Storm Forecast
Valid: Fri 14 Jan 2011 06:00 to Sat 15 Jan 2011 06:00 UTC
Issued: Thu 13 Jan 2011 21:19
Forecaster: TUSCHY

A level 1 was issued for parts of United Kingdom mainly for an isolated funnel/tornado event

SYNOPSIS

Double-barrel low structure south of Iceland gradually morphs into a large sub-970 hPa low pressure area with access to a modified subtropical air mass. Downstream of this feature, geopotential increase is noted and this will cause thunderstorm-hostile conditions over most parts of Europe. Lower heights over the far eastern Mediterranean foster scattered showers/thunderstorms due to an augmented BL(SST)-mid tropospheric temperature gradient. Weak shear keeps activity disorganized with no severe risk forecast (despite an isolated spout risk although slim risk diminishes during the afternoon hours, as drier airmass filters in from the north, dissolving marginal LL-CAPE).

DISCUSSION

... Ireland, Scotland and UK ...

Numerous time frames occur, where thunderstorms could evolve:

a) Morning hours mainly over UK. First tongue of high PWAT content overspreads the area with gradually cooling mid-levels as region resides at the cyclonic side of an intense mid-/upper streak. Cold front passage during the morning hours could spark a few isolated thunderstorms as MUCAPE increases from the SW and even surface based convection can't be ruled out given BL quality. However confidence in strong updraft evolution is still marginal at best, so severe risk will be on the lower end side with gusty winds and marginal hail. Don't want to mention explicitly the word "tornado" in this part despite marginally enhanced LL CAPE along the front itself, but overall conditions don't hint on any enhanced tornado risk right now.

b) Next round of probably DMC is bound to the passage of a cold thermal trough around peak time heating. This results in better SBCAPE of 100 to locally up to 300 J/kg, decreasing onshore due to mixing. EL temperature drop to levels, where deep updrafts are possible with attendant electrification, so a few thunderstorms are forecast. Shear especially at LL is supportive for some storm organization and we somewhat like increasing directional shear in the BL along the coast and onshore. Focus therefore will be a funnel/isolated tornado risk with strong wind gusts and marginal hail. A low-end level 1 was issued for those areas, where best CAPE/shear overlap is forecast.

During the evening hours, deep WAA regime (with second high PWAT content tongue from the SW) overspreads the area and brings any ongoing convection to an end. Dependant on the final track of those synoptic features, a limited time frame for somewhat enhanced convection over Scotland exists during the morning hours (15th). No thunderstorm risk seen, but with intense pressure gradient present, a few strong to severe wind gusts could accompany showers (probably not discernable in the background wind field and therefore not considered in this outlook).