Storm Forecast
Valid: Wed 21 May 2014 06:00 to Thu 22 May 2014 06:00 UTC
Issued: Tue 20 May 2014 21:20
Forecaster: PUCIK
A level 2 was issued for Southern to Central France mainly for severe wind gusts, large hail, tornadoes and excessive precipitation.
A level 1 was issed for Northern France, BENELUX, Northwestern Germany and Denmark mainly for large hail, severe wind gusts and to the lesser degree for tornadoes.
A level 1 was issued for Northern Russia mainly for large hail and severe wind gusts.
A level 1 was issued for Central Ukraine, extreme Eastern Belarus and parts of Western Russia mainly for large hail and excessive precipitation.
SYNOPSIS
The main macrosynoptic feature will remain in the form of a cut-off low and long-wave trough residing over the eastern Atlantic. Center of the cut-off will move towards western Iberia, with strong mid to upper tropospheric flow covering its southern and eastern flanks. A belt of southerly flow will stretch from northern Africa all the way towards western Scandinavia. With this flow, a plume of steep mid-level lapse rates is advected northwards. Towards east, a ridge will cover much of the Central Europe and the Mediterranean.
Closer to the surface, center of the low will remain off the coastline of NW Iberia. A wavy frontal boundary will stretch from Spain towards W France, SE England, Denmark and further into Scandinavia. Ahead of it, a WAA regime will continue over much of Western and Central Europe.
DISCUSSION
... Southern to Central France ...
Models generally agree on quite potent situation regarding the degree of vertical wind shear, with DLS ranging from 20 to 35 m/s and SREH values around 200 m2/s2 as southeasterly surface flow will be overspread by the strong southerly flow aloft. Strengthening wind field towards the afternoon and evening is expected to contribute to strong LLS, with values above 10 m/s in the 0-1 km layer. Such conditions would certainly be conducive to well organised convection.
Degree of instability developing during the day is more questionable - models show that the steepest lapse rates will be displaced to the east of the best low-level moisture GFS seems more optimistic regarding CAPE values while ECMWF stays less optimistic with several hundreds J/kg of CAPE with spots of CAPE above 1000 J/kg. One of the issues might be the abundance of "cloud debris" limiting the daytime heating.
Convection may already exist early in the forecast period, with several outflow boundaries and convergence zones over the area. Towards 12 UTC, first isolated cells might develop, taking on the supercellular character thanks to the favourable wind shear. These storms could pose risks of large hail, severe wind gusts and also tornadoes. With shear parallel to the frontal boundary, quick clustering of storms is possible, which might limit the supercell potential later in the period. Between 15 and 21 UTC, as subtle short-wave ejects from Spain northward, rapid storm clustering might yield a forward propagating MCS producing a swath of severe wind gusts and possibly also excessive precipitation.
Despite many questionable factors which might significantly limit the severe potential, Lvl 2 is introduced due to the combination of threats.
... Northern France towards BENELUX and Denmark ...
In the WAA regime, favourably veered vertical wind profiles will yield moderate to strong DLS and enhanced SREH, that could support well organised multi or supercells. Models agree on the CI and destabilisation especially in the late afternoon hours. CAPE will likely stay below 1000 J/kg, increasing towards east of the highlighted area, where steeper mid-level lapse rates are simulated. One downside of the situation might be that initial storms will move towards the more stable (northern) part off the boundary. Especially the right moving cells could survive longer in the unstable airmass. In case that isolated supercells manage to develop, large hail may occur, as well as severe wind gusts and possibly also tornado as LLS will increase towards the evening.
... Ukraine to Western Russia ...
Even though vertical wind profiles will have weakened considerably compared to the previous forecast periods, scattered DMC is still expected to develop in moderate CAPE and weak to moderate DLS. Stronger multicells may be capable of large hail and excessive precipitation.
... Northern Russia ...
Strong zonal flow will contribute to the favourable DLS for well organised multi or supercells. Large hail and severe wind gusts might occur, as storms develop in moderate instability ahead of the slowly propagating cold front.
