21st Sept 2011 Storm/convective Forecast Discussion And Reports

[Coast]

I'll start today off with ESTOFEX's level one forecast for Northern England!

Storm Forecast

Valid: Wed 21 Sep 2011 06:00 to Thu 22 Sep 2011 06:00 UTC

Issued: Tue 20 Sep 2011 22:21

Forecaster: KOROSEC

A level 2 was issued for parts of Greece and western Turkey mainly for excessive convective rainfall, large hail, strong to severe wind gusts and a tornado threat.

A level 1 was issued for Greece, Ionian sea and Aegean sea mainly for waterspouts and locally excessive rainfalls.

A level 1 was issued for parts of northern UK mainly for strong/severe wind gusts and brief tornado or two.

SYNOPSIS

The main synoptic features today will be two cyclonic vortices, one over SE Mediterranean and another vortex situated over the Northern Atlantic with its short-wave trough progressing over northern UK towards Scandinavia. At surface, a broad surface low with cold front moves across Aegean sea into western Turkey while deep surface low makes only slow progress beneath the vortex near UK. A cold front pushes eastwards across North sea. Elswhere, stable conditions prevail.

DISCUSSION

... parts of northern UK ...

Quite impressive mid-level jet rounds the trough axis moving across the northern UK, providing strong shear at all levels. Models are in an agreement of marginal instability release along the rapidly moving cold front. Some convective activity is expected with a convective line possibly to form along the frontal convergence. With very strong mid/low level wind profiles and shear, some organized storms could take place within the line. The main threat will be strong/severe wind gusts while given the strong LL shear and SR helicity available, a tornado or two could occur as well. Front should rapidly move across the North sea and reach SW Scandinavia in the early night.

and from the UKASF:

Forecaster: Dan

Last Updated: 22:43 Tuesday, 20th September 2011

Valid: 00:00 Wednesday, 21st September 2011 - 23:59 Wednesday, 21st September 2011

Areas Affected:

SLGT: Scotland, Northern Ireland, N England

Synopsis:

Upper trough axis swings across the British Isles during Wednesday. Any significant convection will be found in the base of said trough behind a rather active cold front.

Discussion:

During the forecast period a cold front will move southwards and eastwards across the highlighted areas. >60kts DLS and >40kts LLS along the cold front enchances the tornado risk greatly, and thus there is a considerable chance for a tornado to occur along what is expected to be quite an active cold front with locally torrential downpours. There is certainly potential for some quite noteworthy line convection along the front, and perhaps a LEWP event/Bow Echo. A very steep temperature gradient will also be present said front in the mid-levels (typically >8C temperature difference on either side at 500mb) which is likely to lead to some very heavy downpours in places.

Behind the cold front, cold mid-levels overspread the aforementioned regions (as low as -28C at 500mb). This, combined with daytime heating, will steepen lapse rates and lead to the formation of widespread heavy showers and a few thunderstorms, particularly across western Scotland and later in the day across the Northern Isles. DLS decreases significantly behind the front, but LLS remains high at >20kts locally and thus the potential for a funnel exists. Given the cold air aloft, hail is likely in many showers, perhaps an isolated event of hail diameter ~1.5cm.

Showers will persist well into the night and are likely to penetrate quite far inland across Scotland given the strong westerly upper flow.

SkyWarn:

SKYWARN UK SEVERE WEATHER WARNING #053

ISSUED: 0000UTC WEDNESDAY 21ST SEPTEMBER 2011

SKYWARN UK HAS ISSUED A SEVERE WEATHER WARNING FOR THE FOLLOWING REGIONS:

WESTERN ISLES WESTERN SCOTLAND NORTHERN SCOTLAND CENTRAL SCOTLAND SOUTHERN SCOTLAND

EASTERN SCOTLAND NORTHERN ENGLAND NORTHERN IRELAND

IN EFFECT UNTIL 1700UTC WEDNESDAY 21ST SEPTEMBER

STRONG JETSTREAM AND VERY DRY MID LEVEL AIR ENHANCING CHANCE OF VORTICES WITHIN AN ACTIVE COLD FRONT SEVERE WEATHER IS EXPECTED IMMINENTLY, OR OCCURRING, SOMEWHERE WITHIN THE REGIONS INDICATED. THREATS WITHIN THIS WARNING INCLUDE, BUT ARE NOT LIMITED TO: HEAVY RAIN...STRONG GUSTS...FUNNELS/TORNADOES...HAIL...FLASH FLOODING

DISCUSSION:

THERE IS GOOD MODEL CONFIDENCE AND PARTNER AGENCY AGREEMENT ON AN INCREASED RISK OF A SEVERE WEATHER EVENT DURING WEDNESDAY. AT 07Z ONWARDS AN ACTIVE FRONT NORTHWEST OF THE UK IS MOVING INTO AN AREA OF FAVOURABLE SHEAR AT ALL LEVELS, WITH 60KTS DEEP LAYER SHEAR AND 20KTS OF LOW LEVEL SHEAR ALONG THE FRONT ITSELF. VERY DRY AIR AT 700MB AND ABOVE IS BROUGHT DOWN TO 850MB ENHANCING INSTABILITY IMMEDIATELY BEHIND THE FRONT, WITH CONVECTIVE ELEMENTS LIKELY EMBEDDED WITHIN. THESE COULD INTRODUCE THE 925MPH GALE TO THE SURFACE, AND/OR GENERATE VORTICES. AS A RESULT STRONG AND POTENTIALLY DAMAGING GUSTS ARE LIKELY, AS IS THE POSSIBILITY OF BRIEF STRONG TORNADOES, ANYWHERE ALONG THE FRONT BUT MOST LIKELY CENTRAL TO EASTERN SCOTLAND. FURTHER SOUTH INTO NORTHERN ENGLAND THE FRONT FRAGMENTS UNDER THE SUPPRESSIVE JETSTREAM MAX ABOVE, AND INTERACTIONS WITH THE LAND. THERE STILL EXISTS A SLIGHTLY LESSER CHANCE OF STRONG GUSTS FROM CONVECTIVE VORTICES AND LEE WAVING, WITH A GREATER RISK OF FLOODING FROM SUCCESSIVE HEAVY SHOWERS. THERE ALSO EXISTS A MARGINAL RISK OF LARGE HAIL ALONG THE FRONTAL ZONE AND IN POST-FRONTAL CONVECTION OVER WESTERN SCOTLAND AND NORTHERN IRELAND.

[Coast]

Nothing of note from 21st OWS:

However the UKMO interpretation shows the areas of interest:

Strong KO index:

and some convective cloud:

Impressive CAPE throughput the day in the North and Scotland:

+ some shear:

Wowsers!!!!!

Good chance of a funnel or small tornado the further North you are today - keep the cameras loaded folks!

[P-M]

lol I will believe it when I see it but cheers for the head's up Coast! :-))

[Coast]

lol I will believe it when I see it but cheers for the head's up Coast! :-))

Looking at the pretty colours for today this seems to be more of a Scottish thing, but frankly after the funny year we've had, anywhere in the North may see some action.

One thing i do have confidence in today, is that the SE and the Channel coast in particular will miss out again.....

[Stuart]

Thank Coast

[Coast]

Thank Coast

Good luck Stu, you should be in a good position for something today - get your camera ready!

[Stuart]

I got it reday coast but you know my luck

Edited September 21, 2011 by Stuart

[Coast]

Is nothing happening yet with you guys at the other end of the Island?

[P-M]

Is nothing happening yet with you guys at the other end of the Island?

Nothing here just yet apart from the odd big gust of wind. Sky is turning very dark from my west right now and breeze is picking up but other than that - nada! Looks like Scotland is the best place to be although I am a happy that the north of the UK is in a L1 Estofex warning lol

[P-M]

Update from Tony Gilbert UKww the blue box extends over NE England into Cumbria and the borders:

http://www.ukweatherworld.co.uk/forum/index.php?/topic/86660-convective-outlook-weds-21st-sept-11/

UPDATE 8.40am Weds

As per earlier forecast with enhanced tornado risk as per blue box below. UKMO FAX chart suggests a sharpened baroclinic zone for the evolving cold front

In addition, a mean average calculation between 800mb and 600mb shows severe category convective gusts of 60 kts possible anywhere within the red box. Most sferic activity could well remain across Scotland, though by far the stronger risk for severe weather will be associated with northern England where a combination of surface friction inland increases the vector velocity between the ground and cloud base. The blue enhanced risk box is aligned with the dry 10% humidity nose at 500mb where potential instability is utilised best. My only concerns here is the lack of convergence which can act as a trigger to launch stronger cell development and tornadoes. In view of this one missing atmospheric condition the overall risk should remain 'SLIGHT' in nature. This is nevertheless a serious threat here for a strong isolated tornado event!

A more relistic time frame for Blue risk box would say 11Z-16Z

Edited September 21, 2011 by Paul-Michael

[NUT]

Not thundery but we have a pretty sharp squall running through us now. Had hail along with it

[P-M]

I was going to say looking at some of those radar returns I am surprised nobody is reporting anything too severe. Looks like a definite squall line has appeared on the radar.

[Coast]

So absolutely nothing out of all this so far then? Not a bit of helical wind, a small crack of thunder or a brief squally shower? So much for all the potential across the board this morning (actually pretty good agreement for once!) Or are all the communications to Scotland down because of a tornado?

[Isolated Frost]

Temperature fell 5C in 30 minutes during an intense shower - that's something noteworthy, yes?

[Coast]

that's something noteworthy, yes?

Was there a tornado that gave widespread uprooting of trees etc?

Somehow I was expecting a little bit more, but thanks! I guess that pesky frontal convergence came to nothing?

[Stuart]

not much happend here today just a few sharps showers so a let down

[Aaron]

A huge squall made its way down England but vanished.. above me.. uhh