Storm Forecast
Valid: Mon 21 Oct 2013 06:00 to Tue 22 Oct 2013 06:00 UTC
Issued: Sun 20 Oct 2013 20:54
Forecaster: TUSCHY

A level 1 was issued for far NW Portugal and Spain, mainly for an isolated tornado event and excessive rain.

A level 1 was issued for N-Italy and the N-Adriatic Sea, mainly for an isolated tornado event and excessive rain.

SYNOPSIS

Westerly flow regime transforms into a more amplified pattern with a southward digging trough over the E-Atlantic and ridging over C-Europe and the Mediterranean. An unseasonably warm air mass from the Iberian Peninsula spreads far N/NE.
Cold and stable conditions prevail over far NE Europe.

DISCUSSION

... N-Italy and the N-Adriatic Sea ...

A moist and unstable air mass still resides over the C-Mediterranean, but rising mid-level heights and surface pressure induce a gradual downswing of organized convection during the forecast.

This forecast period begins with ongoing clusters from the overnight hours. Clusters will be effective rainfall producers due to their slow motion and temporal back-building tendencies. DLS remains in the 10-15 m/s range for most the time, with a gradual weakening trend expected during the night. Same with LL shear. So expect organized thunderstorms mainly during the daytime hours with excessive rain the main risk. More discrete storms or tail-end storms will be able to produce an isolated tornado/large hail event. During the night, mainly onshore convection weakens considerably, but convection with access to the offshore areas could survive the night with only gradual weakening expected. Excessive rain remains the primary risk.

... Portugal to UK to the North Sea ...

Large-scale trough over the E-Atlantic advects a warm and moist air mass far north. This plume of energetic air is characterized by weak mid-level lapse rates and moist LL. Hence, slim CAPE will be the main problem for sustained and long-lived updrafts. Betimes, drier and cooler mid-level air overspreads the air mass mainly over the W-Bay of Biscay to the north and south. This should increase the risk of electrified convection substantially.

Once again, a NE-ward progressing warm front (UK to the North-Sea) was highlighted by a 15-% lightning area, which covers a broad region. Nothing severe is forecast with that convection.

For W-Portugal and extreme NW-Spain, shear parameters become very supportive for organized convection, but meager CAPE remains a concern. However, onshore moving storms can temporarily ingest high helical and moderately unstable air, which could support an isolated tornado risk. A forecast sounding of the NW-coast of Portugal at 00Z (in the proximity of a strong eastbound moving warm front) reveals 66 kt DLS, 270 m^2/s^2 SRH-3 and 500 J/kg MLCAPE with an LCL of 960 hPa. However, it will be hard to get discrete storm structures going with near front parallel flow, so expect heavy to excessive rain the main risk with clustering storms.
For SW-Ireland, the environment looks supportive for temporal and shallow organized convection with a short-lived funnel/isolated tornado risk. However, effective PWs of 20-25 mm also point to an heavy to excessive rainfall risk. This hazard looks too marginal for a level 1.

... N/NE Poland and W-Belarus ...

Eastward sliding mid-level wave with a -25 °C 500 hPa layer overspreads eastward fanning moisture in the lower troposphere. 300-600 J/kg MLCAPE is forecast with 15-20 m/s DLS and 15 m/s LL shear. Although meager CAPE probably keeps longevity of convection low, one or two storms could become severe with an isolated severe wind gusts and tornado risk. This convection rapidly vanishes after sunset.