Storm Forecast
Valid: Sun 19 Apr 2009 06:00 to Mon 20 Apr 2009 06:00 UTC
Issued: Sat 18 Apr 2009 21:30
Forecaster: DAHL

SYNOPSIS

Quasi-stationary large-scale upper cut-off cyclone will be covering southwestern parts of Europe and the W Mediterranean through the period. At low levels, a weak cyclonic circulation is accompanying the upper feature, advecting a tongue of warm/moist air around the eastern and northern periphery of this feature at low levels. Otherwise, meandering westerly upper flow will exist over northern Europe, supporting building high pressure over Scandinavia/N Europe.

DISCUSSION

... Balkans through Alpine regions into E France ...

Weak instability is anticipated in the warm/moist air wrapping around the E and N periphery of the deep-layer circulation associated with the SW European upper low. Appreciable QG-forcing for upward motion is expected, and scattered, mainly imbedded storms should occur in a strip from the S Balkans across the Alpine areas into E France. Indications are that CAPE, as well as kinematic fields will be quite marginal, and severe thunderstorms are unlikely.

... central France ... Spain ... Italy ... western Mediterranean Sea ...

Another focus for convection exits beneath the upper low in a deep, modified polar air mass. This activity will be mainly diurnally driven and hence tied to land masses, though isolated activity may also occur over the Mediterranean Sea. Severe threat should be quite weak given very marginal shear/CAPE.

... N Algeria ... Tunesia ...

A strong vort max moving along the S periphery of the upper low will affect the N Maghreb States during the period. Convective scenario is uncertain given that CAPE is not simulated within the vort maximum's path. This may be due to low-level dry-air advection, though the one available upstream sounding (DAOR) does feature weak instability. If at least minimal instability materializes, storms will likely initiate given strong lift ahead of the vort max. DLS will be well in excess of 20 m/s, so development of a severe MCS would be possible. The convection may be slightly elevated, but marginally severe hail could not be excluded. However, the threat is very conditional, and currently is not supported by the GFS. Will thus not include this region in a severe-thunderstorm forecast, though an upgrade may become necessary on Sunday.