Extended Forecast
Valid: Thu 01 Aug 2013 06:00 to Fri 02 Aug 2013 06:00 UTC
Issued: Wed 31 Jul 2013 05:03
Forecaster: TUSCHY

A level 1 was issued for Ireland mainly for heavy rain, severe wind gusts and tornadoes. An isolated large hail event is possible, too.

SYNOPSIS and DISCUSSION

A potent upper trough continues to amplify west of Europe with strengthening ridging downstream over S/C Europe. Another upper trough over N-Europe weakens and breaks open into a closed low which exits the N-Baltic Sea to the southeast.

An extensive surface high pressure area over most parts of Europe keeps conditions stable regarding initiation of deep moist convection.

Scattered thunderstorms are expected to develop from C-Sweden, S-Finland further to the SE. This activity evolves along and ahead of a gradually eastward moving surface cold front. DLS of 15 m/s might support an isolated organized thunderstorm event with large hail the primary risk. A marginal level 1 may be added in later outlooks in case CAPE either remains in the 500 to 800 J/kg MLCAPE range or even improves over S-Finland.

An augmented severe risk might evolve over Ireland and parts of UK. A warm front continues to lift north with a N-S aligned cold front approaching slowly from the west. Hence a broad warm sector covers most of Ireland and UK during the forecast. Some improvement of the mid-level lapse rates is likely with a northward spreading EML plume, which finally covers S/C-UK during the afternoon and evening hours. Rich BL moisture with mean BL mixing ratios in excess of 11 g/kg and improving lapse rates offer increasing MLCAPE over most parts of UK in the order of 500 to 1000 J/kg. Further west, over Ireland, the characteristics of a true subtropical air mass with weak lapse rates remain, which also lowers MLCAPE build-up substantially. Only SW/W Ireland might see 400 to 800 J/kg MLCAPE due to deepening BL moisture along the gradually eastward moving cold front.

Initiation over Ireland will be most probable due to the eastward sliding cold front and the placement beneath the left exit region of a powerful 40 / 50 m/s 500 / 300 hPa jet streak. 20 to 30 m/s DLS overspread the cold front and should offer plenty of shear for temporarily organized storms. The near parallel alignement of the cold front to the jets should support rapid thunderstorm clustering and upscale growth with numerous clusters affecting and leaving Ireland to the north. A sub-1000 hPa depression just west of Ireland increases ageostrophic deflection of the BL wind field, so substantial LL shear (15 m/s and 200 m^2/s^2 SRH-1) is forecast. Combined with LCLs blow 800 m, an enhanced tornado risk exists with any more discrete thunderstorm. Otherwise, severe wind gusts and flash flood producing rain is possible. The level 1 was extended towards E-Ireland mainly for the rainfall risk.

Further east over UK, missing forcing and moderate capping keep CI probabilities low for now. Foci for isolated initiation could arise along inland moving sea-breeze fronts or along a confluent LL flow regime over N-UK/Scotland during the afternoon hours onwards, but confidence in that scenario remains too low for issuing a level 1 that far out. In case of CI however, strong shear and moderate CAPE would support organized convection with large hail, strong to severe wind gusts and an isolated tornado event. A 50-% lightning area was added for the offshore areas just E of UK due to persistent signals of an overnight thunderstorm cluster.