Storm Forecast
Valid: Fri 25 Nov 2011 06:00 to Sat 26 Nov 2011 06:00 UTC
Issued: Thu 24 Nov 2011 17:28
Forecaster: TUSCHY

A level 1 was issued for far NE Morocco/NW Algeria mainly for heavy rainfall.

SYNOPSIS

The overall pattern becomes more zonal with strong westerlies affecting N-Europe. Further south, high pressure builds in from the west and keeps most parts of W/CNTRL Europe dry. Residual upper trough over the W/CNTRL Mediterranean sparks numerous showers and thunderstorms.

DISCUSSION

... NE-Morocco, NW Algeria all the way to the Ionian Sea ...

High pressure builds in from the E-Atlantic and causes a gradually strengthening easterly flow over the W/CNTRL Mediterranean. With still cool mid-levels present, some modest MLCAPE is forecast over the highlighted area. Slow storm motion causes thunderstorms with an heavy rainfall risk. Storms may repeatedly move ashore along the NE coast of Morocco and far NW coast of Algeria and hence a level 1 was issued to reflect the augmented heavy rainfall risk.

Another area of enhanced shower/thunderstorm activity runs from far S-Italy to the south, where models indicate somewhat stronger convergence. Again, heavy rainfall will be the main risk with those storms. Some weak LLCAPE is present so an isolated waterspout event can't be ruled out. No level 1 was issued due to ongoing discrepancies in the model data (e.g. placement and amount of the convective precipitation)

... W-coast of Norway and offshore...

Isolated thunderstorms are possible in the cold, maritime air mass with 500 hPa temperatures dropping below -30 °C. Yesterday's lightning data already indicated sporadic activity S of Iceland (24th Nov. at 17 Z).
As SSTs increase to 10 °C and more north of Scotland, DMC becomes increasingly likely with EL temperatures below -30°C. The main hazard will be strong to severe wind gusts as 850 hPa winds exceed 20 m/s during the forecast. No level 1 was yet included, because we would like to see an environment supportive for more widespread DMC or some focus for better organized DMC (e.g. lined up along a front).

A sub-960 hPa depression moves towards the N-coast of Norway with 850 hPa winds increasing to 30-35 m/s. As GFS indicates weak MUCAPE signals in this wind field, severe to damaging wind gusts will be possible in N-Norway, as shallow convection may mix those winds down to the surface. However, we do not reflect that risk in our warning scheme, as no/only very sporadic DMC is expected with that activity.