[Storms and Convective discussion - November 2023 onwards]

THUNDERSTORM WATCH - TUES 12 DEC 2023

Issued 2023-12-12 08:46:32
Valid: 12/12/2023 0600 - 13/12/2023 0600

Forecast Details

Upper and collocated surface low will move east across England and Wales on Tuesday, upper cold pool will steepen lapse rates across southern Britain, particularly over relatively warmer SSTs which are forecast to generate 200-500 j/kg CAPE, this instability moving inland from the coasts in moist southwesterly flow for a time across Wales, central and southern England ahead of surface low centred over Wales by 15z. Increasing forced ascent of this unstable airmass ahead of the low moving east will support heavy showers and a few brief thunderstorms – which may produce hail, gusty winds and intense rainfall leading to localised flooding. A few funnel clouds or even a brief waterspout/weak tornado can’t be ruled out with stronger updrafts near coasts where relative warmer SSTs will create more buoyancy of air.

Issued by: Nick Finnis

Severe Convective Weather & Storms Forecast - Netweather.tv

WWW.NETWEATHER.TV

 

[UK Storm and Severe Convective Forecast]

UK Severe Convective & Storm Forecast - Issued 2023-12-12 08:46:32

Valid: 12/12/2023 0600 - 13/12/2023 0600
THUNDERSTORM WATCH - TUES 12 DEC 2023

Click here for the full forecast

[Model Output Discussion - Into Winter]

2 minutes ago, Jacob said:

Honestly I saw this coming. While people were all doom and gloom about warm weather, you were treating it like it was a few days away when it’s well known that weather models flip all the time, especially in FI

It will lift the doom and gloom for 6 hours, unless the pub run follows suit with a similar stonker of a northerly, then we can wait for a reality check in the morning 

[Model Output Discussion - Into Winter]

Oh no, here we go again, just when we thought it was all over! The 4x daily GFS rollercoaster

[Model Output Discussion - Into Winter]

The trop PV looks too well-organised and strong to allow any amplification of ridges to build north to allow a downstream northerly of any potency or longevity. I guess we'll need to carry on kicking that can down the road for the next cold and wintry fix, let's hope it's not too long a wait. 

[Model Output Discussion - Into Winter]

I've always thought the GFS had a knack of picking up northerlies best at range that do come off in the 20 years+ of weather model watching, so certainly worth entertaining the idea in the run up to Xmas, despite the ups and downs last few days. How long lasting and potent it will be is too far off to contemplate though.

[Model Output Discussion - Into Winter]

Every year we come back and get on the same roller coaster of emotions tugged by 4x GFS daily runs blowing cold or mild when there's hints of a pattern change on the horizon 

Not a bad effort by 12z GFS to get coldies on a high to start the evening ...

[Model Output Discussion - Into Winter]

9 minutes ago, swfc said:

Good points. The tripole as you say is evident but the mid atlantic could do with a drop in temps. The mixing up from the hurricane season as vanished. The atlantic ridge imo isnt atm going to make much headway towards greendland. Stronger chance as the pv moves towards siberia and the far north of europe. Zonal winds are under pressure without a full on ssw and the 10hpa strat is looking pretty good in that respect. The period until 2024 looks with all caveats hopefully a lot drier ans seasonable. Maybe an outside chance for those who havnt been naughty seeing some wintry precipitation

From the studies I've read, the N Atlantic tripole tends to have have a positive feedback later in the winter if it's only recently appeared. The tripole likely appeared due to the -NAO from mid-September through to late November, with low pressure systems on a southerly track over the N Atlantic cooling waters during autumn. It's feedback to the atmosphere is not an immediate impact, but can be linked to further episodes of -NAO in coming months. 2009-10 and 2010-11 winters featured the tripole. Will not be the only factor that may bring a -NAO though, SSW / -eQBO weakening the PV will help too.

[Model Output Discussion - Into Winter]

9 minutes ago, BartyHater said:

Apparently some in the US forums are bemoaning the complete lack of cold this Christmas….looks to me like for 50,000,000+ in the NE, things are going to come good just at the right time once again….what a surprise eh?

Yes, but that would be dry cold for most of them, away from downwind areas of Great Lakes - which may get Lake Effect snow. NE USA normally needs a low hugging the eastern seaboard, of which there isn't around then, otherwise it's mostly dry and bitterly cold other than flurries.

For us, could be a lot worse than the 06z GFS op, some cold outbreaks in there, but also some mild sectors toppling in from the NW. At least that troughing is digging deep to our east - which will help pull in arctic air from the north and maintain the deep cold to our east and northeast, should it come the time to tap in the coming weeks,

[UK Storm and Severe Convective Forecast]

UK Severe Convective & Storm Forecast - Issued 2023-12-10 10:20:29

Valid: 10/12/2023 1000 - 11/12/2023 0600
SEVERE CONVECTIVE WATCH - SUN 10 DEC 2023

Click here for the full forecast

[Model discussion highlights]

Just now, LRD said:

Thanks Nick. That's really strange. SW wind, high uppers and only producing those temps.

Quite an inversion going on, especially in the south. Clear skies at night lead to temperatures dropping like a stone near the longest night of the year if the air is dry, which it would be under a strong upper level ridge with dry air subsiding from high up - which also evaporates any cloud.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4972905

[Model Output Discussion - Into Winter]

Just now, Dennis said:

late dec23 will show cold days 

Always the optimist!

The trop PV is not one for splitting to allow height rises over Greenland on recent GFS runs, maybe a new trend that won't reverse and get worse just before Xmas, or it may revert back to decent splitting into separate vortices over N Canada and NE Europe like the more colder runs of previous days. More runs needed.

12z GFS does at least drag down some cold arctic air, but with the TPV not splitting to allow the Atlantic ridge to build north over Greenland, any cold incursion would be prone to being toppled from the NW by mild sectors.

[Model Output Discussion - Into Winter]

Just now, LRD said:

Thanks Nick. That's really strange. SW wind, high uppers and only producing those temps.

Quite an inversion going on, especially in the south. Clear skies at night lead to temperatures dropping like a stone near the longest night of the year if the air is dry, which it would be under a strong upper level ridge with dry air subsiding from high up - which also evaporates any cloud.

[Model Output Discussion - Into Winter]

1 minute ago, LRD said:

The trend towards that is obvious though. Exact details to be determined but an exceptionally warm spell from about the 16th or 17th is the front runner. The south might escape the worst of the warmth under an inversion but even that's doubtful

I looked at the long draw SWly flow at 850 hPa, with T850s of 10C reaching the UK from the Azores around the 17th and all's not what it seems at the surface ...

Even the highs in the afternoon of 17th aren't screaming overly mild - more average.

All an educated guess that far off though ...

 

[Model Output Discussion - Into Winter]

Morning. Nice to see 00z GFS op in a festive mood with a wintry look to start Christmas Eve.

I suppose, ideally, we see the high building NE from the Azores gain high enough latitude to keep it calm and chilly, with night frosts and develop some surface cold, before the high migrates NW and an upper trough drops SE into Scandinavia and thus opens up the doors to the arctic in time for Xmas.

However, 00z EC continues to show the high not gaining enough latitude so we end up with a mild and possibly cloudy flow off the Atlantic - especially further north, south maybe clearer and frosty.

[10 years on from the North Sea storm surge which brought the worst coastal flooding for 60 years]

A look back on the North Sea storm surge which brought flooding to eastern coastal communities, 60 years after the Great North Sea Flood of January 1953.

View the full blog here

[Model Output Discussion - Into Winter]

So, a week of changeable weather from the west off the Atlantic, potentially deep low on Sunday crossing the north needs watching, it looks like the weather will settle down again from Thursday week with a strong signal from 00z EPS/GEFS for a rise in pressure.

High pressure building in from Azores looks favoured, but where it eventually ends up positioning as an anticyclone could determine if it will be mild & dry or chilly & dry with frost. It could end up being milder in the north with flow from the southwest nearer low pressure to the northwest and colder towards the south.

But some differences between models and uncertainty in the shape and orientation of the strengthening tropospheric polar vortex will affect the position of the jet stream and thus determine where the anticyclone will position.

[Model discussion highlights]

22 minutes ago, BartyHater said:

Once again the overnight runs look like chalk and cheese compared to the expected mid-month pattern touted by the background teleconnections.

Anyone without access to the latter would be forgiven for thinking another cold spell is a very long way off this morning, with the GFS ENS doing little to counter that argument - mass scatter being the only crumb of comfort.

Lets hope the flip comes in the next few days, because trying to find comfort in the ECM 240hr becomes very draining.

 

I wouldn't be expecting much of a shake up from the zonal / unsettled outlook by mid-month. More like around Christmas time for the pattern to change to a more blocked one, *if* the MJO reaching P7 and 8 mid-month sufficiently amplifies the upper flow over the N Atlantic and Europe after around a 10 day lag. But no guarantees.

No point getting too hung up over most model runs over the next week or two not showing cold scenarios. The current model outlook is bog standard default fare for December, what we normally see, until  wave driving from an uptick in tropical forcing over the Pacific to high latitudes picks up mid-month to change the mid to high latitude upper flow patterns upstream. 

.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4969697

[Model Output Discussion - Into Winter]

22 minutes ago, BartyHater said:

Once again the overnight runs look like chalk and cheese compared to the expected mid-month pattern touted by the background teleconnections.

Anyone without access to the latter would be forgiven for thinking another cold spell is a very long way off this morning, with the GFS ENS doing little to counter that argument - mass scatter being the only crumb of comfort.

Lets hope the flip comes in the next few days, because trying to find comfort in the ECM 240hr becomes very draining.

 

I wouldn't be expecting much of a shake up from the zonal / unsettled outlook by mid-month. More like around Christmas time for the pattern to change to a more blocked one, *if* the MJO reaching P7 and 8 mid-month sufficiently amplifies the upper flow over the N Atlantic and Europe after around a 10 day lag. But no guarantees.

No point getting too hung up over most model runs over the next week or two not showing cold scenarios. The current model outlook is bog standard default fare for December, what we normally see, until  wave driving from an uptick in tropical forcing over the Pacific to high latitudes picks up mid-month to change the mid to high latitude upper flow patterns upstream. 

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[More snow on the way for parts of England & Wales]

Areas of rain spreading in off the Atlantic will turn readily to snow as it bumps into cold air over the UK tonight and again on Sunday night and Monday, with a risk of travel disruption.

View the full blog here

[SE, London & EA - Weather Discussion]

37 minutes ago, 3cats said:

Alexa has just alerted us that there’s a 75% of snow at 4am tomorrow morning here in Redhill. 

Better set the alarm for 4am then

Looks dry on most models for that time. Afternoon may see some flurries.

[Model Output Discussion - Into Winter]

3 minutes ago, TSNWK said:

Well that’s something.. maybe still a small chance of sliders if we can keep the cold in…

Don't think we can totally discount the idea of a more southerly tracking jet taking lows under the UK and Ireland which stay in the cold air in the day 7-10 range, fair few GEFS postage stamps show this option, but its not the favourite evolution in the medium range for now.

[Model Output Discussion - Into Winter]

30 minutes ago, Catacol said:

Pretty sure of its arrival. For all of the excitement, and the pleasing context of a week of cold, we have known all along that core drivers are not favourable for high lat blocking of any sustained nature. It is interesting to contemplate why it looks as though it will last 7-10 days - pretty damned good really - but until we see the next pacific wave arrive alongside potential impacts on the vortex proper this very early spell is a bonus. 

Yes, since looking at the 00z EPS and clusters, it does look like the writing is on the wall, as per last post. It seems upstream forcing over N America and Pacific will start to overwhelm any blocking over the N Atlantic later next week.

MJO RMM plot which combines the standard MJO with the current 120-day signal of the background base tropical forcing pattern suggest MJO getting into P7 on Dec 15, P8 on Dec 20, P1 on Dec 25. So perhaps for the festive period of late December we may see a return of more blocking. Scandi high?

Phase 7 in Nino in Dec

Phase 8 in Nino Dec

[Model Output Discussion - Into Winter]

Does look like the writings on the wall for the demise of the cold spell from EPS mean and also clusters in 2 camps, the Atlantic trough taking charge having largest support by a big margin.

[Model Output Discussion - Into Winter]

Looking a dry cold spell for most, any wintry showers limited to northern Scotland & eastern coastal areas in the main, perhaps a dangler down the Irish Sea into Pembrokeshire and Cornwall too.

May feel like a waste of cold enough air for some snow, but it is what it is, and I guess it makes a change from mild, wet and windy weather that we've endured most October and November. The breakdown from the Atlantic keeps showing, but still too far out to be sure of its arrival for now.