Model Discussion - 10th March Onwards

[Polar Maritime]

A very unsettled end to the weekend and the begining of next week, With some quite cold air pushing down from the North -8 uppers into scotland and -6 uppers pushing down into northern England.

With a deep Low swinging in of the Atlantic into this cold air, i would expect some quite significant snowfall over the Scottish Highlands, and down to lower levels for a time. Also a risk over the high ground on the Pennines to.

Most areas will get some good rainfall totals out of this system.

With winds turning again to a N/N/E flow from Wednesday, and continuing cool/cold and unsettled.

[Summer Sun]

Well it looks like I got the water butt linked up to the Greenhouse today just in time as it should start to get filled up early next week quite nicely

[Summer Sun]

ECM continues to the unsettled theme bringing plenty of April showers

http://www.null/ds/ecm.htm

[J10]

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[damianslaw]

A cold unsettled outlook for the foreseeable future. All models agree on a deep low pressure system zipping into the country by Tuesday bumping into cold uppers so a particularly wet raw day could be in store for many on Tuesday and with heights building firm to the north, the low will be tracking SE across the country during Wednesday.

Thereafter - all models continue to suggest the trough will become limpet across the country with further low pressure system moving across the country by the weekend - so staying unsettled and cold and also windy.

If it was Dec, Jan or Feb and even March the current outlook would be a snow cold lovers dream... alas it isn't, but it is good news for the parched ground and for anyone who doesn't particularly like warm weather in April.. I think by the month end many will be saying how March swapped places with April this year.. reminding me of April and May last year and in 2007, this April is certainly helping to change my opinion of the year so far being an absolute snoozefest... and is living up to its reputation of the being the cruellest and least atlantic dominated tropical maritime infested month..

[johnholmes]

Anyone wanting the current spell of coldish and unsettled weather to alter any time soon had beest not look any further in this post.

All 3 500mb anomaly charts for way into the 3rd week of April, out to the 27th on the NOAA version, showing much the same upper air pattern.

Fairly cold 500mb air over the country with the flow north of west and quite a marked trough just about sitting over the UK.

Summer definitely on hold for 2-3 weeks it would seem!

ECMWF-GFS version this morning for 23 April

and the NOAA version for 8-14 days issued this evening for 21-27 April, pretty much the same as the 6-10 day version.

Whatever variations the synoptic 2x or 4x daily ouputs show the probability of the upper air pattern being much as the above two sets of charts suggest is very high; minor and brief variations but nothing more than that.

Edited April 13, 2012 by johnholmes

[Cloud 10]

The MJO is forecast to do a u-turn back into phase 8 for the end of the month,so yet another indication

of the cool and unsettled weather continuing.

[Robbie Garrett]

The MJO is forecast to do a u-turn back into phase 8 for the end of the month,so yet another indication

of the cool and unsettled weather continuing.

What phase is an ideal hot/humid thundery summer??

[Thundery wintry showers]

It varies according to the summer month but for high summer (July/August) phase 6 looks pretty good for hot/humid/thundery weather with composites indicating troughing in the mid-Atlantic and relatively high pressure over central Europe. For June, phases 1 and 8 look optimal for the same reasons. Therefore for a consistently hot/humid/thundery summer you'd probably want the MJO to drift backwards, starting at phase 1 in early June, and then falling slowly through phase 7 and into phase 6 during July and August- seems pretty unlikely to me though!

Meanwhile the MJO composites for phase 8 in April suggest frequent troughing to the east and winds from a generally northerly quarter, which is consistent with what we've seen during the past fortnight but it has to be said that next week looks set to be rather more Atlantic-dominated with westerly winds, though with temperatures continuing mostly below normal.

[Bottesford]

A few off topic posts have been removed - please stick to model discussion and leave the drought talk to the relevant thread and the personal digs off the forum completely!

Thanks.

[rob48]

Anyone wanting the current spell of coldish and unsettled weather to alter any time soon had beest not look any further in this post.

All 3 500mb anomaly charts for way into the 3rd week of April, out to the 27th on the NOAA version, showing much the same upper air pattern.

Fairly cold 500mb air over the country with the flow north of west and quite a marked trough just about sitting over the UK.

Summer definitely on hold for 2-3 weeks it would seem!

ECMWF-GFS version this morning for 23 April

and the NOAA version for 8-14 days issued this evening for 21-27 April, pretty much the same as the 6-10 day version.

Whatever variations the synoptic 2x or 4x daily ouputs show the probability of the upper air pattern being much as the above two sets of charts suggest is very high; minor and brief variations but nothing more than that.

Good.

I didn't think it was supposed to start for another six or seven weeks.

[Robbie Garrett]

It varies according to the summer month but for high summer (July/August) phase 6 looks pretty good for hot/humid/thundery weather with composites indicating troughing in the mid-Atlantic and relatively high pressure over central Europe. For June, phases 1 and 8 look optimal for the same reasons. Therefore for a consistently hot/humid/thundery summer you'd probably want the MJO to drift backwards, starting at phase 1 in early June, and then falling slowly through phase 7 and into phase 6 during July and August- seems pretty unlikely to me though!

Meanwhile the MJO composites for phase 8 in April suggest frequent troughing to the east and winds from a generally northerly quarter, which is consistent with what we've seen during the past fortnight but it has to be said that next week looks set to be rather more Atlantic-dominated with westerly winds, though with temperatures continuing mostly below normal.

Thanks for that detailed explanation.

I take it this would mean high pressure situated to our east over mainland Europe (which flows in a south easterly direction up into the UK. With low pressure/disturbance situated to our far southwest?

[TonyH]

Weather guide blog for the coming week:

http://forum.netweather.tv/blog/309/entry-4386-weather-guide-sunday-15th-to-saturday-21st-april-west-wales-and-central-midlands-specific/

[bluearmy]

Hints that an extra bit of retrogression could take the centre of the main trough further west and turn the flow south of east/west in the 10/14 day timescale. I wondered about this a few days ago but since then, the favoured ens picture has been for most of nw Europe to be under the mean trough. Wonder if we'll see this variation in the mean solution gathering pace over the next few days?

[Thundery wintry showers]

Thanks for that detailed explanation.

I take it this would mean high pressure situated to our east over mainland Europe (which flows in a south easterly direction up into the UK. With low pressure/disturbance situated to our far southwest?

Pretty much- we're talking a setup with a weak area of high pressure over mainland Europe and troughing in the mid-Atlantic which gives us frequent southerly winds and Atlantic systems struggling to make inroads against the high pressure, sparking off thundery activity as weakening fronts cross the country. For south-western areas you are best off with a south-easterly type (so that continental storms track towards SW rather than SE England) but in the north-east a south to south-westerly is optimal (to prevent 'stabilising' winds off the North Sea) and elsewhere anything vectored between SSW and SE is generally favourable.

It's a volatile setup because if the Atlantic trough heads eastwards we can end up with cool changeable westerlies, but then again the same sort of thing applies to the much-lauded "Azores High ridging" setup, for if ridge retreats back to the Azores we end up with the same result.

Talking of the Azores High, it looks like staying well out to the south-west over the coming days. Interestingly despite the strong Atlantic influence the emphasis on sunshine and showers currently looks likely to continue, with frontal rain belts tending to pass through overnight (a particularly potent frontal system on Monday night for example).

[Robbie Garrett]

Pretty much- we're talking a setup with a weak area of high pressure over mainland Europe and troughing in the mid-Atlantic which gives us frequent southerly winds and Atlantic systems struggling to make inroads against the high pressure, sparking off thundery activity as weakening fronts cross the country. For south-western areas you are best off with a south-easterly type (so that continental storms track towards SW rather than SE England) but in the north-east a south to south-westerly is optimal (to prevent 'stabilising' winds off the North Sea) and elsewhere anything vectored between SSW and SE is generally favourable.

1015-1025hpa? or so I am trying to think back to the late 90s and the sort of conditions that we had back then, as I've probably mentioned before conditions where fantastic for overnight MCS/Supercells. (Whatever they where) and I don't think I've experience that in at least 12 years + I take it the ideal synoptic would be direct southerly winds?? so storms can penetrate far into Land from the french mainland.

There conditions are far more ideal, and I take it most of the decent Summer storms we've had where imports (7th July 2009 - funny enough was the day I landed back in the UK from Holiday) I remember the storms came from the direct south after a continuous heatwave, that and last years MCS where the only times I remember conditions being partially ideal.

Also a quick question I understand what toughing and ridging is, but why is the 500hpa trough important for the low pressure = crap weather?

Is the 500hpa ridge and trough very similar to the surface high pressure/surface low?? but on a bigger scale at the 500hpa level.

Edited April 14, 2012 by Robbie Garrett

[Thundery wintry showers]

Troughing at 500hPa is closely correlated both with low pressure at the surface and with cold airmasses in the upper atmosphere.

Some members (notably Weather09 recently) have referenced cold pools at 500hPa- this is because a strong reduction in temperature with height implies instability (and thus convective potential) and if the temperatures at 500hPa are very cold this implies deep instability.

High pressure actually suppresses thunderstorm chances as it implies subsiding air near the surface and reduced likelihood of 500hPa troughing and cold pooling. I've seen quite a few members claim that amazing thunderstorms are often preceded by prolonged anticyclonic spells, but I think this is nothing more than confirmation bias (reflecting the preference for prolonged anticyclonic weather and thunderstorms)- although it does help if there is a certain amount of subsidence near the surface to help provide a "cap" (which temporarily suppresses convection, before a forcing mechanism breaks through the cap and produces explosive convective development). I think pressure of around 1010-1020hPa is normally conducive but above 1025mb we will struggle to get enough instability.

The main source of cloudy wet weather is Atlantic frontal depressions (typically associated with a strong jetstream and Atlantic depressions moving across at intervals). There is an association with 500hPa troughing in the sense that it encourages low pressure formation, as we often hear about regarding the polar vortex in winter, but it is really the amount of associated frontal activity that determines whether the weather with low pressure close by will be predominantly dull and wet or brighter with showers (and possibly thunderstorms).

[shedhead]

Looking at the latest model guidance along with the teleconnective signals, any fine, dry and warm weather looks a very long way away, with the current cool, unsettled and often wet pattern persisting through the rest of this month and probably extending into next. As ever you can't please all of the people all of the time, but as I've mentioned previously in this thread the balance had to come, personally I'd far prefer to see said balance in April/May rather than July/Aug.

Edited April 14, 2012 by shedhead

[johnholmes]

Hints that an extra bit of retrogression could take the centre of the main trough further west and turn the flow south of east/west in the 10/14 day timescale. I wondered about this a few days ago but since then, the favoured ens picture has been for most of nw Europe to be under the mean trough. Wonder if we'll see this variation in the mean solution gathering pace over the next few days?

Its something that might happen ba, and the 500mb anomaly charts will give us the first clue; overall they have nudged the trough just a touch west of its predicted position 3-5 days ago.

regarding my comment a while ago with this

Summer definitely on hold for 2-3 weeks it would seem!

not my highlighting

but to be totally correct the phrase might have been better to say - summer like conditions.....

Picking up on the comments made by TWS (Ian) in reply to RG about 500mb patterns.

The upper air is always, easier, if that is the right term to predict than the surface, the two are inter related as is everything in meteorology. Nothing can be taken in isolation, but as TWS explains where there is a 500 or 300mb trough one will normally find surface weather systems. Where there is an upper ridge its usual to find a surface high under it or close by.

The 500mb charts are at what meteorologists call the level of non divergence something we don't hear much about now but our understanding of upper air has moved on considerably since those days. Plotting surface charts and the 500mb chart then 'gridding' to obtain the 1000-500mb 'thickness' was, prior to computers, the only way to get any idea out to 24 hours ahead what the weather systems might do, where they might move to, would the surface high intensify/decline, would the low deepen/fill. The same data is still used by the models but more recent terms like vorticity are much more to the fore with them.

Lots of information, in non technical terms is available in the Glossary of the web site below

http://weatherfaqs.org.uk/

This explanation below seems to be quite a good one about the level of non divergence

(meteorology) A level in the atmosphere throughout which the horizontal velocity divergence is zero; although in some meteorological situation there may be several such surfaces (not necessarily level), the level of nondivergence usually considered is that mid-tropospheric surface which separates the major regions of horizontal convergence and divergence associated with the typical vertical structure of the migratory cyclonic-scale weather systems.

Read more: http://www.answers.c...e#ixzz1s18RiRIV

I hope that has not muddied the waters too much-it is very complex but the 500mb level is a very good guide to what the weather patterns may be and the anomaly charts are very good IF consistent over several days for showing the pattern at the upper levels 7-15 days or so ahead. How useful I am hoping to find out with the checks I am running on them.

Edited April 14, 2012 by johnholmes

[Robbie Garrett]

Just to add to TWS's earlier post (#688) re. the ideal wind direction concerning convective potential for different regions of the UK, the 500hPa height contours bear the most relevance concerning thunderstorm motion- that is, the track storms are likely to take. So, as TWS rightly points out, for western and southwestern parts of the country a flow from east of south would to be more beneficial than a flow west of south (so a trough with a negative "tilt")- though it doesn't always tend to be as "clear cut" as that.

However, if it's severe convective weather (such as MCS/Tornado potential) you're looking for, a SW'erly flow at 500hPa and S'erly flow at 850hPa (preferably strong), combined with a light S'erly to SE'erly winds as you descend from 850hPa to the boundary layer, is the ideal setup for such a scenarios (provided the atmosphere is sufficiently unstable) as such varying degrees in wind direction and strength increases vertical wind shear.

Could you provide a diagram? As I am interested in what you mean... doesn't have to be too detailed. Just gives us noobs a better idea as to what you mean, I am not fully onboard.

[johnholmes]

Could you provide a diagram? As I am interested in what you mean... doesn't have to be too detailed. Just gives us noobs a better idea as to what you mean, I am not fully onboard.

try this link-its first class and prepared by Nick F senior forecaster for Net Wx-as good an explanation as you will find I think.

http://forum.netweather.tv/topic/29497-a-guide-to-uk-thunderstorm-setups/

[Robbie Garrett]

I'll have a read thanks.

[johnholmes]

the thanks are not to me really but to Nick F-quite first class and may explain to those who don't know Nick why he is the one that provides the Net Wx storm forecasts.

[Summer Sun]

GFS 12z remains unsettled once more tonight

http://www.null/ds/gfs.htm

UKMO is also unsettled

http://www.null/ds/ukmo.htm

[damianslaw]

The models show little change from yesterday with the trough anchored firmly across the country. The coming week should deliver much more windier conditions than the week gone and more persistant belts of rain.

This thread has become very long - 36 pages starting from the 10th March - mods perhaps time for a new one...