Storm Forecast
Valid: Fri 19 Aug 2011 06:00 to Sat 20 Aug 2011 06:00 UTC
Issued: Thu 18 Aug 2011 19:37
Forecaster: TUSCHY
A level 2 was issued for parts of Poland mainly for a concentrated swath of damaging wind gusts and a few tornadoes.
A level 1 was issued for far E-Germany, the Czech Republic, Poland, far N-Italy, parts of Lithuania and W-Belarus mainly for severe to damaging wind gusts and isolated large hail.
A level 1 was issued for Portugal and parts of Spain mainly for large hail and severe downbursts. A few very large hail events may occur.
A level 1 was issued for parts of central France, the Alps, N-Italy and parts of Germany mainly for severe wind gusts and large to isolated very large hail.
SYNOPSIS
The strengthening vortex over the NE-Atlantic sends strong westerlies to parts of NW Europe, as numerous smaller disturbances cross UK, the North Sea and S-Norway/Sweden from west to east. Another cut-off low remains nearly stationary just southwest of Portugal and advects a hot air mass northwards. Most parts of the Mediterranean remain hot and dry/stable.
DISCUSSION
... E/E-CNTRL Germany, Poland, the CNTRL/N-Czech Republic, Lithuania and W-Belarus ...
Progressive pattern continues, as diffluent and negative tilted trough moves in from the west. A 35 m/s mid-level jet evolves along its southern flank and advances eastwards over Poland during later afternoon hours onwards. QG forcing also increases over Poland as upper trough sharpens, so ingredients are set for scattered to widespread thunderstorm initiation.
Ongoing storms along an eastward moving cold front may weaken temporarily during the morning hours over E/E-CNTRL Germany, but eventually start to gain strength as diabatic heating starts. Still unsure where cold front will be situated at 06 Z due to ongoing convective enhancement. Decided to go further southeast to account for the possibility of an active prefrontal convergence zone. Increasing forcing and moderately unstable air mass ahead, beneath diffluent mid/upper levels assist in an organized line of storms to cross far E-Germany, the CNTRL/N-Czech Republic and Poland from west to east. Decoupling of decaying EML plume and best BL moisture may keep MLCAPE in the 500 - 800 J/kg range with decreasing values over N/NE-Poland, which is still more than adequate for the upcoming strongly forced set-up. As 35 m/s mid-level speed max moves in from the west during the afternoon hours, 0-6 km bulk shear exceeds 20 m/s. Also, LLJ deepens betimes with 15-20 m/s all the way up to 700 hPa.
This favors an organized MCS to cross Poland/the Czech Republic from west to east during the day with severe to damaging wind gusts. Large hail will be confined to either more discrete cells ahead or a few stronger cores within the line. The same for the tornado risk, which seems to be somewhat enhanced over central/NE-Poland (although model wind field may be modulated by convective feedback problems). Thunderstorms gradually decay after sunset due to loss of daytime heating/instability. However, forcing over W-Belarus and Lithuania may offset that effect and hence the level area was expanded well to the east up there.
A level 2 was added for the region where strongest height falls, shear and CAPE overlap due to an augmented chance for a concentrated swath of damaging wind gusts and a few tornadoes.
... S-CNTRL France, S-Germany, the S-Czech Republic and the Alps ...
As convection is ongoing during the writing of this outlook, uncertainties remain high, where frontal boundary finally resides. However, models agree in a fairly active baroclinic zone to set-up in the area of interest, embedded within modest westerlies. Moisture pooling along this boundary may assist in a concentrated swath of 500 - 1000 J/kg MLCAPE within 10-15 m/s DLS....increasing to 20 m/s towards NE-Austria/Czech Republic. Of note is the 700 hPa wind field, which remains fairly strong, so well organized multicells are expected, producing large hail and strong wind gusts. Would not want to exclude an isolated very large hail event especially over far NW-Italy and W-CNTRL France. Despite an overall decrease in activity during the night, weak disturbances continue to cross the baroclinic zone from W to E, so isolated thunderstorms may occur all night long.
... Portugal and Spain ...
A quasi-stationary cold-core low just west of Portugal continues to advect a hot and dry air mass northwards. As can be expected in such a deep WAA pattern, very steep lapse rates cover most of the Iberian Peninsula, while air mass remains capped over most parts of the area. This idea is supported by latest forecast sounding data (strongly curved hodographs, elevated CAPE exceeding 1kJ/kg) but QPF forecasts still differ significantly. Sporadic storms can't be ruled our over S-Spain despite strong cap, as numerous disturbances move through. However, augmented chances for initiation arise over Portugal and NW-Spain, as BL cools down somewhat beneath diffluent upper flow (the same offshore with cooler marine BL). Degree of MUCAPE all night long hints at an augmented large to very large hail risk with each storm, next to strong to severe downbursts due to high LCLs and well mixed subcloud layers. We went ahead and issued a broad level 1 with different thunderstorm probs. to highlight the highest chance for severe.
