Storm Forecast
Valid: Tue 28 Apr 2009 06:00 to Wed 29 Apr 2009 06:00 UTC
Issued: Tue 28 Apr 2009 04:32
Forecaster: VAN DER VELDE
SYNOPSIS
The weather pattern for today shows a low pressure area and upper cold pool over southern England and France in which many showers will develop, here and there with thunder. At its east flank, a stationary frontal band with a warmer slightly unstable airmass stretches between Austria and the North Sea.
DISCUSSION
...western France..
The airmass predicted by GFS has high low-level CAPE values (>150 J/kg) and steep low level temperature lapse rates. In combination with the absence of mechanical mixing (wind) and a source of vertical vorticity, the environment appears conducive to landspouts.
In southwestern France, the flow is stronger at all levels and pulls in slightly more moist air from the ocean. A shortwave in the upper flow passes during the afternoon and the northwesterly jet creates deep shear vector magnitudes >20 m/s. Some SREH is present and 0-1 km shear is predicted >10 m/s. LCL height is low around 500m. A slight tornado threat is present. Gusts can be strong but will likely remain under the severe criterion.
Hail will be possible to common in most parts as result of quite steep low/mid level lapse rates and low freezing levels, with sizes not likely to exceed 1-1.5 cm.
...Albania and western Greece...
As a mid level trough passes during late morning/mid afternoon over an Adriatic/Ionian band of relatively high theta-e and strong convergence, storms are likely to form in this region, but MLCAPE vs ICAPE and MU-EL suggest instability will be elevated inland. Overlapping jet creates DLS of 15-25 m/s while LLS can reach 15 m/s... inland. This may not be tapped, but instead up to 300 mē/sē SREH at the edge of the instability zone could yield a supercell or two, with large hail, gusts, and a tornado is not excluded. This threat level is marginal given the not very high CAPE and uncertain overlap by low/mid level shear and elevated instability over land.
