Storm Forecast
Valid: Fri 03 Jul 2015 06:00 to Sat 04 Jul 2015 06:00 UTC
Issued: Thu 02 Jul 2015 22:17
Forecaster: GROENEMEIJER

A level 2 was issued for NW France and the channel region, for very large hail, severe wind gusts and tornadoes and to a lesser extent for extreme precipitation.

A level 1 was issued for a greater part of Northern and Western France and the Southern United Kingdom for large hail and extreme precipitation.

A level 2 was issues for parts of Northern Germany and Southern Denmark, for large hail, and to a lesser extent for severe wind gusts and extreme precipitation.

A level 1 was issued for Northern Denmark, Southern Germany and the Alps for for large hail, and to a lesser extent for severe wind gusts and extreme precipitation.

A level 1 was issued for the Southwest Ukraine, Southeast Moldova and East Romania for extreme precipitation and large hai.

A level 1 was issue for parts of Northwest Russia including Southern Karelia, and the oblasts Leningrad, Vologda, Tver, Novgorod and Yaroslavl, mainly for severe wind gusts and tornadoes.

SYNOPSIS

Friday morning at 06 UTC...a ridge extends from the Western Mediterranean to the Baltic States and a cold core low is slowly drifting southwestward over the western Black Sea. Very high lower-troposperic temperatures are present the ridge. An eastward intrusion of cooler near-surface air has made its way into Germany ahead of a second northward impulse over France of particularly hot air.

DISCUSSION

NW France...

The forecast for this area is particularly difficult. NWP guidance suggests that a warm front will set up over the area during the afternoon and evening, and that steep lapse rates and moisture will lead to ample surface-based CAPE of 1000 (ECMWF) to 2000-3000 J/kg (GFS). In addition, a substantial elevated CAPE is forecast to develop during the evening over the Channel and Southern England.

During the evening, the warm frontal zone features easterly surface winds, a brisk 10-18 m/s southerly flow at 850 hPa and 15-20 m/s southerwesterlies at 500 hPa. This yields a large curved low-level hodograph with 400 m2/s2 of SREH. Hence, strong rotation is expected in any boundary-rooted storm that might develop. A risk of very large hail producing supercells with - given the 10-15 ms/ 0-1 km shear forecast in the 18-21 time frame - a risk of (strong) tornadoes exists.

The difficulty is caused by the fact that NWP models are not uniformly producing surface-based convective storms. Indeed, a significant hindrance to surface-based development is the warm air at 850 hPa of around 18 C. The GFS model initiates what is likely surface-based convection within the level 2 area during the afternoon, but there is very little precipitation in ECMWF.

I have decided to go with a level 2, with the understanding that there is a risk that surface-based convective over NW France may not occur at all, in which case the severe weather threat is limited to large hail and extreme rainfall from any elevated storms.

England, Channel region, Benelux...

Elevated convection should move in from the south during the late evening and after midnight. There is a small risk of large hail and extreme precipitation with these storms.

Germany, Denmark...

The boundary of cooler air generated on Thursday will likely be the focus of new convective development during the late afternoon and evening. For this area, the level 2 was issued. Along that boundary, relatively high surface moisture should be available, that in combination with the steep lapse rates lead to 2000 or more J/kg CAPE. 0-6 km shear is weak, but with 10 m/s it should sustain multicellular modes. The strong updrafts will likely cause large hail in some places, and the deep boundary layer may lead to strong local downbursts.

Away from the boundary the probability of convective initiation decreases, but ample buoyancy still implies a large hail and downburst threat. Across Denmark, convective coverage is not a concern, but the risk of severe weather should decrease with the somwhat lower CAPE forecast there compared to Germany.

Alps...

In an environment similar to that over Germany, i.e. very hot with a relatively deep, dry boundary layer, scattered storms should develop during the day in response to the mountain circulation. The sizable CAPE forecast to develop over the Alpine slopes indicates that these storms, despite a lack of shear may produce very isolated severe weather, specifically extreme precipitation and large hail, and possibly strong downbursts.

NW Russia...

Within a strong west-northwesterly flow some CAPE is expected to develop to the warm side of a WNW-ESE oriented frontal zone. NWP guidance indicate modest signals for convective coverage. Nevertheless, the high shear prompts a level 1 to be issued for severe wind gusts but also for tornadoes.

Eastern Balkans...

On the western fringe of the md/upper-level low ample large-scale upward motion is forecast that should aid storm initiation. Indeed, NWP models simulate widespread convection. The shear is in the 10-18 m/s range, indiicating that some storm systems may be rather well-organized multicells. Some marginally large hail and, in particular extreme precipitation are forecaste to occur on an isolated basis.