morning and the Easter weather has played out much as expected so far. There have been some warmer interludes with sun shine but the persistent cool breeze hasn't helped and while it's been dry here in lowland East London there has been a little rain elsewhere as weak weather fronts have slipped SE in the flow.

The models have been suggesting a fine anticyclonic spell before a renewed push of Northerlies but let's see where we are this morning with the picture out to Thursday April 27th:

Starting then with the GEM 00Z OP output:

gem-0-240.png?00

A very anticyclonic run from GEM this morning. This week the HP tries to set up to the North but pressure from the jet shifts the core HP south to lie across southern Britain by Thursday before a half-hearted retrogression into the weekend after which the HP sets up close to the SW early next week before moving back across the south of the country during midweek.

Little or no rain is the obvious conclusion so the incredibly dry spell continues. LP systems move round the HP to the North and SW and then slide SE down into Scandinavia where it stays quite cold and wintry. Perhaps a little rain to the NE on occasions bu little else. On the other hand, nothing desperately warm wither and overnight ground frosts under clear skies remain a hazard to farmers and growers.

ECM 00Z at the same time:

ECM1-240.GIF?17-12

Perhaps a sign of a more defined change at the end of the ECM output. This run has made a little more of the N'ly incursions and brought a couple of fairly weak and transitory Arctic outbreaks in the next week or so but ends with the HP being moved east as the Atlantic trough gathers but that draws in warmer S'ly winds so a lift in temperatures before the possibility of something more unsettled.

GFS 00Z OP at the same time:

gfs-0-240.png

Something a little different from GFS which has chopped and changed quite a bit in recent times. The immediate evolution is a recognisable blend of GEM and ECM but with a more pronounced N'ly early next week as the LP comes closer to NE Scotland then other models suggest but after that GFS creates a large LP to the far north just as ECM and this runs due east on a more N'ly track across to northern Scandinavia bringing warmer SW'ly winds across all of NW Europe.

However, that LP marks quite a change in the pattern as heights rise again over Greenland and the jet shifts back south in furthest FI with heights lowering across the British Isles and a more unsettled but warmer outlook greets the start of May with the next Bank Holiday likely to be warmer and wetter,.The coldest of the northern hemisphere air shifts over to Siberia by this time.

The 00Z GEFS are fascinating this morning - the OP has some support but the spread is fascinating:

gens-22-1-240.png

There's a huge difference between those like the OP which develop LP around SE Greenland and those which maintain heights to the NW and there are to my eye at least 9 members keeping significant heights to the NW up over Iceland and Greenland so the OP is by no means a done deal.

As an example, this is P9 at T+240:

gens-9-1-240.png

This is typical of the significant cluster maintaining heights to the NW via retrogression and that would see much colder and more unsettled condition than the OP with a sharp N'ly flow.

To conclude, a quiet week up;coming with HP moving round the British Isles before ending up across the south. Nothing terribly warm and indeed chilly nights possible under clear skies with rural frost.

From there, a significant divergence - some models keep HP very close with LP systems and the attendant cold N'ly plunge far to the east while others offer the occasional glancing blow of a transitory N'ly. There is however a significant group among GEFS suggesting more emphatic retrogression and a build of heights to the NW with the trough sinking south over Scandinavia and N'ly winds impacting the British Isles. 

So nothing warm in the immediate future but probably nothing wet though the possibility of a colder evolution to a more unsettled spell especially for the East of the British Isles can't be ruled out.

 

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

A glorious morning here in London and after a benign 00Z OP yesterday, 06Z OP picked up the retrogression scenario and offered a strong full on N'ly for next week. The possibility and intensity of retrogression and a N'ly outbreak dominated the output yesterday but let's see where we are now.

Today's output takes us to Friday April 28th:

Starting as always with the GEM 00Z OP:

gem-0-240.png?00

Subtle changes from yesterday's output. The LP which develops to the west of Iceland next weekend dives more south then south east through the British Isles and heights rise strongly to the north behind the LP which fills but maintains a shallow trough to the SW linking back to a mid Atlantic LP. An E'ly flow for most and quite unsettled in the south with rain or showers while the north turns fine. Not terribly warm either.

ECM 00Z at the same time:

ECM1-240.GIF?18-12

ECM continues to play the retrogression card quite strongly and now has it clearly to T+168 which is the cusp of low-res so more likely to verify (perhaps). A ridge from Greenland to Iceland linking back to heights over Svalbard and the Pole and the trough is essentially a cut off feature over the northern North Sea. Cool and unsettled for much of the British Isles away from the far west with rain or showers.

GFS 00Z OP at the same time:

gfs-0-240.png

Very different evolution backing away from retrogression on the OP to a large extent. Next week's LP heads not south but SE to Denmark so there is a brief N'ly before pressure builds back in from the west. Thereafter, a new and far more vigorous LP forms to the east of Greenland and pushes the HP back south while maintaining the ridge over much of the British Isles so a benign spell after the transient N'ly - warming up by day but with still chilly nights.

It all gets a bit messy further into FI with the HP moving east across the British Isles into Scandinavia but then heights fall and a weak trough moves down over the British Isles so we end on a cool and unsettled note.

GFS 06Z OP at T+234:

gfs-0-234.png?6

Not too dissimilar and very different from yesterday with a pulse of very cold air (not quite the PV but not far removed) over NE Canada firing up the jet and encouraging cyclogenesis off the east Greenland coast. The main trough heads east but a secondary feature heads south lowering heights and with the Azores HP displaced far to the west the trough is in charge so very different weather for early May with rain or showers.

The 06Z GEFS at T+240 shows less spread then yesterday's 00Z and most of the Members develop the LP around Iceland but some send the trough further SE more quickly with the HP retrogressing further with time. I do think there's more uncertainty at the end of this month/early May but the move, while it's been away from the coldest of Northerlies, has been to a wetter, more unsettled evolution with the trough dropping more or less over the British Isles.

In summary, a pleasant week ahead for many and the first attempt at a N'ly will hardly be felt but the course of the second LP set for early next week remains far from settled. It could drop right through the British Isles (GEM) or set up a N'ly (ECM) or a more transient spell as the LP heads SE (GFS). Thereafter, the trend looks ominous for the next Bank Holiday with the trough threatening to set up near or over the British Isles bringing a significant change to a more unsettled scenario with rain or showers though rain would no doubt be welcome for many.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Another glorious morning here in London but a decidedly chilly night with only 3c at 7am suggesting rural areas will have had quite a sharp ground and indeed air frost around dawn which isn't good if you have (or indeed are) a tender sapling.

Yesterday morning's models suggested considerable uncertainty towards the end of the month with the strength and duration of any N'ly spell far from decided,

This morning's output is for the start of Saturday April 29th:

Starting with GEM 00Z at this time:

gem-0-240.png?00

A thoroughly cold evolution from GEM this morning and were this December or January, there would be salivation all round (except from Sidney). HLB very much in evidence with a strong ridge from an HP cell developing over SE Greenland augmenting the N flow and as the trough backs SW, a very cool and changeable spell midweek next week before the HP builds into Scandinavia and the British Isles ends on an E'ly with a cold pool covering much of NW Europe so a far from exciting end to the month if you are after warmth.

ECM 00Z at the same time:

ECM1-240.GIF?19-12

About as different an evolution as you can get. LP is in charge with no real sign of HLB. Yes, there's a N'ly early next week but the trough lingers further north keeping a weak cool NW'ly over the British Isles with small secondary LP forming augmenting any rain or showers. Perhaps a sign of a return to a more Atlantic-based pattern in the further outlook. Nothing warm here either and likely to be showery if not wet at times.

GFS 00Z OP at the same time:

gfs-0-240.png

Different again. Next week's N'ly is fairly brief but sharp. However, the HP cell is never far from the SW and builds back across the country before transferring NE into Scandinavia albeit with the ridge persisting over the British Isles so a fine and dry outlook. Again, not sure how warm this would be with a weak flow from the NE and perhaps sea fog would be an issue to coasts but not too bad overall.

Further into FI, the Scandinavian HP holds off the Atlantic LP systems until at the very end of FI, more general HLB builds to the north leaving the British Isles in a warmish E'ly flow.

The 00Z GEFS at T+240 tell a different story again and are supportive of the Control which wants to resume a more traditional Atlantic-based mobility with deep LP to the NW and any ridging further south. It's a strong signal but runs contrary to the OP and other model output. We often see GFS, almost as a default, attempt to restore mobility from the west into FI and this often occurs when there is no clear signal and we may be at that point where the evolution for the end of the month is far from clear. The Mean illustrates this by keeping LP to the west of Iceland over SE Greenland but that's nowhere near either the GEM or ECM evolutions.

In summary, uncertainty rules (probably). After a benign few days, there's general agreement on the HP moving west or north west and allowing a plunge of N'ly air early and into the middle of next week. After that, it's fair to say it's confusion.

Both GEM and GFS build an HP into Scandinavia and both offer it as the main way forward with HLB in charge. ECM is nowhere near that and its T+240 chart looks an unlikely evolution to this observer. The GEFS strongly hint at a return to a more mobile and less amplified pattern but again is that a default response in lieu of no strong signal or a hint the effects of the final warming are ending and the default summer pattern is re-emerging ?

As ever, more runs are needed.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Nearly afternoon all :)

Well, another chilly start in London Town and this cool but dry weather looks set to continue for a while before a colder spell next week.

Another Bank Holiday looms so where will be we be on April 30th ?

Let's have a look at the models:

Starting with GEM 00Z:

gem-0-240.png?00

A very different evolution to yesterday. The Greenland heights rapidly collapse in the middle of next week as the Atlantic fires up to the north but heights remain low over Europe so the British isles is in a col with a nagging NE'ly across the far SE which will keep it cool and possibly showery but fine for many.

ECM 00Z at the same time:

ECM1-240.GIF?20-12

A not dissimilar evolution. The Greenland heights collapse as the Atlantic fires up but perhaps orientated differently to GEM. Again, there's a col but the British Isles are more to the east so a more defined E'ly and cooler conditions with a Scandinavian HP in charge.

GFS 00Z OP at the same time:

gfs-0-240.png

Thus has mobility triumphed (it seems). Again, Greenland heights swiftly collapse as new LP develops over NE Canada and heads east. Residual weak northern blocking remains  but under a transient ridge, the British isles would be mainly fair but with Atlantic weather systems approaching.

Further into FI, the LP continues SE and disrupts as heights build over Scandinavia and the British isles gets an E'ly which, as the HP transfers west, becomes a NW'ly at the very end of the run.

GFS 06Z OP at T+234:

gfs-0-234.png?6

Not a huge difference from the 00Z and clear the long trough extends right into Scandinavia keeping the UK in a mild SW'ly but with plenty of rain or showers.

Further into FI, a large Scandinavian HP develops and Britain enjoys a warm SE'ly but this is brief as pressure falls (aided by the Azores HP returning) and Britain ends up under LP with rain or perhaps thundery showers with the best of the warmth and heat shunted to the east.

Looking at the 06Z GEFS and the OP is a fair blend of the various solutions in the suite. There's still a lot of blocking in the suite but the emphasis has shifted more to the NE than the NW though the OP solution of a more Atlantic-dominated evolution in the medium term is obviously well represented.

In summary, we've seen a major shift overnight in the models away from a prolonged spell of Greenland heights and blocking to a (at least temporary) more mobile solution. It appears the encroachment of warmer air over Greenland to pump up the heights isn't sufficient to maintain the block in the absence of any cross-polar ridging and therefore colder air re-establishes to the north of Greenland and that inspires cyclogenesis which sends active LP systems across from the west and collapses the heights.

From there, the evolution is much more uncertain though a number of options keep low heights over Europe and build an HP over Scandinavia leaving the British Isles in a col by the end of the month with the possibility of northern blocking resuming into May so it looks as though this could be a temporary phase rather than a significant pattern change.

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

A less inspiring day in London Town but all eyes are turning toward next week's "Arctic blast" and where we go from there. There was a clear shift away from Greenland heights in the medium term on yesterday's models though plenty of HLB still in evidence as we moved into May.

This morning's output takes us to the early hours of Bank Holiday Monday, May 1st:

Starting as always with the GEM 00Z output:

gem-0-240.png?00

Significant changes from yesterday morning's output. The brief but quite sharp midweek N'ly next week is replaced by falling heights to the west and rising heights to the NE sending the jet back south. The British Isles becomes warmer, true, but much more unsettled with periods of rain or showers for all and with quite light winds any showers could be heavy and prolonged for the south and west. Pressure remains high to the east of Europe and it will be interesting to see if following LP systems take a more southerly track into France and Iberia.

ECM 00Z at the same time:

ECM1-240.GIF?21-12

A variation on yesterday's theme of the British Isles in a col between pressure systems. After the midweek N'ly pressure rises from the west as the Azores HP ridges in but it doesn't hold and the trough moves in instead with frontal systems nearing the west of Ireland by T+240.

Further east, there's a more interesting scenario with a large HP cell over northern Scandinavia but pressure still low over Denmark, Germany and central Europe. It's not hard to imagine from the alignment of the trough a NW-SE move of the trough into Europe going right through the British Isles so potentially the far north of Scotland could have ESE'ly winds and the south a more W'ly flow. None of it screams settled and the likelihood is an increasingly unsettled pattern with rain or showers developing as pressure falls.

On then to GFS and while 06Z OP is rolling out, I'll look at 00Z OP at T+240:

gfs-0-240.png

A different evolution again. GFS develops a stronger ridge than ECM and holds the Atlantic trough well out to the west allowing pressure to build through the British Isles to link to heights to the far north. The cut off LP is over western Poland helping to build the heights to the north and west. In terms of weather, this would leave the British Isles generally fine and dry and warming up nicely as we draw in warmer air from the south.

Into FI and heights build strongly to the north and the British Isles moves into first a cool NE'ly with the cut off trough advancing back SW and then a warmer air flow from the East.

The 06Z OP has rolled out in the interim and here is T+234:

gfs-0-234.png?6

Very different again - a much shallower and weaker Atlantic feature and no northern blocking at all with a pulse of cold air moving down from Labrador and Newfoundland into the Northern Atlantic. For the British Isles, HP is in charge with fair if not fine conditions and pleasantly warm. FI looks to keep a strongly anticyclonic theme but with the HP drifting to the West or North West cooler temperatures and an onshore breeze for the south and east.

Back to the 00Z GEFS and it's interesting to note the Control follows the OP in broad terms which is always worth noting. The suite though contains a myriad of options and very broadly three options - one and possibly the majority cluster has heights to the east as the dominant feature while another significant cluster brings in the Atlantic quite strongly. The OP looks a little out of line as only three or four members build the core of the HP over the British Isles.

To conclude, a brief N'ly blast next midweek is followed by extreme uncertainty. The form horse looks to be a build of HP back across the country from the east and/or south but a more unsettled Atlantic-dominated pattern can't be ruled out if the HP builds too far north and the trough is able to extend either from west or east across the British Isles. We'll know more after the weekend but the next Bank Holiday could either be glorious or a washout and both options are currently modelled.

 

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

A dry morning to start the new week in London but I suspect tomorrow and Wednesday are going to be a real shock to the system with a plunge of very cold Arctic air taking us well below the seasonal temperature numbers.

That doesn't last but the coming Bank Holiday was far from clear and the extended forecasts suggested a big change in early May to extensive northern blocking which might or might not bring some warm air across from the SE or keep things cooler from the NE.

Let's see where we are this morning with the outlook out to the early hours of Thursday May 4th.

Starting as always with the GEM 00Z output:

gem-0-240.png?00

A fascinating evolution from GEM this morning. The short-lived N'ly is snuffed out by HP collapsing SE in the face of a new trough over Eastern Greenland digging south. However, as that happens, pressure builds over Scandinavia and the Atlantic is repulsed by a cool E or ENE'ly flow bringing disappointing conditions for the Bank Holiday (especially for the south). LP remains near the south early in the new week bringing rain or showers but that fills and we are left at T+240 with weak but extensive northern blocking with the core heights just to the east of Iceland and a large mid-Atlantic LP which you would think would start to shift NE toward the British Isles.

ECM 00Z at the same time:

ECM1-240.GIF?24-12

A very different evolution. The trough coming SE from Greenland fills and disrupts to the west of the British Isles so the Bank Holiday weekend isn't too cold but has rain or showers for most especially on Monday. After the Bank Holiday pressure builds strongly first from the south and then more robustly from the NE and at T+240 we have a classic Scandinavian HP centred over Southern Sweden with a ridge SW over the British Isles. This will mean dry conditions for most but it's hard to be certain about the temperatures. The problem is with central Europe quite cool it may be the air is not as warm as we think so we're left with a cool HP but that's far from clear.

GFS 00Z OP at the same time:

gfs-0-240.png

Another model, another evolution and it seems FI could be early as T+96 as there's no clarity for the immediate aftermath to the midweek cold snap. GFS holds the trough to the NW as HP builds in toward the end of the week but over the weekend the HP declines and the trough takes over and the Bank Holiday Monday chart from the GFS OP isn't that far removed from the ECM chart at the same time. The flow is a more defined SW'ly so temperatures reasonable but a risk of rain or showers for all.

After the Bank Holiday GFS builds pressure from the south but that switches to the NE but not Scandinavia and more over the Northern North Sea and we get this curiously oriented HP and a flow which may look superficially favourable from the SE but in fact the air is sourced back over NW Russia (note the small LP over Finland) so may not be that warm. It's a curious evolution and some of the modelling of pressure systems to the east over Scandinavia is a little off-putting (one LP moves north for seemingly no reason).

FI is a nightmare for those wanting warm spring sunshine as retrogression moves the core of the HP to the NW and the trough starts to encroach from the east keeping the winds N or NE throughout so little or no warmth and by the end of the run a very real threat of rain with snow to altitude coming from the east and NE.

Looking at the 00Z GEFS at T+240 and I can only see a couple of members keeping LP near Iceland and the rest have blocking and the majority have HLB albeit of many and varied positions and orientations so the signal for northern blocking of some flavour is very strong. Resolving the position and orientation of the HP is of course critical to determining our weather and that's far from clear at present.

In summary, a morning of model confusion and uncertainty past this week's brief colder interlude. GEM is slightly on its own wanting to build the Scandinavian HP more quickly than ECM or GFS which have a milder but more unsettled flow for the Bank Holiday. However, both these models agree on the return of strong HLB next week with an attendant E or ESE flow which GEM keeps as a weaker feature.

The possibility of HLB dominating the further outlook is very high and while if the 00Z GFS Control is a guide could bring us some fine and warm conditions, the OP would provide a dry but much cooler if not colder scenario with winds from the N or NE and a risk of the trough encroaching back as we move further into May.

 

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Quite a shock to the system this morning to leave the house at 6.45am and realise the winter hat was required. A sparkling morning here in London Town but the much anticipated colder snap has duly arrived and we'll see if convection builds enough later on for us to see a hail shower down this way.

Moving on toward and into May and plenty of evidence of northern blocking in the outputs yesterday with perhaps a warmer interlude from an ESE next week trending colder as the wind shifted round more to the NE into the second week of May.

Today's model output takes us to the early hours of Friday May 5th:

Starting with GEM 00Z:

gem-0-240.png?00

This morning's GEM evolution matches some of the GFS/ECM evolutions present on the 12z outputs yesterday. The current cold spell is followed by a brief ridge before the trough to the NW digs SE which in turn throws up heights to the NE and the British Isles is the battle ground with a flow of warm SE'ly winds. The weather would be fine to the north and east but the south and especially the SW would be at risk of rain and showers.

From there, the HP pushes west to the north of the British Isles and the SE becomes more of an E'ly as warmer air pushes into Greenland and the dreaded Greenland HP sets up. The T+240 chart has a strong ridge coming SE so for many it's a dry and warm prospect.

ECM 00Z at the same time:

ECM1-240.GIF?25-12

The actual evolution isn't dissimilar to GEM and while the final synoptic set up is also similar it's worth noting the source of the air over the British Isles is from the north not the Continent so this would be a much cooler E'ly for most.

On then to the GFS 00Z OP:

gfs-0-240.png

Superficially, only slight differences but the GFS takes a different evolution to get there. The weekend battleground ends with an LP moving across the south on Bank Holiday Monday which would be a washout for many but heights rise behind that LP from the south and the north and its the northern arm of the heights which becomes the dominant feature as the trough stalls well out to the west. The HP strengthens just NE of the Faeroes into a major HP cell by T+240.

Further into FI the heights move west then south west and this brings a cooler ENE'ly flow across the British Isles with colder air from Scandinavia generating a small LP which goes down the east coast bringing a spell of rain or showers for eastern counties.

GFS 06Z OP at T+234:

gfs-0-234.png?6

A cleaner evolution to the E'ly but not much different to the 00Z in all honesty. Further into FI and while the heights retreat to the NW, we get something akin to a negative west-based NAO setting up as the trough comes back in from the SW and the winds over much of the British Isles fall light before backing WSW in the south.

Looking at the 06Z GEFS at T+240, there's a lot of northern blocking and the OP is a fair guide as to the rest of the suite though as said yesterday the position and orientation of the HP are varying from run to run and will continue to do so.

In conclusion, more certainty this morning. The brief cold spell will be followed by an equally brief drier interlude before the trough approaches from the NW just in time for the Bank Holiday. GEM and ECM limit the effect of the trough to the south and west under a brisk SE'ly but GFS takes and LP through the south of the country on Monday which would be a poor day for many.

Pressure builds from next Tuesday from the north with a strong HP setting up by the end of the week though exact position and orientation aren't settled - it could be a warm E'ly or something much cooler depending on air source. From there, broad agreement on some form of retrogression into the second week of May but as kid month approaches, there's huge scatter and uncertainty and nothing reliable in the way of a forecast.

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Very much like the bleak midwinter here in London with a cold N or NE'ly wind and doubtless close to a ground frost outside the capital. I saw a shower of rain and soft hail yesterday afternoon in SW London and I suspect those with altitude have seen something more wintry.

This is a snap, however, not a spell and the trend to build HLB to the NE was very strong yesterday leaving the British Isles in a warm or potentially very warm SE'ly flow.

Let's see where we are now with the model outlook to the early hours of Saturday May 6th:

Starting with the 00Z GEM:

gem-0-240.png?00

The evolution through the Bank Holiday weekend and early next week is pretty well set with the trough splitting next Tuesday as heights rise strongly to the NE. Part of the trough recedes back into the Atlantic while a smaller element remains around the Low Countries helping to prop up the northern HP. That passes close to or just north of the British Isles by this time next week before receding into Greenland.

Battle is then joined between the colder NE flow and the returning Atlantic trough and the latter prevails bringing more unsettled conditions back in from the SW. It's worth noting the very strong Greenland heights which may keep the jet running well to the south (the Azores HP is out of the picture) through the British Isles so once set in, it could be quite a prolonged unsettled spell.

ECM 00Z at the same time:

ECM1-240.GIF?26-12

A number of similarities with GEM but some key differences. The position and orientation of the Atlantic trough is different - it is weaker and further south. This maintains a broad NE or E flow across the British Isles at T+240 though the risk of showers in the far SW in the strengthening E'ly breeze can't be ignored. There's little sign of a coherent Scandinavian trough but the possibility of disturbances in the flow to the NE is another factor. The jet is very weak and held a long way south. It would however be a fine and dry outlook especially for NW Scotland.

GFS 00Z OP at the same time:

gfs-0-240.png

As is often the case, GFS goes a different route. Initially it's much the same as you might expect but GFS has hinted for the past few days at low heights penetrating further into Europe next week so while the HP builds strongly to the NE before retrogressing NW in tune with the other models, the development of the trough is very different.

Rather than holding the trough in the Atlantic, GFS OP brings it into western Europe and specifically over France with a change to a cooler and more unstable ENE flow across southern Britain with a heightened risk of rain or showers especially in the south, SW and the Channel Islands. Scotland would stay mainly dry, warm in the west, cooler with haar to the east coast.

Further into FI and the pattern remains broadly unchanged with strong HLB and the jet sent a long way south. A chance of the jet moving back at the very end of the run with LP approaching the south of Britain from both east and west accentuating the chance of rain or showers.

Looking at the 00Z GEFS at T+240 and it's hard not to be surprised by the lack of disagreement. The OP is strongly representative of the suite and the location and orientation of the HP around Iceland now seems to be the favourite option. Needless to say, there are more options further into FI but while the Control goes for a more Atlantic-based breakdown, a number of the members develop a strong Scandinavian trough or LP and a return of cold N'ly winds around or just before the middle of May.

To conclude, more agreement than I've seen for a while. The ending of the cold spell presages a more unsettled weekend with the trough attempting to move in as heights build strongly to the NE. This brings in a warm but brisk SE'ly wind and the likelihood of rain or showers for southern and south western areas.

The heights win out into next week but the trough moves across the south on Bank Holiday Monday bringing a potentially disappointing day before conditions improve from the north and the whole of the British Isles enjoys a few days of dry and warm weather albeit with sea fog or haar a threat for eastern coastal areas. Further into next week and with LP close to the south on some evolutions, rain or showers for southern counties are quite possible while the best weather will always be to the far NW - that trip to the Western Isles you've always promised yourself, a good time to take it !!

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Posted
  • Location: East Ham, London
  • Location: East Ham, London

Morning all :)

Didn't expect to have to scrape ice off the car this morning but this final hurrah of winter is now at hand and on its own way out to be replaced by, well, a confused weekend. Perhaps best in the north and east but the details still to be worked out.

Yesterday, the models were strongly in favour of a build of pressure to the NE gradually transferring to the NW over time but there were many permutations and much to resolve.

This morning's output takes us to the early hours of Sunday May 7th:

Starting with the GEM 00Z at T+240:

gem-0-240.png?00

Quite a simple story this morning. After a brief unsettled hiatus through the early part of the coming weekend, pressure builds strongly from north and south on Tuesday and the northern HP becomes the dominant feature first to the NE of the British Isles and then retrogressing to be centred over Greenland but with a ridge SE back to the British Isles. The air flow over the British isles is sourced from a long way north so this would be a cooler NE'ly but the weather would be fine for most and the further west the better in terms of sunshine.

ECM 00Z at the same time:

ECM1-240.GIF?27-12

Some subtle differences to GEM - note the orientiation of the Atlantic trough - but the net effect is much the same. The HP starts in the northern North Sea but retrogresses to Greenland with the ridge more ESWE and that means a more defined NE'ly for much of the British Isles sourced from some cold air over Scandinavia and NW Russia. Dry weather dominates the British Isles with the best of the weather in the west and north west.

GFS 00Z OP at T+240:

gfs-0-240.png

Subtle differences again starting with the Atlantic trough which is further south and with a different orientation to the GEM and ECM. The HP evolution is broadly similar though GFS creates a small LP to the far north which runs east into northern Scandinavia before sinking south. This cuts off the relatively warm E'ly and introduces a weak NE'ly flow but pressure falls across the south of Britain as LP moves NE from Biscay so  by T+240 the south would be unsettled with rain or showers at times while the far north and north west holds on to drier conditions.

Little changes further into FI with pressure always high to the north and low to the south and the wind flow oscillating between a warm SE'ly and a cooler NE'ly. The south would be vulnerbakle to showers or longer spells of rain while Scotland stays largely dry.

The Control brings the trough up into the British Isles in furthest FI.

The 00Z GEFS at T+240 show strong agreement for the overall evolution and for strong HLB to the north and north west. The position and orientation of the trough has a number of variants but the majority produce a NE'ly flow of some degree.

In summary, while the details of the weekend may still need finalising, the trend to build strong heights to the NE early next week is confirmed this morning. All models take these heights toward Greenland though with varying orientation of the ridge. Some bring the ridge back SE over the British Isles keeping us broadly dry and fine while others keep the ridge further north allowing pressure to fall to the south and less settled conditions to encroach from the south and south west.

The net effect is, after a brief warmer E'ly interlude to swing winds to a cooler NE'ly direction. 

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