Storm Forecast
Valid: Wed 20 Jul 2016 06:00 to Thu 21 Jul 2016 06:00 UTC
Issued: Tue 19 Jul 2016 22:52
Forecaster: PUCIK

A level 1 was issued for N England and Scotland mainly for excessive precipitation and to the lesser degree for large hail.

A level 1 was issued for N France, BENELUX and NW Germany mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for parts of E Spain mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Russia mainly for excessive precipitation and to the lesser degree for tornadoes and large hail.

SYNOPSIS

At mid to upper troposphere, a ridge will stretch from the Central Mediterranean towards Germany and S Scandinavia. To the west, a large cyclonic vortex resides over the Atlantic and its short-wave will cross Great Britain / NW France / BENELUX during the forecast period. A plume of steep lapse rates has already advected over much of the region and in combination with rather moist lower troposphere ahead and along cold front should contribute to development of latent instability. Another region with DMC activity will be over Russia, underneath a large, quasi-stationary closed low that is forecast to deepen during the forecast period. Both regions deserve a closer inspection regarding potential for severe weather.

DISCUSSION

... Norhern England, Scotland ...

In conjunction with the passage of a pronounced cold front, widespread thunderstorm activity is forecast over the region already in the morning hours. Forecast soundings point to the threat of excessive rainfall with skinny CAPE profiles and saturated troposphere. Despite the presence of DLS above 15 m/s and a potential for well organised DMC, large hail threat does not seem to be as pronounced as the threat of heavy rainfall due to the lack of high CAPE values and steep mid-tropospheric lapse rates. Nevertheless, especially over the southern half of Lvl 1, isolated large hail event may occur with stronger updrafts.

... Central France, BENELUX, NW Germany ...

It is difficult to put all details together for the setup over this region as model solutions diverge to some degree. Moderate CAPE values (1000 - 2000 J/kg) are forecast towards 18 UTC as 2 m dewpoints reach upper teens and are ovespread by moderately steep mid tropospheric lapse rates (around 7 K/km between 800 and 600 hPa layer). DLS will reach 10 to 20 m/s, increasing southwards and towards the evening hours. Hodographs look quite messy or with veer-back-veer pattern, suggesting that environment is not very conducive to well developed, persistent supercells. Thus, expected storm type is a mixture of multicells, perhaps with one or two brief supercells. Storms will form especially in the evening to night hours as the decaying cold front approaches the area. Pre-frontal convergence zones could also aid in the initiation, but forecast soundings suggest pronounced cap early into the day. Large hail or severe wind gusts will be possible especially with the first storms (forming in higher LCLs and steeper lapse rates environment) while the later, overnight and likely elevated storms will primarily bear a heavy precip threat.

... Spain ...

High-based storms (LCLs over 2500 m) forming in the environment of moderate to strong DLS (15 - 25 m/s) may be capable of severe wind gusts and/or large hail. Coverage of storms is questionable atm, but at least isolated storms are likely to form. The highest threat will be over the Ebro Valley (N Spain), where the strongest DLS is simulated.

... Russia ...

In the strong WAA regime to the north of the low, marginal to moderate latent instability is forecast by NWP with CAPE values locally exceeding 1000 J/kg. Around 15 m/s of DLS is simulated especially over the western part of the Lvl 1 with most of the shear confined below 850 hPa. Together with quite saturated profiles and abundant low-level moisture, this points to the high precipitation intensity and efficiency of storms. Therefore, excessive precipitation will likely be the primary threat. If brief supercells manage to develop, tornado threat can not be ruled out as LLS values reach or even exceed 10 m/s, especially just north of the surface low. Large hail may occur as well with briefly rotating cells.