Storm Forecast
Valid: Sat 27 Jul 2019 06:00 to Sun 28 Jul 2019 06:00 UTC
Issued: Fri 26 Jul 2019 20:51
Forecaster: PUCIK

A level 2 was issued across W Italy coastline and N Italy mainly for excessive rainfall, large to very large hail, severe wind gusts and to the lesser extent for tornadoes.

A level 1 was issued in a belt from NE Spain, through SE France, parts of BENELUX, S Germany, Czech Republic, Slovakia, Austria, Hungary and Slovenia mainly for excessive rainfall, large hail and severe wind gusts.

A level 1 was issued for NW Belarus and Lithuania mainly for large hail and severe wind gusts.

SYNOPSIS

Synoptic-scale pattern is undergoing a major shift during this period with a ridge over Scandinavia being cut off by a low centered over Poland and a short-wave trough that will move from the Bay of Biscay towards N Italy by the end of the forecast period. Closer to the surface, cyclogenesis is simulated over Netherlands and a wavy frontal boundary will run from BENELUX through E France into N Spain.

Ahead of the frontal boundary, warm sector will be characterised by abundant lower tropospheric moisture and mid-tropospheric lapse rates of around 6.5 K/km. Widespread thunderstorm activity is forecast in the uncapped and moderately to strongly unstable airmass with considerable severe weather risk, especially over parts of Italy.

DISCUSSION

... BENELUX, W Germany E France, NW Italy...

Widespread thunderstorms are forecast to initiate during the day in uncapped airmass with low LFCs. While weak vertical wind shear will preclude well organised storms, forecast profiles suggest high precipitation efficiency in the environment of high RH throught the deep layer of troposphere, low LCLs and significant warm cloud depth. Excessive precipitation will be primary concern and the threat seems to be elevated especially over Belgium / S Netherlands along the stalling boundary and over the Alps, where numerous rounds of storms will be possible.

... SE Germany, Czech Republic, Slovakia, Austria, Hungary, S Poland, N Italy...

Similar to the previous area, widespread convective initiation is anticipated and numerous thunderstorms may already be ongoing at the beginning of the forecast period, particularly over the Alps and over E Czech Republic / Slovakia. This poses a question of the airmass recovery for some of the areas affected by early storms. Nevertheless, NWP agrees on MLCAPE of around 1000 J/kg, increasing southward towards Italy, overlapping with 10 to 15 m/s of DLS, yielding at least isolated well organised and severe thunderstorms. Early into the forecast period, a belt of stronger DLS is forecast across S Poland, E Czech Republic, Slovakia and Austria, so that transient supercells would be possible. Stronger cells will be capable of large hail and isolated severe wind gusts, but the main threat will be excessive rainfall. This threat will increase towards the evening hours as storms cluster and mean cell motion decreases. Widespread thunderstorm activity is forecast to persist throughout the evening and early night over N Italy as the short-wave trough arrives from W, exacerbating the heavy rainfall potential and warranting Lvl 2 for this area.

... NW to W Italy coastline ...

Compared to the areas N and NW-ward, this region will be under both higher CAPE and stronger DLS as moisture pools along the coastlines in S to SW-ly flow and mid-tropospheric flow increases ahead of the approaching short-wave trough. Thus, the likelihood of supercells and well-organised convective systems will be higher here. Liguria may experience the first storms already early in the forecast period, but the coverage along the remaining coastline will increase in the evening to night hours as storms propagate SSE-ward along the coastline. The main threat will be excessive precipitation, but the degree of CAPE and shear overlap also point ot the risk of very large hail and damaging wind gusts. While tornado risk will be non-zero, it will be diminished by the lack of strong lower tropospheric shear.

... Belarus into Lithuania ...

A belt of enhanced mid-tropospheric flow is simulated north of the cut-off low, enhancing DLS to 15 to 20 m/s, favoring well organised multicells and some supercells. Initiation remains questionable as some of the NWP simulates no precipiation across the area, but due to the non-zero risk of large hail and severe wind gusts, will maintain a Lvl 1 in case that storms form here.