Storm Forecast
Valid: Fri 08 Nov 2013 06:00 to Sat 09 Nov 2013 06:00 UTC
Issued: Thu 07 Nov 2013 22:25
Forecaster: TUSCHY

SYNOPSIS

The European pattern is characterized by a gradually deepening and rapidly approaching large-scale trough from the E-Atlantic and strong ridging over the central Mediterranean towards the Black Sea.

In the lower to mid troposphere, an extensive baroclinic zone runs from the N-Bay of Biscay to the Baltic Sea. Interaction of a mid-layer impulse with that frontal zone results in cyclogenesis, entering the English Channel around noon and approaching Benelux until midnight. Besides, pressure also drops over S-France and more significantly over N-Italy beyond 00Z as lee-cyclogenesis will be underway.

DISCUSSION

... SW/C-France to Germany ...

Extrapolating the path of the eastward racing patch of layered clouds (representing the approaching impulse, 21Z) with the location of the extensive frontal cloud band confirms model expectation regarding placement of interaction/cyclogenesis. This evolution resembles an instant occlusion cyclogenesis, which often is characterized by some secluded warm air inside its center (enhanced wind risk along its S/SW fringe) and a temporal split front appearance with the eastward moving cold front....both having some impact on our forecast.

First, expect isolated thunderstorms along the still untouched baroclinic zone over the S-Bay of Biscay into C-France during the morning hours. Modest mid-level lapse rates and pooling of BL moisture within that converging flow regime should offer some low-end CAPE within a strongly sheared environment. Strong to severe wind gusts will be the main hazard with any storm.

Thereafter, until noon, falling pressure over NW France induces a developing warm sector, which covers most of C/E France and C/S Germany. The main focus for a few thunderstorms arises along the backside of the sharpening/N-ward lifting warm front and this activity should move from E-France to W-C Germany and further to the NE during the night. Effective PWs, although decreasing, still support heavy bursts of rain with this convection. Strong wind gusts are also possible. Of interest is the increase of SRH-1 to more than 200 m^2/s^2, some LL CAPE and 300 J/kg SBCAPE between Nancy and Treves during the afternoon hours. An isolated funnel/short-lived tornado event in this region during the afternoon hours can't be ruled out.

Then, during the afternoon/evening hours, the cold front becomes the focus for isolated CI, too. Expected split front character of that boundary induces an isolated risk for storms over most of C/E France into Switzerland and SW Germany during the evening and overnight hours. With decrease of mid-layer forcing due to falling heights over the NW Mediterranean and south of the Alps, not much activity is forecast however. Any stronger downdraft is able to bring strong gusts down to the surface.

Most interesting part of this scenario likely evolves over Benelux during the overnight hours. Some secluded warm air in the depression's core should assist in a tight gradient along its SW-fringe with strong to severe wind gusts. Right now, this scenario is supported by a few high resolved models. It has to be monitored regarding the chance for a confined area with severe (non-convectively induced) wind gusts.

We decided to stick with the frontal zone regarding the 15-% lightning area. However, a very isolated short-lived event can also occur north and south of that highlighted area.

... N-Italy...

Around 21Z onwards, conditions over far SE-France and NW Italy gradually improve for scattered coastal thunderstorms, which build eastwards. 20 m/s DLS assists in a few organized storms with gusty winds and marginal hail. No level 1 will be added right now although some temporal back-building from Genova to Milano is possible (even to Trento with enhanced usplope flow). Same for the NE Adriatic Sea into extreme NE Italy/W-Slovenia. Heavy rain will be the main hazard.

...Rest of the lightning areas ...

Marine convection with 850 hPa flow aob 15 m/s should result in gusty winds and some sleet (H85 below 0°C). Nothing severe is anticipated.