Storm Forecast
Valid: Mon 19 Aug 2013 06:00 to Tue 20 Aug 2013 06:00 UTC
Issued: Sun 18 Aug 2013 18:38
Forecaster: TUSCHY

A level 1 was issued for parts of Italy, Switzerland, W-Austria, S/C Czech Republic and parts of Germany mainly for an isolated large hail, strong to isolated severe wind gust and heavy rainfall event. A low-end tornado risk exists over NW Italy.

A level 1 was issued for N-Tunisia mainly for isolated large hail.

SYNOPSIS

No surprise to see a rather strong positive NAO signal in ensemble forecasts with a 595 gpdm high placed just east of the Azores and a 530 gpdm low over Iceland. A double polar front jet structure evolves between both pressure systems with one jet affecting offshore areas between Scotland and Iceland and the other one approaching Ireland and UK from the west. An upper wave over UK continues to move east with a constant amplification forecast in response to a 30 m/s mid-level jet maximum moving into its base. Further amplification and mid-level height drops are forecast over Italy and the feature will decelerate during the forecast while crossing Germany.

High pressure over SE Europe persists with stable conditions.

DISCUSSION

... Belgium, the Netherlands, Germany, parts of W-Poland and the W-/C-Czech Republic, Austria, Switzerland and parts of N-Italy ...

At mid-/upper-levels, a strong impulse crosses those regions from west to east with a slow down noted during the forecast period in most model data. This feature will be accompanied by a pronounced frontal boundary, which runs from E/SE France all the way to Poland. The interaction of both features results in a gradual drop of surface pressure over Germany and a weak surface low could evolve along that boundary while drifting to the east. Placement and intensity of that low remain uncertain due to an ongoing spread in model/ensemble data. This depression however results in a wavy frontal structure with a cold front moving southbound towards Italy and Austria and a warm front lifting gradually north towards N-Poland.

The combination of a weakly capped air mass and strong synoptic forcing in addition to the healthy looking baroclinic zone all foster rapid and scattered thunderstorm initiation. The least severe activity is forecast over Belgium / the Netherlands and also over NW Germany during the afternoon hours. Placed beneath the trough, shear at all levels remains weak with modest SBCAPE on the order of 400 to 800 J/kg. With plenty of BL moisture advected beneath the upper trough and BL dewpoints in the lower to mid tens, enhanced LLCAPE may be realized. Not much LL convergence is seen in model data, but combination of mid- and low-level CAPE and expected convergence zones should support an isolated funnel/tornado risk. This activity peaks during the afternoon and evening hours before decreasing after sunset due to BL stabilization. No upgrade was performed due to the spotty nature of potential events and expected low coverage of severe.

Of more interest will be the evolving warm sector, which covers the Alps, S/C Germany, the W-Czech Republic and W-Poland. An overlap of 15-20 m/s DLS and 500 to locally 1000 J/kg MLCAPE (maximized over SE Bavaria) should support organized multicells as early as noon. The hail risk will be tempered by weak mid-level lapse rates and expected short duration of discrete storm structures, but an isolated large hail event will be still possible with any mature multicell. Given nearly uncapped conditions, strong upper divergence / QG forcing but also the progressive frontal boundary itself, expect rapid upscale growth into a messy picture of various shower and thunderstorm clusters. We don't want to rule out some more organized line-ups, especially over SE-Bavaria, where some models indicate a stronger postfrontal pressure rise signal. There, swaths of strong to isolated severe wind gusts are possible, which could extend all the way into N -Austria. During the evening hours, an elongated line of showers and thunderstorms is expected from NW Poland to W/C Czech Republic into N-Austria. Embedded stronger cells may still pose an isolated hail and wind risk. However the main hazard transforms to heavy rain and some localized flash flooding. The reason for the enhanced flood risk will be forecast effective PWs of 30-40 mm and increasing training probabilities along the front, as convective elements start to move from southwest to northeast, affecting the same spots for some time due to an overall slow-down of the eastward propagating cold front. Heavy rain may also affect parts of W-Austria and E-Switzerland with the front squeezed along the Alpine Rim. However, decreasing CAPE and shear should start to reduce the overall severe risk .

Despite some signals of stronger cells over E-Germany and NW-Poland, no level 1 was issued for those areas due to rapidly decreasing CAPE.

During the afternoon hours, the southbound moving cold front arrives in NW Italy and taps into increasing BL moisture with 0-1 km average mixing ratios in excess of 13 g/kg. This amount of BL moisture offsets the weak mid-level lapse rates and results in robust at or above 1000 J/kg MLCAPE. 20 m/s DLS overspread the surface front and assist in well organized multicells and isolated supercells with large hail and a strong to severe wind gust event. Even an isolated tornado event can't be ruled out with 100-200 J/kg 0-3 km CAPE forecast by local WRF. This activity grows upscale into a large cluster of thunderstorms during the night while moving to the south and east. The hail and wind risk continues, although heavy rain becomes the predominant risk betimes.

... Rest of the lightning areas ...

Isolated thunderstorm initiation over N-Tunisia is possible during the evening hours with a SE-ward moving upper low atop. 500-1000 J/kg MLCAPE and 15 m/s DLS could result in a better organized multicell event with large hail and strong wind gusts. A marginal level 1 was issued. The risk diminishes during the second half of the night.

Over E-Spain and N-Sweden, isolated thunderstorms are forecast, but an unfavorable overlap of either weak shear or weak CAPE (or both) keeps the severe risk low.