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Convective Outlook - Sun Jan 18th

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martinastro
Martin Mc Kenna
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« on: January 17, 2009, 08:50:41 pm »

From Tony Gilbert of TORRO and UKWW



7.30pm Sat

Slight Risk Tstorms 18Z Sat 17th Jan-18Z Sun 18th Jan West Ireland, West Scotland , West & Southern Sector UK.

Slight Risk Severe Convective Gust west region Ireland And Southern Sector UK 06Z-15Z

Synopsis;

Low NW of Scotland deepens to 938 mb. Cold front brings continued heavy rain (non convective) across Ireland and UK overnight. Deep upper trough traverses NW Europe through Sunday enhancing pockets of convection in the wake of the initial cold front. 500 mb jet realigns and increases vertical shear for localised convection.

Discussion;

 A few general thunderstorms are possible as per convective map for the next 24hrs.

My attention remains connected with showers within the wake of the cold front and not the overnight cold front itself which I believe to be primarily non convective. After the initial subsidence depletes there is some minor scope for a few convective showers developing along the south UK coastline during late morning. In addition to this GFS bolster strong CAPE values along western coastal region Ireland. Mean average winds between 800 mb and 600mb  suggest convective gust possible within severe category.

My personal perspective is that given the now  non sharp cold front derived from the model output. Winds could be severe overnight but not truly convective in nature! Winds through this period will in fact be within a climate of  negative shear and much less likely to produce tornadoes within the parameters that we would normally associate such activity. Moreover particular attention is given to the depth of the upper trough and its potential influence on any building convective cell by way of lapse rate potential. All in all I am not convinced that the conditions ATM are favorable for tornado genesis!...This is not to say that they cannot occur, it is merely to say that based on the various model output they do in fact seem unlikely ATM. I am still nevertheless  keen to update based on the early morning model update which could prove to be more conducive maybe!
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martinastro
Martin Mc Kenna
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« Reply #1 on: January 17, 2009, 10:26:46 pm »

From ESTOFEX...



Storm Forecast
Valid: Sun 18 Jan 2009 06:00 to Mon 19 Jan 2009 06:00 UTC
Issued: Sat 17 Jan 2009 22:14
Forecaster: SCHLENCZEK
SYNOPSIS

A strong zonal upper flow pattern has established over the North Atlantic with an impressive 100 m/s westerly jet streak southwest of the British Isles. In the vicinity of the jet's left exit region over the North Sea, an intense 940hPa surface low will shift northeastward while occluding. The next intense cyclone will develop further upstream on sunday afternoon / evening. Its cold front will cross western UK and northwestern France during the night hours and rather shallow but organized convection may develop along the cold front.

High over low blocking will persist for some more days over Russia and an upper level cut-off low over the N Maghreb States moves southward during the period. Over the Mediterranean area, stable conditions will continue.

DISCUSSION

...Ireland, W UK, NW France...

On Sunday morning, showers and thunderstorms are expected in the unstable post-frontal airmass over the British Isles where low-level winds in order of 20 - 25 m/s will be in place. Weak directional shear should minimize the tornado threat but isolated severe gusts may occur. In the afternoon, the approaching warm front of the next cyclone and upper WAA will lead to stable conditions. This will change again around 00 UTC on Monday when the upper vort-max of the next shortwave trough arrives. In an environment with marginal CAPE but very strong (35 m/s) LL shear and locally more than 600 J/kg SRH1 / 1000 J/kg SRH3, shallow convective lines may develop, capable of producing severe convectively-enhanced gusts, in some places even more than 32 m/s, and maybe a few tornadoes. Uncertainties with respect to instability and forcing exist and an update may follow in the evening.

...Other regions...

Showers and thunderstorms with sporadic lightning and gusty winds may also develop over the North Sea, N France, Belgium, the Netherlands and NW Germany. Another area with possible thunderstorms is southern Norway.
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