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Deep Low Pressure Thurs 11th-Fri 12th November

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Author Topic: Deep Low Pressure Thurs 11th-Fri 12th November  (Read 1816 times)
martinastro
Martin Mc Kenna
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« Reply #15 on: November 10, 2010, 08:35:54 pm »

SKYWARN UK SEVERE WEATHER WATCH


ISSUED: 1800UTC, WEDNESDAY 10TH NOVEMBER 2010.

SKYWARN UK HAS ISSUED A SEVERE WEATHER WATCH FOR THE FOLLOWING REGIONS:
ALL REGIONS IN SCOTLAND, ENGLAND, WALES AND NORTHERN IRELAND.

IN EFFECT FROM 2100UTC ON WEDNESDAY 10TH NOVEMBER UNTIL 1200UTC ON FRIDAY 12TH NOVEMBER 2010.

ANOTHER STRONG LOW PRESSURE SYSTEM TO CROSS THE UK, BRINGING WET AND WINDY WEATHER TO MOST PLACES.

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...WIND GUSTS TO 80mph.

DISCUSSION:
ANOTHER VERY DEEP LOW PRESSURE SYSTEM IS EXPECTED TO TRACK ACROSS SCOTLAND BRINGING THE RISK OF SEVERE WEATHER TO A LARGE PORTION OF THE UK. A VIGOROUS, 160KT UPPER JET STREAK WILL LIE ACROSS CENTRAL PARTS OF THE UK ON THE SOUTHERN SIDE OF A STRONG EXTRATROPICAL CYCLONE - 'CARMEN' - THAT WILL TRACK EASTWARDS ACROSS SCOTLAND. A BLEND OF MODEL DATA SUGGESTS THAT THE MINIMUM PRESSURE OF THE CYCLONE WILL BE AROUND 950hPa, PERHAPS EVEN SUB-950hPa WITH MET OFFICE DATA SHOWING A LOW PRESSURE OF 946 hPa JUST WEST OF THE SCOTTISH COASTLINE. THE FRONTAL SYSTEMS ARE EXPECTED TO MOVE QUICKLY, WITH MUCH OF ENGLAND LOCATED WITHIN THE WARM SECTOR OVERNIGHT AND INTO THURSDAY MORNING. THE FAST MOVING NATURE OF THE FRONTS SUGGEST THAT SEVERE RAINFALL ACCUMULATIONS ASSOCIATED WITH THEM IS UNLIKELY ACROSS MOST OF THE UK. CONVECTION ASSOCIATED WITH THE COLD FRONT MAY POSE A RISK THAT WILL BE MONITORED FOR THE ISSUANCE OF AN APPROPRIATE WATCH - A SURGE OF DRY AIR AND VERY STRONG WINDS AND LOW LEVEL SHEAR INDICATE THE POTENTIAL FOR A LINE OF CONVECTION TO FIRE WITH STRONG REAR-INFLOW AND THE ASSOCIATED RISKS OF SEVERE WIND GUSTS, AND PERHAPS A FEW TORNADOES. AT THE LEFT EXIT REGION OF THE JET, VORTICITY PRESENT SHOULD TRIGGER CONVECTIVELY ENHANCED RAINFALL EMBEDDED IN A WRAP-AROUND OCCLUSION, AND IT IS IN THIS REGION OF SOUTHWEST SCOTLAND THAT THE HIGHEST RAINFALL TOTALS WILL OCCUR - UP TO AROUND 45mm ACCUMULATION. THE MAIN RISK WITH THIS SYSTEM IS ASSOCIATED WITH SEVERE WIND GUSTS, ESPECIALLY POST COLD-FRONTAL. A VERY STRONG PRESSURE GRADIENT WILL SEE 900hPa WINDS INCREASING TO 65-70 KNOTS OVER A WIDE AREA. WIND GUSTS TO 70 KNOTS ARE POSSIBLE ALONG COASTAL AND EXPOSED AREAS, AND ISOLATED HIGHER GUSTS ARE A RISK. INLAND GUSTS SHOULD BE SOMEWHAT LESS, STILL REACHING POTENTIALLY UP TO 55-60 KNOTS IN PLACES. COASTAL AREAS SEEING WINDS SUSTAINED UP TO 45 KNOTS, WITH 20-30 KNOTS INLAND. STRONGEST WINDS OCCURRING INTO THURSDAY EVENING. GUSTS EASING BELOW SEVERE LEVELS BY EARLY AFTERNOON ON FRIDAY.

SPOTTER ACTIVATION IS REQUESTED AND SPOTTERS ARE REQUESTED TO REPORT RAINFALL ACCUMULATIONS AND ALL FACTORS EXCEEDING ACTIVATION CRITERIA.

A WARNING WILL LIKELY NEED TO BE ISSUED SHORTLY AS THE ZONE OF SEVERE WEATHER ENTERS THE BRITISH ISLES.
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