Storm Forecast
Valid: Sat 10 May 2008 06:00 to Sun 11 May 2008 06:00 UTC
Issued: Sat 10 May 2008 06:45
Forecaster: DAHL

SYNOPSIS

Upper block pattern persists across Europe ... with feature of main interest being the intensifying cut-off cyclone over the Iberian Peninsula which will make slow eastward progress on Saturday. Another upper cut-off low is present over eastern Europe, which exhibits smaller-scale vort maxima at its periphery, which are expected to affect the Belarus region late on Saturday. At low levels ... weak cyclonic circulation is accompanying the Iberian upper low, maintaining north- and eastward advection of a weak Saharan EML plume into the southern Mediterranean regions. Otherwise, SFC conditions over Europe are rather quiescent.

DISCUSSION

... Iberian Peninsula ... NW Mediterranean ...

Air mass beneath the southern and eastern periphery of the Iberian upper low, as well as beneath its center, is weakly unstable, and rather weakly capped. Convection may first develop over the southern parts of the peninsula early in the day, but main activity should shift to the off shore regions as the unstable air mass deepens with the eastward progression of the upper low ... and as DCVA-related forcing for ascent increases during the day. Model guidance suggests that there will be a small overlap of decent DLS and CAPE over the N parts of the W Mediterranean, as well as over NE Spain. In addition, LLS is simulated to become rather favorable (10 m/s) along the Spanish NE coast. Well-organized multicellular storms and mesocyclones may occur, posing a threat ranging from large hail and severe wind gusts to isolated tornadoes. Given rather late timing and lack of isolation, storms may struggle to persist as they move inland, but the at least coastal regions should be affected by the severe threat. Bulk of activity may remain off-shore, however.

... NE Poland ... NW Belarus ... S Baltic States ...

Model guidance suggests that weak instability will evolve over eastern Poland and W Belarus along a weak frontal boundary which is associated with the western periphery of the E-European upper low. Weak large-scale upward motion is expected to overspread this region late in the day and scattered thunderstorms are expected. DLS is simulated to be about 15 m/s, along with locally up to 10 m/s LLS. This suggests that a marginal threat for severe multicellular storms, and possibly also a few mesocyclones may exist. However ... the loss of daytime heating may limit the extent of convective evolution, and the best shear is expected around 21Z, well after sunset. Confidence is rather low that convection will be widespread and that it will persist long enough to benefit from the favorable shear. Hence, a categorical risk is not issued, though an isolated large hail event or even a tornado or two cannot be completely discounted.

Otherwise ... there will be numerous foci for mainly diurnally-driven deep convection over Europe, but shear and thermodynamic fields suggest that organized severe threat will be rather low with this activity.
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