Storm Forecast
Valid: Fri 03 Apr 2009 06:00 to Sat 04 Apr 2009 06:00 UTC
Issued: Thu 02 Apr 2009 16:23
Forecaster: TUSCHY

SYNOPSIS

The European weather pattern stays stable as a weakening upper low is placed over the west-central Mediterranean with no significant shift expected for the next 24 hours. Strong CAA over NE-Europe keeps environment hostile for deep, electrified convection. High geopotential heights in-between also keep thunderstorm chances limited. A weak trough approaches from the west and affects UK/Ireland and Scotland during the end of the forecast period.

DISCUSSION

... Western / central Mediterranean and adjacent areas ...

Quasi-stationary upper low stays put, commuting its more SW-NE elongated circulation into a more circular one, which confines coldest mid-levels roughly west of Corsica/Sardinia. This is worthwhile to mention as thunderstorm probabilities seem to decrease offshore compared to 24h ago, e.g. over the Tyrrhenian Sea and most of the Adriatic Sea. So the western Mediterranean is the most favorable region for electrified convection (offshore) and models like GFS/WRF have plenty of that over the warm waters. The foci are:

i) a tongue of better instability around the Balearic Islands during the morning hours/noon as diffluent mid-level zone overlaps and

ii) the region west of Sardinia, where better LL moisture/instability exist.

For the islands and continental areas we went close to model guidance as neither synoptic scale support nor significant changes of the quality of the boundary layer are forecast. Climatology was heavily weighted in this forecast.

Weak shear and marginal CAPE release hint on sub-severe hail/gusty winds with strongest showers/thunderstorms. An isolated waterspout can't be excluded as LL CAPE is somewhat enhanced and background flow very weak.

... SE-/E-France, W-Switzerland and S-Germany ...

Persistent signals for convective initiation were present in past few model runs like GFS/WRF/UKMO, atmosphere at mid-levels gets more humid, a weak thermal trough crosses the area from the eastsoutheast during the favorable time of strongest diabatic heating while a decaying short-wave spreads westwards during the noon/afternoon hours. Also, persistent boundary layer moisture recovery was noticed over E-France and W/SW Germany during the past 48h, so confidence is high enough to include SW-Germany and parts of E-France, strongly confined to coldest atmospheric stratification. Weakly organized ordinary cells are forecast with gusty winds and marginal hail.

A tricky forecast for extreme SE-Germany as overall conditions are still too hostile for electrified, deep convection. However a slight cool-down of the 700-850hPa layer during peak heating and reducing geopotential heights could indeed result in a very sporadic thunderstorm along the orographical favored areas. Another pro could be, that thetae values at lowest 2km are maximized over extreme E/SE-Germany. No general thunderstorm area was included as both coverage and intensity of thunderstorms should be too sparse for the issuance of such an area.

Thunderstorms rapidly decay after sunset as boundary layer stabilizes and instability vanishes.