Storm Forecast
Valid: Mon 26 Sep 2011 06:00 to Tue 27 Sep 2011 06:00 UTC
Issued: Sun 25 Sep 2011 17:27
Forecaster: TUSCHY

A level 1 was issued for parts of Norway and Sweden mainly for severe wind gusts and an isolated tornado event.

A level 1 was issued for Sicily and surrounding areas mainly for excessive rainfall and local downbursts.

SYNOPSIS

A progressive pattern continues over N-Europe as a strengthening upper wave crosses Norway and Sweden during the forecast. Further to the south, temporarily weakening ridging is in place over central Europe and keeps thunderstorm probabilities limited. Main focus for an isolated, short-lived storm will be an eastward moving cold front over Benelux/W-Germany, accompanied by a thermal trough at 500 hPa. Also, postfrontal onshore flow may allow a sporadic storm or two to form over N-France. Over the central Mediterranean, a slow moving/quasi-stationary upper low sparks scattered to widespread thunderstorms with an excessive rainfall risk.

DISCUSSION

... Tyrrhenian Sea, Sicily, S-Italy, Sardinia and Corsica ...

As ridging over W-Europe strengthens during the day, an upper trough over the central Mediterranean becomes quasi-stationary with little net movement to the SE. This feature can be easily identified by its cyclonic streamline pattern but the thermal trough is fractured into numerous pieces. This assists in widespread destabilization over the central Mediterranean with high res. models and global models all indicating up to 1 kJ/kg MLCAPE with locally higher maxima. Despite some weakening of the mid-level lapse rates due the to diffuse thermal trough, rich BL moisture is trapped beneath that feature with PWAT exceeding 35 kg/m^2. No steering flow is present with DLS in the order of 10 m/s or below. High resolution models still disagree where QPF maxima will be centered with model solutions favoring places either W/NW of Sicily or Sicily itself. Forecast soundings indicate deep/moist profiles with broad CAPE profiles and storm motions of 2-5 kt. Combined with high-res. model peaks of 50-80 l/qm/3h we expect an augmented chance for slow moving storms with excessive rainfall the primary risk. However, wet downbursts in stronger storms can't be ruled out either. The thunderstorm threat keeps going all day long with the maximized activity gradually occurring offshore after sunset.

We decided to stick with a broad level 1 due to ongoing model discrepancies and the more isolated nature of potential excessive rainfall events. However a serious flash flood risk may accompany each mature thunderstorm over Sicily.

... Parts of Norway and Sweden ...

A sharpening upper trough crosses Norway during the daytime hours from the SW. Accompanied surface trough and cold front will be the focus for some enhanced convective activity within the highlighted area. Overall model guidance agrees well as a sharp cold front races E/NE-ward between 09-15Z. This boundary is placed somewhat ahead of the upper wave, so a cool down first starts below 600-500 hPa. Forecast soundings reveal rapidly steepening lapse rates beneath 500 hPa as cold front moves in and some low-end CAPE build-up seems reasonable. A strongly forced line is expected to rapidly organize along the cold front with 15-20 m/s flow in the lowest 3 km. Strong to severe wind gusts will be the main hazard with that line, but somehwat better curved hodographs below 3 km just ahead of the cold front also indicate a chance for some storm rotation and an isolated tornado risk can't be excluded. Due to marginal CAPE and quite warm EL temperatures, not much lightning activity is expected and hence only a 15% TSTM area was added next to the level 1. A big uncertainty will be the quite rough topography, which may suppress a well structured MCS but may help to increase SRH significantly on the meso-gamma scale.
The models weaken the CAPE field over Sweden as diabatic heating wanes. However strong forcing may help to offset that scenario for a few hours and hence the level 1 was expanded well to the east/into the late evening hours.