Uk Convective General Discussion & Forecasts - April 2012 Part 4

[Jane Louise]

Here you go folks a new thread . An exciting day awaits tomorrow and tonight, Good luck y'all

[MP-R]

Not sure what happened to the cell that went over Taunton and Bridgewater but it missed here.

As for tomorrow, Alex Deakin really did like the word 'thunderstorm' on the BBC forecast and Ian Ferguson was quite confident for 'beefy downpours' so could get interesting. The sooner the frontal rain clears northwards the better really. Will tomorrow be the day for a real thunderstorm? We'll see.

[CreweCold]

Not sure what happened to the cell that went over Taunton and Bridgewater but it missed here.

As for tomorrow, Alex Deakin really did like the word 'thunderstorm' on the BBC forecast and Ian Ferguson was quite confident for 'beefy downpours' so could get interesting. The sooner the frontal rain clears northwards the better really. Will tomorrow be the day for a real thunderstorm? We'll see.

Looks like they'll develop and then trundle north towards the evening time. I noticed he really emphasised the word vicious.

[davehsug]

I'll wait & see. If carol says "humdingers" in the morning it's game over

[Raidan]

Ha ha some of you guys should do stand up,some of the comments on here had me laughing especially you AJ some of the things you come out with lol,back to tonight I'm wondering if I should stay up after reading the skywarn warning but living in Bristol and constant let downs and dissappointment I don't know if it would be worth it,or should I wait to see what tomorrow daytime brings,probably nothing as always the case like it is for others,hmmm decisions decisions.

[Costa Del Fal]

Indeed Alex was very keen on ThunderSTORMS not Thundery showers, however I really do seem sceptical with these systems and too much cloud within them. We shall soon find out. Could be interesting here tomorrow evening!

[Robbie Garrett]

Lord have mercy....

EGKK 241702Z 2418/2524 27010KT 9999 SCT030 BECMG 2418/2421 20007KT PROB40 TEMPO 2418/2420 7000 SHRA BECMG 2502/2505 13013KT 8000 -RA TEMPO 2505/2510 4000 RA BKN012 BECMG 2506/2508 14020G30KT PROB40 TEMPO 2506/2509 +RA BKN008 BECMG 2509/2511 17017G27KT 9999 NSW TEMPO 2510/2524 5000 +SHRA TSRA PROB40 TEMPO 2510/2524 18025G37KT 2000 +TSRAGS BKN014CB

You don't need to know what it says, just the length would purely show you tomorrows not a good day for flying a kite, let alone an airplane let's put it that way.

[Robbie Garrett]

Also, next Monday looks of real interest....

and wow, Wednesday.......

Plenty of time for upgrades and downgrades, but even the most experienced members would know that if that was to come off with a LP in the bay of biscay your asking for an MCC/MCS or Supercell... etc I'd be seriously shocked if we don't see at least a Thunderstorm in that situation.

[Sprites]

Indeed Alex was very keen on ThunderSTORMS not Thundery showers, however I really do seem sceptical with these systems and too much cloud within them. We shall soon find out. Could be interesting here tomorrow evening!

when it comes to forecasting thunderstorms, i dont have much faith in the T.V. weather forecast, ive found the online weather forums are more accurate...

[Coast]

Morning weather fans!!!

Shall we start with a level 1 for Southern England from ESTOFEX?

Storm Forecast

Valid: Wed 25 Apr 2012 06:00 to Thu 26 Apr 2012 06:00 UTC

Issued: Tue 24 Apr 2012 22:59

Forecaster: PUCIK

A level 1 was issued for NW France, S England and Belgium mainly for severe wind gusts and tornadoes.

SYNOPSIS

A complex synoptic setup is forecast over Europe during the given day. Major feature will be a deep cyclonic vortex featuring very strong low level wind fields (over 30 m/s at 850 hPa level) over the eastern Atlantic, centered just south of the British Isles as of 12 UTC. Strong mid (40 m/s) and upper level (60 m/s) jet is forecast on the southern and southeastern flank of the vortex. Towards the east, a mid/upper level trough is forecast to decay into a short wave trough crossing Poland during the day and a cut-off low, which will traverse across Romania during the afternoon hours. A mid-level ridge is expected to amplify between these two features.

Closer to the surface, a main frontal zone will run from Balkans towards Belarus and the Baltic states. Along this zone, numerous separate low pressure centers will contribute to the wavy nature of the front, especially regarding northern Romania/southern Ukraine. Another frontal zone will be linked to the deep low pressure system, crossing western Europe during the day.

DISCUSSION

... NW France to S England / Belgium...

Behind the large stratiform precip shield of the warm front, in the strong southwesterly flow, models predict weakly unstable airmass, featuring almost indifferent vertical temperature gradient spreading across this region. Thanks to the very deep low, with its central pressure close to 970 hPa, intense low level wind field is forecast, with 925 to 850 hPa wind speeds approaching 35 m/s in the region of NW France and in the range of 25-30 m/s elsewhere. Forecast soundings/hodographs reveal very skinny CAPE profiles with unidirectional strong shear confined mostly to 700 hPa, with no further significant increase in windspeeds above this level. MLCAPE values should not exceed 500 J/kg, with more realistic predictions between 100-200 J/kg (per ECMWF/WRF simulations).

Deccelerating mid and upper level low, along with unfavorable position of the jet-streak during the development of instability over the region point to the reduced possibility of strong, concentrated forcing favoring rapid development of QLCS along the cold front. Nevertheless, scattered DMC is forecast, which should pose risk of severe wind gusts and tornadoes, especially thanks to the LLS exceeding 15 m/s. Threat should be the most prominent between 12-15 UTC timeframe, virtually ceasing after 18 UTC. An upgrade to Lvl 2, especially over NW France is possible if forcing or instability prove to be more favorable.

SkyWarn have:

SKYWARN UK SEVERE THUNDERSTORM WATCH #014

ISSUED: 1400UTC TUESDAY 24TH APRIL 2012 (GJ)

SKYWARN UK HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR THE FOLLOWING REGIONS: ENGLAND WALES CHANNEL ISLANDS

IN EFFECT FROM 0100UTC UNTIL 2359UTC WEDNESDAY 25TH APRIL 2012

UNSEASONABLY DEEP LOW PRESSURE SYSTEM SWINGING INTO THE SOUTHWEST, BRINGING A WELL-SHEARED OCCLUDING FRONT AND POST-FRONTAL INSTABILITY WITH INTENSE PRECIPITATION.

THERE IS A POSSIBILITY OF SEVERE WEATHER AFFECTING THE INDICATED REGIONS IN THE TIME PERIOD SPECIFIED. THREATS WITHIN THIS WATCH INCLUDE, BUT ARE NOT LIMITED TO:

HEAVY RAIN...STRONG GUSTS...FLASH FLOODING...FUNNELS/WEAK TORNADOES...LIGHTNING

DISCUSSION:

THERE IS STRONG MODEL CONFIDENCE ON THE POSSIBILITY OF A SEVERE CONVECTIVE RISK THROUGH TOMORROW. A LOW PRESSURE ~972MB DEVELOPS FROM THE SOUTHWEST INTO THE CENTRAL SOUTH OF ENGLAND DURING THE MORNING. AHEAD OF THIS, A RAPIDLY OCCLUDING FRONT COULD BRING RAINFALL ACCUMULATIONS OF UP TO 40MM OVER A SHORT PERIOD OF TIME ACROSS SOUTHERN ENGLAND AND WALES, ESPECIALLY NEAR TO COASTS. A STRONG LOW TO MID LEVEL SHEAR ENVIRONMENT EXISTS AS 850MB WINDS OF 50KTS ROUND THE CIRCULATION UNDER UPPER DIVERGENCE AND STRONG VORTICITY AS THE CYCLONIC JET NOSES UP FROM THE SOUTH, PROVIDING AMPLE DIRECTIONAL SHEAR AND DYNAMIC FORCING FOR ORGANISED CONVECTIVE ELEMENTS THOUGH INSTABILITY IS INITIALLY LIMITED TO LOW TO MID LEVELS ALONG THE BACK OF THE FRONTAL ZONE. WITHIN THIS FRONT, MEAN WINDS OF 40MPH WITH CONVECTIVE GUSTS UP TO 60MPH ARE POSSIBLE, ENHANCING RAINFALL, ALSO ACROSS HIGHER GROUND FURTHER NORTH IN ENGLAND AND WALES LATER IN THE PERIOD. BEHIND THE FRONT, A DEEPER BUT ONLY MODERATELY UNSTABLE ENVIRONMENT WILL ALLOW FOR SUBSEQUENT ISOLATED CONVECTION BRINGING FURTHER LOCALLY HEAVY RAIN TO SATURATED AREAS. THIS COULD BECOME MORE ORGANISED ALONG CENTRAL TO EASTERN PORTIONS OF SOUTHERN ENGLAND, UNDER INCREASING DLS OF UP TO 30KTS THROUGH THE AFTERNOON; WHILE SUFFICIENT LLS EXISTS ACROSS THE WATCH AREAS FOR BRIEF FUNNELS AND WEAK TORNADOES. THIS WATCH MAY BE UPDATED OR UPGRADED. PLEASE MONITOR WEATHER AND TRAVEL INFO AS NECESSARY.

UKASF:

Forecaster: Dan

Last Updated: 2012-04-24 23:58:00

Valid: 2012-04-25 00:00:00 - 2012-04-25 23:59:00

Areas Affected:

SLGT: Midlands, Wales, E Anglia, SE, CS + SW England

Synopsis:

A new vertically-stacked area of low pressure will approach from the southwest, with a set of fronts moving northwards and eastwards. Post-frontal instability will be the main interest for convection.

Discussion:

Along the occluded front exists the potential, with an overlap of >35kts LLS and >60kts DLS, of line convection/LEWPs capable of producing localised torrential downpours and gusty winds.

Main attention however will be behind these fronts, with a large zone of instability wrapping around the area of low pressure. As LSTs rise during the afternoon, a few hundred J/kg CAPE will develop, allowing scattered showers and thunderstorms to occur. While an overlap of 40kts DLS and >25kts LLS is likely during the afternoon, particularly over SE England, due to the vertically-stacked nature of the low pressure system there is a distinct lack of directional shear. Thus, although convection may become organised, this does somewhat hinder the chance of any long-lived sferic activity, and also any tornadic potential.

Nevertheless, there is the chance for a tornado event to occur, with hail also likely in many of the showers, perhaps up to 2.0cm in diameter locally in stronger cells. Convection could become quite messy with excessive cloud associated with occlusion debris and a rather moist vertical profile, coupled with a strong steering flow. For now we only go with a SLIGHT level for lightning coverage, but this will need to be monitored during Wednesday and may be upgraded if necessary, along with the potential for severe weather.

Showers will persist throughout the night in places, merging into longer spells of rain along sections of the wrap-around occlusion along with a continued risk of lightning (albeit <35%).

I'll get some chart information out in a moment for you to decide where the potential and threats are.

[ardsar]

Heavy showers around the plymouth area with thunder and lightning. Very Squally conditions. Had 19.4mm of rain over the last 6 hours.

[SteveB]

Looks like some storm firing up already.

Anyone lucky enough to catch one could have a good display.

[Coast]

OK, a bit more information on the possibilities for today and I think convection-wise, it's more favoured in the South:

Nice blob of red in the South for possible lightning:

Are we going to see any thunderstorms??

Bay of Biscay is filled, Wales (like yesterday) maybe and possibly something on the Southern coast/Channel:

Bit of CAPE creeping in later, but not significant:

Big dollops of shear and convergence:

Spout potential in the Channel approaches mid afternoon:

LAPSE rates OK:

Tornado possibilities reasonable all day:

Larger wind gusts generally more to the South of us in France, but the headland along the coast in Sussex may get a bit of attention:

I'll stick some separate stuff in about winds and gusts associated with the low in the Atlantic storms section shortly.

[Marcus_surfer]

Looks interesting today

[Paul Sherman]

If Nick F Was Issuing a Convective Forecast today should be firmly in the "Slight Risk" Category today!!

The Showers that follow the Cold Front look lively enough and with 40kts of Flow should be capable of Multi Cell Capabilities in Central Southern England, could also be some Tornado Reports near the Coasts later on. One To Follow Later today if Cells can gather strength and that rare beast the LLJ Can Impact Southern England moving up from France.

[Kain]

Looks like its kicking off down Devon!

[Azores Hi]

If

One To Follow Later today if Cells can gather strength and that rare beast the LLJ Can Impact Southern England moving up from France.

*puts hand up*

Sir! Sir! Please Sir! What's LLJ Sir?

[Coast]

Sir! Sir! Please Sir! What's LLJ Sir?

Low Level Jet?

A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer). The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

[Paul Sherman]

Sorry Azores it is "The Low Level Jet"

It helps lift the Parcels to create Convection and lots of it.

It happens almost daily in the USA usually from 5-6pm and helps substantailly in development of Tornadoes over there or it can create an Overnight MCS Affecting the EML (Elevated Mixed Layer)

It is a rare beast in the Uk but can happen on odd occasions but more prevalent in France and Continental Air Masses

[Weather Wizard]

Torro just put out a warning and I am in the red area

This is begining to look intresting

http://www.torro.org...te/forecast.php

[Paul Sherman]

Pretty much as I expected really.

With 40kts of Flow and Deep Layer Shear present Supercells are a real possibility today more especially near Southern Coastal Counties.

That Tornado Watch is warranted and would also be issued in the US With these parameters, remember it is just a risk though but things are falling into place quite rapidly atm

[Marcus_surfer]

Some big showers moving up from the south, see how gusty it gets soon in the showers

[Azores Hi]

Thanks for the explainations re LLJ!

[Kain]

Its actually quite exciting today to see what happens abit later. I don't have alot of faith in torro usually as they forecast tornadoes at a drop of a hat, but I cant help but hope they are right today (as long as no one gets hurt of course)

[Lynxus]

WOW. There really are some intense showers down south at the moment.

Wow, Just wow..

Its very windy here but rain stopped an hour or so ago..

Time to get my radar subscription I suggest everyone does