Storm Forecast
Valid: Fri 13 May 2011 06:00 to Sat 14 May 2011 06:00 UTC
Issued: Thu 12 May 2011 21:23
Forecaster: TUSCHY

A level 1 was issued for N-Portugal and N-Spain mainly for large hail and severe wind gusts.

A level 1 was issued for N-Italy mainly for excessive rainfall and isolated large hail

A level 1 was issued for W-Ukraine and S-Belarus mainly for severe wind gusts

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

SYNOPSIS

Another trough approaches Europe from the west with downstream ridging affecting most parts of south/central Europe. Focus for thunderstorm development will be a west-east aligned quasi-stationary boundary, running from the Bay of Biscay towards the Alps.

DISCUSSION

... N-Portugal and parts of Spain ...

Active frontal boundary becomes quasi-stationary somewhere just along the N-coast of Spain (model discrepancies still left). Cross sections reveal strong thetae gradient present with LL-CAA resulting in some stabilization of the lower troposphere. Hence focus for initiation remains onshore. Yesterday, BL moisture over N-Portugal/Spain was modest at best with warm mid-levels. No significant change is forecast for the forecast period. Also, strongest PVA max approaches late in the forecast period with subtle short waves present throughout the daytime hours. Hence diabatic heating and favored mountainous areas will be the main foci for initiation. Mid-level lapse rates remain modest with marginal shear/instability overlap. However, I don't want to exclude a few large hail reports as wind field strengthens throughout the day. Also, a few severe wind gust reports are possible due to the deep/dry subcloud layer. Thunderstorms keep going well into the night, also spreading into SW-France. No severe is forecast during the night.

... N-Italy ...

Ongoing BL recovery during the daytime hours beneath somewhat steepening mid-level lapse rates allow troposphere to acquire modest instability ... roughly 800 J/kg MLCAPE. With weak wind fields present, thunderstorms reveal slow storm motion vectors with a risk for locally high rainfall amounts. Also, isolated large hail will be possible especially during the initiation stage. A level 1 was introduced mainly for the heavy rainfall risk. Clusters with embedded thunderstorms and heavy rainfall may persist well into the night.

... United Kingdom and Ireland ...

A well mixed maritime air mass is present with low LCLs and daytime driven LL CAPE. Thunderstorms are forecast despite uncertainty of final coverage. SBCAPE of 200 - 500 J/kg and cold EL temperatures (mainly over Ireland) indicate a chance for a few deep updrafts within a strong sheared environment (0-6 km bulk shear of roughly 20 m/s). Hence, a few better organized thunderstorms with isolated large hail and strong wind gusts are well possible, next to an isolated tornado event. Despite no real indications of supportive BL helicity/shear, any deviant storm motion may result in good helical flow, given strength of background wind field.

... W-Ukraine and S-Belarus ...

A level 1 was added for the cold front, entering W-Ukraine during the early afternoon hours from the west. Air mass ahead of this front features modest CAPE of roughly 500 J/kg (ML), so a limited time frame for deep updrafts/longer lived thunderstorms exists (probably until the evening hours). Shear vectors are forecact to be oriented near normal to eastward progressing cold front with robust jet max racing eastwards (500 and 700 hPa). Strong forcing and 15 m/s DLS may assist in an organized line of multicells/isolated supercells along the cold front with an enhanced severe wind gust risk. During the evening hours, the severe risk diminishes as CAPE fields decay.

... Rest of the highlighted areas ..

Neither shear nor instability favor an organized severe risk within the highlighted regions. Thunderstorm intensity and coverage peak during the afternoon hours with a weakening trend thereafter.

Estonia will be the only region, where a somewhat enhanced severe risk exists as LL depression moves in. However, bad handling of that feature in the model field and displaced shear/instability fields preclude any level area. Nevertheless an isolated tornado event will be possible, especially if backed surface winds and better BL moisture overlap.