Tornado Risk - S. Britain - Wed Sept 2nd

[martinastro]

Storm Forecast
Valid: Wed 02 Sep 2009 06:00 to Thu 03 Sep 2009 06:00 UTC
Issued: Tue 01 Sep 2009 22:57
Forecaster: PUCIK
A level 1 was issued for Wales, Southern England, Bretagne and Belgium mainly for tornadoes and severe wind gusts.

SYNOPSIS

An unseasonably strong upper level jet-stream will make its way towards Europe in this forecast period with windspeeds over 70 m/s at 300 hPa level. At the same time a mid-level impulse, forming at the delta of the jet, will reach British Isles by Wednesday evening. To the east, a ridge over Central Europe is expected to further weaken and a cut off low will stall over the Black Sea, with rounds of showers and thunderstorms expected over the region. At the surface, a low pressure system will persist over much of Northwestern Europe and a rapid cyclogenesis expected in conjuction with the mid level impulse mentioned above. The rest of Europe will be dominated by insignificant pressure field with a large high over Russia.

DISCUSSION

... Wales, Southern England, Bretagne, Belgium...

Being placed under the left exit region of the strong jet-stream and ahead of the mid level impulse, a favorable forcing should be available in the late evening and night hours. Prediction of potential vorticity shows the likely intrusion of drier, stratospheric air to the rear side of the advancing cold front. Models are not quite in agreement regarding the instability, GFS simulates at least marginal values of MLCAPE, mostly over England within the area of strong forcing. ECMWF is more conservative and at the moment, it is difficult to say, how much instability will materialize during this event. It seems quite probable, that strong low level shear will be present in this situation with its values well over 10 m/s locally. Moreover, a band of enhanced SREH, over 200 J/kg for 0-1 km layer should form just ahead of the cold front. A detrimental factor to the organised convection should be quite weak shear in 0-3 or 0-6 km layer with values around 10-15 m/s, impliyng a low chance that a well organised, strongly forced convective line forms.

Although there are still many uncertainities regarding the evolution of the convection around the cold front, there is a slight chance that some weak tornadoes occur with stronger convective elements that might acquire rotation briefly. Also with more than 20 m/s at 850 hPa level, severe wind gusts might occur as well. The threat will be most prominent around 21Z for England and between 03Z-06Z for European part of Level 1, probably continuing into the next forecast period. An update might become necessary later on if situation changes.

[markt]

Sounds rough, best batten down the hatches, autumn is here...

[martinastro]

TORRO TORNADO WATCH 2009/006

A TORNADO WATCH has been issued at 12:25GMT on Wednesday 2nd September 2009

Valid from/until: 12:45 - 01:30 on Wednesday 2nd/Thursday 3rd September 2009, for the following regions of the United Kingdom & Eire:

Parts of (see map)

Wales

Midlands

E Anglia

All of southern England

Channel Is

THREATS

Tornadoes; wind gusts to 60mph; hail; occasional CG lightning.

SYNOPSIS

Deepening Atlantic low will push its frontal system across the British Isles over the next 24 hours. Several small dry intrusions/short-waves are evident on WV imagery: one is surging towards W Wales; another is pushing towards SW England; a third is passing S Eire. As the first two of these overspread the tropical moist sector, instability is expected to be released - indeed, a couple of sferic have recently been detected in the SW approaches. In addition, the leading edges of the dry intrusions seem to be promoting some line convection ('ropes' evident on vis satellite imagery).

Low-level shear just ahead of the expected convection looks to be sufficient for low-level rotation, bringing the risk of a few tornadoes, should convection root near the surface. In addition, strong wind gusts are likely with any organsied convection, e.g. lines. The intensity of the low-level flow, especially SE of S Wales to the Wash, means that a strong tornado is possible.

Forecaster: RPK

[Padraig OBrien]

have had some very heavy rain here today

[martinastro]

UKww Weather Watch – Heavy Rain & Gales

Valid from: 1800z Weds 2nd - 1800Z Thurs 3rd September 2009;

Areas affected:  England, Wales Weds-Thurs am then N England/Scotland thereafter

General evolution: Humid, moist air associated with the remnants of Tropical Storm Danny is caught up in an Atlantic depression with a rather southerly track (centered 992 mb Cork at 1600 BST; tracking NE and deepening 988 mb, just off Tyneside, by 0600 Thursday).


Forecast: Rain already into parts of Wales and SW England continues and extends N and E overnight, with heavy and prolonged bouts, risk local thunder and strong gusty winds. Totals of 25-40 mm (50+ over hill country) enough to cause poor driving condituions and local flooding with rivers already high this week in many areas. Gales along the south coast tonight into tomorrow, easing towards midday, with gusts over 60 mph possible especially from the Isle of Wight eastwards - enough to cause tree damage given full leaf and wet ground as added factors. The heaviest rain moves NE into Thursday to give Scotland in particular a very wet day (again 50 mm+ over hill country) before it clears into the North Sea towards the end of the forecast period.

In such situations two links are useful to monitor developing conditions (as well as reports here on UKww):
Met Office Rainfall Radar: HERE

Environment Agency Flood/Severe Flood Warnings: HERE 



UKww will monitor this Warning and update it if necessary.

Issued by JSM for UKww, 1750Z  02/09/2009

The user assumes the entire risk related to its use of this data. UKww (UK Weatherworld) is providing this data "as is" and UKww disclaims any and all warranties, whether express or implied, including (without limitation) any implied warranties of merchantability or fitness for a particular purpose. In no event will UKww be liable to you or to any third party for any direct, indirect, incidental, consequential, special or exemplary damages or lost profit resulting from any use or misuse of this data.

The UKww Warnings Team use Net Weather Extra when compiling forecasts.
-----
www.geologywales.co.uk/storms
Director, TORRO Extreme Rainfall Events Division

[martinastro]

Latest update from Tony Gilbert...



Slight Risk of Thunderstorms UK regions as per map 18Z-03Z

Moderate Risk of Brief Tornado Development 20Z-03Z Central South and SE UK regions .

Probably a bold statement. Though based on the model output we do in fact IMO have a significant risk here for the next few hrs.

Whilst I agree with the earlier forecasts and comments on the thread. For me, I see the primary risk along the post frontal trough as it tracks east in the wake of the triple point.

Particular attention is once again drawn to the rapid influx of dry air at 700mb upwards and its influence on the lower mid level lapse rates. In short any vertical moving parcel should enhance this thermal differential and produce strong though relatively brief updrafts. Low level shear forecast continues to show 45 deg veer up to 850 mb with around 25 kts of speed shear. Such levels of vorticity can deliver enough vorticity into any updraft to produce tornadoes. The suggest of 'brief' is attributed the lack of continued increase in speed shear above this level.

So all in all a specific risk will apply now for the next few hrs central and SE UK. Probably the most significant risk that I have seen for this region so far this year.

[markt]

The weather sure is grim at the moment.  Had lots of intermittent heavy rain today.  There were some ominous looking clouds earlier, but nothing special enough to warrant me getting the camera out.

I guess we have this scenario for a while now:  Ex trpoical storm / hurricane whacks into us, showery and blustery conditions follow and if we're lucky enough a ridge of high pressure before the next system piles in and repeat...  Good old autumunal conditions...