Storm Forecast
Valid: Wed 29 Aug 2018 06:00 to Thu 30 Aug 2018 06:00 UTC
Issued: Tue 28 Aug 2018 22:18
Forecaster: PUCIK

A level 1 was issued across NE Spain mainly for large hail and severe wind gusts.

A level 1 was issued across E France, Switzerland, SW and S Germany and extreme NW Austria mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued across Belarus and W Russia mainly for excessive precipitation and large hail.

SYNOPSIS

As of Tuesday 22 UTC, a large convective system is crossing W France, ahead of a short-wave trough and a surface cold front. These two features are forecast to move E-wards during the day, reaching Switzerland, Germany, Denmark and eventually W Poland, NW Austria and W Poland by the end of the forecast period. Ahead of the cold front, an airmass with modest lower tropospheric moisture will be advected towards E and NE. Towards the E, a low will reside over NW Belarus and W Russia, almost stalling and lifting only very slowly NE-wards. At the surface, a cold front is forecast to initiate scattered convection over the area.

DISCUSSION

... E Spain ...

A broad short-wave will approach the region from W towards the afternoon hours with windspeeds around 10 m/s at 500 hPa level. Combined with easterly flow at the surface, DLS values around 15 m/s are likely near the coastline. Isolated to scattered thunderstorms will initiate in upslope flow in around 1000 J/kg, owing to lapse rates of around 7 K/km and moist lower troposphere. Well organised multicells and perhaps even a brief supercell will be capable of isolated large hail and severe wind gusts.

... E France, Germany, Switzerland, extreme NW Austria ...

The evolution of situation will likely depend on the overnight development of the MCS currently ongoing over France. Exact positioning of the front and northward extent of a tongue of low level moisture is also different in NWP output and thus a broad Lvl 1 was chosen as a solution. Several hundreds up to around 1500 J/kg of CAPE are simulated ahead of the advancing cold front with DLS values ranging from 10 to around 15 m/s. High resolution NWP output simulates development of scattered multicells, including some line segments along the front, progressing NE-wards during the day. Isolated large hail will be possible with stronger cells, the threat being somewhat limited by a lack of steeper lapse rates, along with some severe wind gusts. Along the Alps and over S France, excessive precipitation threat will also be present. Storms will weaken with deteriorating moisture as they progress towards E Germany and W Czech Republic during the Wednesday night.

... Belarus to W Russia ...

Ahead of a slowly moving cold front, a tongue of moisture will contribute to skinny CAPE profiles. Nevertheless, around 15 m/s of DLS may allow for a few well organised multicells capable of isolated large hail and/or excessive precipitation as cells train in flow parallel to the initiating boundary.