Storm Forecast
Valid: Mon 29 Mar 2010 06:00 to Tue 30 Mar 2010 06:00 UTC
Issued: Mon 29 Mar 2010 00:00
Forecaster: DAHL

A level 1 was issued across France for severe wind gusts ... large hail ... and tornadoes.

SYNOPSIS

Main actor this period will be an Atlantic upper trough that will cross the southwestern European states on Monday. This trough features two main CVA maxima, one being associated with a peripheral vorticity lobe that is simulated to eject into the northern Mediterranean by the end of the period ... and another vorticity/CVA maximum near the cyclonic-shear side of the trough, which will cross France late Monday night. Rather strong SFC cyclogenesis is anticiüpated to accompany these upper features. Ahead of this trough a plume of warm/moist air will advect mainly into France on Monday.
Downstream, an upper cut-off low is anchored over the southeastern portions of Europe.

DISCUSSION

... France ...

It seems that some instability will develop over France with diurnal heating and upper-level cooling ahead of the upper trough in the warm-sector region ahead of the SFC cold front. Lapse rates and low-level moisture should be rather marginal, but a few 100 J/kg of CAPE are likely. Convection should initiate in the afternoon hours in advance of the first vorticity maximum. Deep-layer shear is expected to range from about 15 m/s to more than 25 m/s, with low-level shear of about 7 m/s, increasing to more than 12 m/s towards late evening as the cyclogenetical flow regime overspreads the region.
The storms will likely merge into a linear system given strong deep shear and rather weak line-normal deep-layer flow. The threat with this system should shift from a hail/wind threat primarily to a wind threat towards early evening. However, isolated cells, especially towards late afternoon/evening, are likely to become supercellular ... posing a threat for large hail ... damaging wind gusts ... and tornadoes. At the moment, the thermodynamic support seems to be somewhat uncertain, especially the availability of low-level moisture, so will not issue a LVL2 threat at the moment. However, an upgrade may become necessary on Monday.

... N Spain ... S France late in the evening ...

Upstream along the cold front, additional convection is anticipated. Shear should still be sufficient for storm organization, so that an isolated straight-line-wind event or a tornado cannot be discounted.

... SE Europe ...

Rather shallow and weakly electrified thunderstorms may occur in the vicinity of the SE European upper low over the E Aegean, Turkey, and the SE Mediterranean Sea. It seems that some chance for waterspouts exists given rather strong low-level buoyancy. However, overall threat is too low to warrant LVL1 probabilities.