Model Output Discussion 12z 18/10/2012

[Gavin Hannah]

i know the NH doesn't look normal but there is far to much emphasis on northern blocking on here im afraid, the fact is this island just looks to have normal autumn weather next week, this weekend looks quite chilly though

yes ECM FI looks blocked but its FI so until this northern blocking has a real impact on this island, i don;t care how much blocking there is over greenland or the NH, as we all know northern blocking doesn't always mean cold for us, i know it dosen't look milder in the near future but as i said these lows that are shown for next week could end up further north, meaning the south becomes milder, i remember nick sussex saying this often happens and he was right as it happened last autumn and winter

Tis very true. Overall, I'd say the model outputs look better than last year so far. As you mentioned, doesn't guarantee a bean, but I will remain quietly optimistic that even if we do get a zonal winter, it will be a cold zonal winter rather than last years dross.

Edited October 24, 2012 by IBringTheHammer

[phil nw.]

i know the NH doesn't look normal but there is far to much emphasis on northern blocking on here im afraid, the fact is this island just looks to have normal autumn weather next week, this weekend looks quite chilly though

yes ECM FI looks blocked but its FI so until this northern blocking has a real impact on this island, i don;t care how much blocking there is over greenland or the NH, as we all know northern blocking doesn't always mean cold for us, i know it dosen't look milder in the near future but as i said these lows that are shown for next week could end up further north, meaning the south becomes milder, i remember nick sussex saying this often happens and he was right as it happened last autumn and winter

The possibility of rising heights from the south east cannot be ruled out in a west based -NAO although that`s not signaled in medium term outputs as yet.

I can understand the fascination with the state of the vortex though and those Polar heights..

The fact that the PV is modelled as a disorganised feature shows that zonal winds are not yet strong enough at lower levels to induce a robust round shape with a zonal flow.

As we can see from the models the jet stream is fragmented and we still have raised height anomolies above 60N.

I guess as we come into November we are coming into the time where we would expect a stronger vortex in normal years as the Stratosphere continues it`s cooling process.The fact that so far trophopheric modelling is still not showing associated extensive low heights over the pole is fascinating for a lot of observers.

The 64000 dollar question is can this state continue?

We continue to monitor the daily outputs with interest.

[Gibby]

I here it often said that a Greenland High is a major player in snow prospects for the UK. Tonight's ECM latter stages illustrate clearly why a Greenland High alone is not enough to promote widespread wintry weather for the UK. Leaving aside the fact that it's only early November I Personally prefer a Scandinavian Winter High as a better guarantee for Winter cold in the UK, especially on the rare occasions it stretches across from Scandi through Iceland to Greenland too. In this type of scenario it's harder for the cold block which forms in these setups to be dislodged as easily and can be responsible for our heaviest and most disruptive Winter snowfalls in Britain when the warmer air of the Atlantic buffers against the cold block over the UK and Europe. With a Greenland High it's a common occurrence for it to bring cold zonality to our shores with it's attendant wintry type showers and that is what the charts show for next week. The cold Northerly on the eastern flank of the High is responsible for the rapid deepening of the monster Low shown on ECM tonight which as I indicated on my report will deliver below average temperatures despite winds having a Southerly component to them due to the airmass having Arctic origin and that's not what a normal South-Westerly would deliver without the prescence of the Greenland High. Sorry for drifting a bit off topic there at times.

Edited October 24, 2012 by Gibby

[Cloud 10]

And th GP N Pacific LP is there

BFTP

Hi BFTP,just wondered what your thoughts are regarding this deep low pressure signal

on the model output.

Could be a very lively start to November.

[Chris D]

Hi BFTP,just wondered what your thoughts are regarding this deep low pressure signal

on the model output.

Could be a very lively start to November.

Wow that is a big anomaly on the 500hPa charts.

Edited October 24, 2012 by Chris D

[Cloud 10]

Wow that is a big anomaly on the 500hPa charts.

Just looked through the GEFS ensembles for the same time frame and nearly all have a deep low pressure

near the UK.

Meteociel - Cartes Ensemble GFS (ENS/GEFS)

http://modeles.meteo...15-1-180.png?12

[Snow and storms]

what kind of conditions would this forecast give, if it came off? that looks like a very substantial storm. Am I right in thinking the rain would not be as bad as an atlantic born storm?

[summer blizzard]

I don't like the 12z models, the low is too strong and as a result cooler air is mixed out.

[SMU]

_{http://modeles.meteociel.fr/modeles/}gfs_{/runs/2012102418/}gfs_-0-42.png?18

GFS _{further west by a couple of hundred miles & has developed the nice trough that was on the} UKMO_-

_{Heavy snow moving into Eastern scotland sat Eve- possibly hanging on as snow further south- like southern uplands, Yorkshire etc-}

_S

[Isolated Frost]

+30, -44c 500hPa temps off Norway, and that trough is forming, a one spotted by the UKMO model and FAX, the ECM then added it in it's 12z and now the GFS seems to see it.

I look at the wet bulb temperature (Theta-W) off Norway as well.

A noticeable trough behind the potential PL and an unstable N to NE flow. All characteristics of a PL... I'm beginning to wonder whether that trough is more than a trough, but is the first Polar Low of the season?...

[Cloud 10]

what kind of conditions would this forecast give, if it came off? that looks like a very substantial storm. Am I right in thinking the rain would not be as bad as an atlantic born storm?

Far to early for specific details but worth keeping an eye on,as quite unusual for such a strong

low pressure signal at 8-10 days out.

[AWD]

Looks likely, even at this early stage, that Halloween will be a particularly unsettled day.

http://www.meteociel.fr/cartes_obs/gens_display.php?x=476&y=0&run=12&runpara=0&type=1&ext=1

One of the members goes right off the scale!

At least it looks like I won't have to pass over too many biscuits and sweets this year! :p

Edited October 24, 2012 by AWD

[davehsug]

+30, -44c 500hPa temps off Norway, and that trough is forming, a one spotted by the UKMO model and FAX, the ECM then added it in it's 12z and now the GFS seems to see it.

I look at the wet bulb temperature (Theta-W) off Norway as well.

A noticeable trough behind the potential PL and an unstable N to NE flow. All characteristics of a PL... I'm beginning to wonder whether that trough is more than a trough, but is the first Polar Low of the season?...

Would have expected better from you IF. Polar lows can not be picked up by models due to their very nature. Every winter we get this.

[Isolated Frost]

Would have expected better from you IF. Polar lows can not be picked up by models due to their very nature. Every winter we get this.

? They possess all the characteristics of a PL as I've described and we are within 18hrs of it's formation. Matt Hugo also said on Twitter this was a possible polar low.

[Snowy Easterlies]

It looks like northern blocking has gone on the 18z GFS although we stay in cool atlantic air, but i hope we don't see some sort of pressure rise from the south as often happens

[Gibby]

what kind of conditions would this forecast give, if it came off? that looks like a very substantial storm. Am I right in thinking the rain would not be as bad as an atlantic born storm?

Just because the Low is so deep does not necessarily mean anymore rainfall than a Low of 1015mbs would give. It's coverage is greater though and once fronts clear the area some potent convective activity would be the most likely cause of rain with adjacent coastal areas seeing rain and hail with lightning too possible. It would likely be rather cold feeling though temperatures should not be that low.

[davehsug]

Well I beg to disagree. JH would be the best to ajudicate I suggest.

[BLAST FROM THE PAST]

Hi BFTP,just wondered what your thoughts are regarding this deep low pressure signal

on the model output.

Could be a very lively start to November.

Most models are going for something similar....a must watch period. I'm concerned about both rain and wind affect but its almost stagnant for some length of time...That is MAIN concern. The sheer thermsl gradient is close to UK so that sort of intensity used to be reserved for NW of Scotland, the southerly shift of the jetstream and -ve PDO set up IMO puts us in the firing line.

BFTP

Edited October 24, 2012 by BLAST FROM THE PAST

[Cloud 10]

Well Polar low or not,it is looking interesting!

[Thundery wintry showers]

Re. Gibby's post, I think the Scandinavian blocking high is indeed a more reliable source of cold weather for most of the UK (the west-based negative NAO with Greenland blocking is a good case in point) but whether it's more reliable for widespread snowfall is another matter. I remember a lot of cold dull dry spells with limited snowfall associated with Scandinavian blocking highs, usually when they are too far east/south to send arctic continental airmasses our way and we instead get a combination of cold surface air and mild upper air heading up from the eastern Mediterranean, with stratocumulus trapped under an inversion. That said, when the Scandinavian high ridges westwards towards Greenland/Iceland (a scenario that Gibby mentioned) then it is very rare not to get widespread snowfalls as a result, via snow showers heading well inland from the North Sea and/or fronts pushing up from the south and stalling against the block.

Meanwhile, the cold snap for the upcoming week has been toned down substantially, with the main initial cold blast going out into the continent and subsequent cold blasts being sent into the mid-Atlantic. Nonetheless, there will be some wintry showers in counties bordering the North Sea on Friday. Friday afternoon and evening will probably see rain showers over eastern England with sleet and snow to near sea level in eastern Scotland, then on Friday night we may see sleet and snow penetrate as far south as Lincolnshire. The -5C 850hPa isotherm struggles to penetrate as far as East Anglia, so although showers will continue over East Anglia for most of Saturday, I think they will continue to fall only as rain or perhaps hail locally.

Re. the possible Monday/Tuesday snow event, it looks to me as if the main thrust of cold air will be out to the west. After that, the ECMWF has low pressure rooted to the north-west of the UK bringing sunshine and showers and temperatures near or slightly below average, but the GFS sends the low southwards to the west of Britain. From previous experience of these setups I feel that the GFS will probably be nearer the mark with this one, but I'll wait until I see tomorrow's runs before reaching any strong conclusions.

Edited October 24, 2012 by Thundery wintry showers

[SP1986]

A polar low is a very fragile system, and certainly in October, most likely wouldn't deliver the correct ingredients for whiteouts and snow drits, such as Polar Lows have the potential of delivering in deep winter.

If the air was to stay relatively dry, the Polar Low would probably dissapate, as you need humidity to sustain it. They often stay near warm water to get their energy.. the nickname Arctic Hurricane is quite apt!

But in general, I would suggest the only place that would really be affected by a Polar Low at this time of year is Northern Scotland..

But can a Polar Low be called so, if it is delivering rain? I'm only familiar with Polar Lows from the extraordinary amounts of snow they can dump in winter.

[Isolated Frost]

FAX looking quite positive from a convection pov. That trough looks very potent (PL or not), and could deliver some serious precipitation if it moves further west.

[Isolated Frost]

The hi-res NMM has bought the trough into view, available in the fantastic NW Extra suite. Plenty of heavy precip associated with that trough so a movement w'wards could give some very heavy snow on Friday evening.

[Gibby]

Good morning. Here is my review of the 00zs from GFS, UKMO and ECM. for today Thursday October 25th 2012.

All models show a similar pattern to the change in weather over the coming 4-5 days. The curent NE flow over the UK is cooling as I type with the breeze freshening as the pressure gradient over the UK steepens. A cold front will move slowly South over Scotland today and the resk of the UK overnight and tomorrow. It will become much colder and clearer once the front has passed with the persistent cloud of recent days clearing way to allow for sunny periods in a chill North wind. A frost will develop in parts of Scotland tonight and everywhere tomorrow night as the cold air digs in. The weekend is shown to have a Northerly flow over the UK with a few wintry showers in the far North and later East of Britain though all other areas will stay dry. It will be cold but with sunny spells through the day. On Sunday winds back towards the West as Low pressure to the North slides down over the UK cutting off the cold Arctic feed and allowing Atlantic winds, cloud and rain to move from the NW to all areas by Monday. At the same time somewhat clearer and colder weather could return South behind the ran band with Monday seeing a rather cold day for most with the chance of a little rain or showers here and there through the day.

GFS then continues through next week with a deepening Low developing just to the North of the UK by midweek bringing milder Atlantic winds back once more with a spell of rain crossing all of the UK through Wednesday. Once cleared a very showery and cool zonal spell of West or NW winds will follow with hail and thunder a possibility in the showers as well as snow on Northern hills in rather cold conditions generally. This rather cool and showery theme continues into the early stages of FI with some longer spells of rain at times before things quieten down in response to High pressure building to the South feeding a milder WSW flow across Britain and pushing Atlantic rain bearing features more towards the NW of the UK. The end of the run sees a NW/SE split developing with mild and dry conditions in the South and East with rain at times in the NW, sweeping SE at the end of the run to all areas.

The GFS Ensembles show the run described above as a mild outlier later on in FI in the South. The general trend between the members is for a slow recovery to normal uppers after the cold dip this weekend the further one travels through the run with Low pressure dominating with quite a bit of rain about at times for all areas.

The Jet Stream shows the main arm to tilt NW to SE over Britain in the coming days to be lying to the South of the UK near the English Channel and northern France in a week or so time.

UKMO for midnight on Wednesday shows Low pressure just to the North of Scotland with a WSW flow over the UK with a trough approaching Western areas through the upcoming period. Less cold conditions will of arrived with an increase of cloud and rain expected over the UK through the day to follow.

ECM too shows a milder interlude midweek as the cold front to the West is slow to cross the UK. By Thursday it does stagger across the UK bringing cool and showery conditions in from the West with some heavy showers at times in a cold and blustery wind with hail and thunder locally and snow on Northern hills. By the end of the run Greenland High pressure blocking has gone and the weather looks like continuing unsettled and less cold in the period after the end of the run.

In Summary this morning an unsettled spell is on the way. The dilution of this weekends cold blip becomes more apparent with each run with next week's major Low looking to hold a position to the North preventing much in the way of a feed from the North to the UK. Instead we have a West or NW flow and a conventional pattern of showers or longer spells of rain with snow restricted to the mountains of Northern Britain at best. High pressure over Greenland becomes less of an influence over the UK with time and breaks down completely by ECM at day 10 this morning.

[Frosty.]

After the mini cold snap ends on sunday (earlier further north), this was the most wintry chart I could find, until then the gfs 00z op run floods the atlantic with arctic air and the atlantic does become ultra blocked, it's an annoying run because if sunday's/monday's low could push further east, we would have a mega cold spell but instead, the uk becomes locked into a cyclonic pattern with wet and windy weather dominating except in the large eye of the low where winds will be much lighter, it does look rather cool but no deep cold at all despite the origins of the air being arctic, the long sea track mixes out the cold uppers and we are left with temps of 8-10c but maybe cold enough for snow on the scottish mountains, eventually the low fills and the pattern becomes flatter before that late FI tease. It's only an op run though so a colder looking outlook cannot be ruled out.

Edited October 25, 2012 by Frosty039