Storm Forecast
Valid: Fri 17 Aug 2012 06:00 to Sat 18 Aug 2012 06:00 UTC
Issued: Thu 16 Aug 2012 18:22
Forecaster: TUSCHY

A level 1 was issued for parts of the UK mainly for a strong to severe wind gust and tornado event.

A level 1 was issued for Ireland mainly for an isolated large hail event.

SYNOPSIS and DISCUSSION

Downstream of an intense cyclonic vortex west of Ireland, WAA spreads to the northeast with a substantial increase of the 500 geopotential heights, reinforcing another omega-like pattern over Europe. A quasi-stationary longwave trough over E-Europe builds more to the south, reaching the E-Mediterranean with subtle height falls during the end of the forecast. Cool and unsettled conditions prevail over N-Europe.

Many places over N/E Europe see some thunderstorm activity, mainly daytime driven and sub-severe. Marginal hail and strong wind gusts accompany those storms.

A belt of strong SW-erlies (e.g. 30 m/s at 500 hPa) still affects Ireland/UK and Scotland. A first surface wave/trough and attendant pronounced warm front cross those areas from SW to NE during the day with pretty robust moisture advection forecast. Mid-level lapse rates remain marginal next to expected thick cloud cover, limiting any diabatic heating until noon. Thereafter, some clearing in the posfrontal warm sector is expected and forecast soundings indicate not much modification is needed for some MLCAPE build-up. HIRLAM and GFS both show another surface wave, placed just S of Ireland during the afternoon hours, again inserting some more backing in the BL flow. This assists in the infiltration of a better mixed LL air mass from N-France towards S-UK, where a decrease of dewpoints and an increase of the LCL can be seen during the afternoon. LCLs however remain well below 1000 m just along and south of the warm front over N-C UK. Both, the warm sector and the frontal boundary itself reveal great shear (both directional and speed shear), so if adequate clearing along/south of the warm front occurs, an isolated thunderstorm event is possible with a risk of producing severe wind gusts and an isolated tornado. Coverage probably remains limited with ongoing weak NVA over the area of interest. We also monitor the progress of a dry slot feature, which moves towards Ireland during the afternoon, so some better CAPE and enhanced thunderstorm activity is possible over Irland. 20 m/s DLS may allow a few storms to become better organized with isolated large hail and strong wind gusts. Therefore, a broad lightning area and a second level 1 were added. The thunderstorm and severe risk diminishes after sunset.

One or two storms may evolve over C/S-Italy during the day but coverage will be too limited for a thunderstorm area.

During the night, an EML plume spreads northwards over Spain and France with capped 500-1000 J/kg MLCAPE present. We do not want to exclude one or two storms over the Pyrenees during the night.