Storm Forecast
Valid: Wed 25 Apr 2007 06:00 to Thu 26 Apr 2007 06:00 UTC
Issued: Tue 24 Apr 2007 21:55
Forecaster: GATZEN

SYNOPSIS

A high is situated over north-eastern and central Europe. At its south-eastern flank, an upper trough has reached northern Balkans and propagates southward during the period. At the western flank, another sharp upper trough starts to cut-off into Iberian Peninsula region. At lower levels, a tongue of warm air mass advects northward over France and Germany, while cool air is present over eastern Europe.

DISCUSSION

Balkans

Upper trough migrates southward over the Balkans, filled with cold mid-level air mass with temperatures below -20°C at the 500 hPa level. A strong northerly jet streak present at its western flank spreads into northern Aegean, providing QG forcing over northern/central Balkans during the day. At the surface, models indicate that a convergence line/frontal boundary that will shift southward, with cool air mass spreading into northern Balkans. Both warm and cool air mass south and north of the convergence line are characterized by well-mixed low levels as indicated by latest soundings. Rich boundary-layer moisture (about 7g/kg) indicated by latest Zagreb sounding is likely to be present in the range of the convergence line during the period. Given daytime heating, mixed low- to mid-level lapse rates as well as forcing underneath the jet streak and due to low-level convergence, thunderstorms will likely develop. Most convection is expected underneath the trough, where weak vertical wind shear will be present, and organized thunderstorms are not very likely. However, deep layer vertical wind shear increases over western Balkans reaching more than 20 m/s over western Croatia. Favorable veering profiles are not forecast, and current thinking is that multicells will be the primary convective mode. Isolated large hail and severe wind gusts are not completely ruled out with the stronger storms. Thunderstorms will likely merge given dry low-levels and expected cold pool development. During the night hours, convective activity is forecast to weaken given weakening forcing as well as low-level stabilization.

Eastern Iberian Peninsula, France

Western trough starts to cut off into Iberian Peninsula region. Associated cold air mass has spread into Bay of Biscay and western Iberian Peninsula. During the period, a frontal boundary will spread into Spain, associated with stratiform clouds and rain. Given QG forcing, models expect a frontal wave forming over central Iberian Peninsula during the day, moving north-eastward into the Pyrenees. To the east/north-east of this developing wave, WAA is expected to affect southern, and central France. Given steep low- and mid-level lapse rates as well as increasing boundary-layer moisture especially in the WAA regime over western France, instability is likely to increase over a broad region. Although QG forcing will remain weak, isolated thunderstorms are expected to develop over central/eastern Spain as well as over southern and central France, and northern Italy. Best potential for a few rotating updrafts will exist near the frontal boundary over western France, where low-level northerly winds may create favorable veering profiles. Most significant severe threat will be large hail with any isolated storm that forms given locally high buoyancy. Isolated severe wind gusts are not excluded given dry low levels and steep lapse rates. Thunderstorms will likely merge into clusters and will weaken during the night.