Storm Forecast
Valid: Thu 26 Apr 2007 06:00 to Fri 27 Apr 2007 06:00 UTC
Issued: Wed 25 Apr 2007 21:55
Forecaster: GATZEN

SYNOPSIS

Quasi-stationary blocking high centered over northern Germany. Troughs over eastern and western Europe are present that form a well-defined omega flow. During the period, western trough cuts off into Iberian Peninsula. Relatively low geopotential is also present over the Mediterranean. Strong winds are not present at the 500 hPa level over Europe, except for a few 15 m/s jet streaks curving around the Iberian cut off as well as the polar jet that affects Scandinavia. At lower levels, tongue of warm air mass over western Europe is unstable. Over eastern Europe, air mass is rather stable and cool.

DISCUSSION

France

In the range of warm air mass, thunderstorms have formed during the last couple of hours. On Thursday, expect another round of scattered thunderstorms, as air mass will be unstable and CIN will be rather weak locally. Limiting factors again will be weak forcing and lack of rich boundary-layer moisture. Thunderstorms will likely cluster during the day and weakening after sunset. Best potential for a rotating updraft exists over western France, where upper jet streak leads to 20 m/s DLS and veering profiles. Large hail is expected to be the most significant severe threat.

Iberian Peninsula

In the range of the developing cut-off low, widespread convective activity is expected to form. Given weak vertical wind shear over most places, only a small fraction of those cells may organize due to favorable interacting of outflow boundaries. Vertical wind shear will increase over south-eastern Spain, where upper jet streak moves eastward. Given 20 m/s DLS over this region, chance for some organized convective cells is expected to be higher. Although favorable veering profiles are not forecast, southerly surface winds and westerly winds aloft may create some helicity, and mesocyclones are not ruled out, capable of producing isolated large hail. Threat seems to be rather weak, though.

Southern Italy

Rather steep low-level lapse rates and some moisture are present over southern Italy. Daytime heating and sea-breeze convergence will likely support initiation and thunderstorms are forecast. At the southern flank of Mediterranean trough, vertical wind shear increases over southern Italy, and a few rotating updrafts are not ruled out, capable of producing isolated large hail.