Storm Forecast
Valid: Fri 24 Apr 2009 06:00 to Sat 25 Apr 2009 06:00 UTC
Issued: Thu 23 Apr 2009 19:19
Forecaster: TUSCHY

SYNOPSIS

Omega pattern over Europe persists during the forecast. A compact cold-core low over the western Alps moves west/northwestwards while weakening. A more dynamic upper trough approaches western Europe during the night hours with a nice surge of warm/humid air well to the north ahead of this feature. Cool and stable conditions prevail over NE-Europe.

DISCUSSION

... Italy,Ionian Sea and Switzerland ...

Temperatures at 500hPa just shy below -20°C and surface dewpoints in the lower to mid tens result in abundant instability (e.g. 500-800 J/kg MLCAPE). The best environment will be roughly from Rome to the southeast. Shear throughout the troposphere is weak, inhibiting widespread storm organisation. Locally robust instability release and a low WBZ however hint on a possible large hail event. Deep layer shear over Sicily increases to 20m/s with similar CAPE values, so large hail ought to be higher in coverage and therefore a level-1 was issued. The level-was was epanded well to the north (e.g. S-Italy), where GFS has more than 100J/kg LL CAPE. A short tornado event along intersecting outflow boundaries is possible in such an environment, so the combination of an isolated large hail/tornado event made a marginal level-1 necessary.
Thunderstorms also increase in coverage west of Greece during the morning hours, but shear at all levels remains too weak for storm organisation. Meanwhile, thunderstorms over Italy decrease in intensity/coverage after sunset.

A retrograd moving cold core low over the western Alps is the focus for isolated thunderstorms mainly over W-Trention, the Lombardy, Piedmont and the Valle d'Aosta, as boundary layer moisture remains high enough for some instability release. A combination of utterly low dewpoints over Switzerland and the weakening trend of the upper low ( e.g. constant warming especially below 500hPa) bring thunderstorm chances down to near zero. However, global models still have a very bad handling in complex terrain and dewpoints don't have to climb a lot for initiation as lapse rates at mid-level remain steep. We therefore accentuated parts of Switzerland, where at least a very isolated thunderstorm event seems possible. The main hazard will be hail due to strong DLS, steep lapse rates and a low WBZ. Thunderstorms weaken during the evening hours and around sunset at the latest.


... N-Spain, W-France and SW-UK...

A dynamic upper trough over the Bay of Biscay swings eastward during the evening and night hours. At the same time, a north-south aligned cold front shifts eastwards,too, speeding up over SW-UK and NW-/W-Central France during the night hours, while stalling over SW-France and NW-Spain. There are still some uncertainties with exact timing of those features (GFS retarded the arrival during the past 3 runs) and the strength of the developing surface depression just west of SW-UK (ECMWF has the deepest solution, whereas GFS remains more reluctant). Boundary layer recovery in most of the models is impressive with surface dewpoints in the lower to mid tens. Latest synop reports along the W-coast of France and NW-Spain have indeed dewpoints around 10°C, so we think that 9-12°C are reasonable well inland over SW-France, which is somewhat lower what models predict.
Strong WAA during the daytime hours keep atmosphere capped until the evening hours, as cold front approaches from the west. Lapse rates at mid-levels are steep enough for some instability release although it still remains uncertain how strong this release will finally be. ECMWF and GFS agree in an rapid thunderstorm increase over the SE-Bay of Biscay and the coastal areas of NW-Spain/SW-France around 18Z-00Z. 20-30m/s DLS and strong directional shear help thunderstorms to gain organisation rapidly with an attendant large hail/severe wind gust risk (e.g. lapse rates between 800-600hPa in excess of 8K), especially over N-Spain, where stronger cap should keep thunderstorms more discrete.
Thunderstorms then start to line up along the cold front over SW-France. It depends on how much instability exists with ECMWF showing the strongest signals, whereas GFS just has rapidly diminishing MUCAPE over W-France. Strong forcing could offset decreasing instability, so for now, we went with the idea of a forward propagating MCS over W-central France after midnight. We just issued a general thunderstorm area for that region as new model runs have to be evaluated. If indeed more instability becomes available, an upgrade may become necessary. Strong to severe wind gusts and marginal hail are possible over SW/W-France.

During the night hours, some MUCAPE is also forecast over SW-UK/E-Ireland but the most likely scenario is a few embedded storms in a more stratiform rain shield. This area has to be monitored, too as shear would be supportive for storm organisation.