Storm Forecast
Valid: Tue 27 Jun 2017 06:00 to Wed 28 Jun 2017 06:00 UTC
Issued: Mon 26 Jun 2017 21:52
Forecaster: DAFIS
A level 2 was issued for NE Spain and France mainly for severe wind gusts, excessive rainfall and large hail.
A level 1 was issued for N Spain, France, Switzerland, Germany, Austria, S Belgium, Luxemburg and Czech Republic mainly for sever wind gusts, large hail and excessive precipitation.
A level 1 was issued for N Italy, Slovenia, Croatia and Bosnia-Herzegovina mainly for sever wind gusts and excessive precipitation.
A level 1 was issued for E Ukraine and SW Russia mainly for excessive precipitation and severe wind gusts.
SYNOPSIS
A rather complex atmospheric set-up is forecast over Europe on Tuesday, with several mid-level waves and one long-wave trough approaching from the Atlantic Ocean. The positively tilted trough which will cross the Iberian Peninsula tomorrow will provide widespread lift to warm and moist air masses. At the same time, strong vertical wind shear will increase the probability of severe thunderstorms in West and Central Europe. The area with the best overlap of steep lapse rates, moist LL air masses and strong DLS is highlighted with a level 2. A large low pressure system is found over Scandinavia and its cold front hardly moves SE, creating an elongated boundary where thunderstorms will remain almost stationary. Another low-pressure system will develop over France where we expect numerous severe storms in the afternoon and during the night of Tuesday and will be incorporated with the larger cyclone over the British Isles.
DISCUSSION
.... N Spain and France .....
As the sharp long-wave trough approaches from the Atlantic Ocean it will provide lift to a plume of very unstable air mass in W Europe. Latest model outputs convergence to an outbreak of severe thunderstorms in N Spain and France. Steep mid-level lapse rates and mixing ratios up to 12 g/kg will create MLCAPE of 1.5 2 kJ/kg. Winds will increase by the afternoon to 20 m/s, and DLS is calculated to exceed 20 m/s, thus storms should organize in MCSs and supercells, able to produce all kinds of severe weather. Mesoscale models suggest the development of a linear MCS in S France, travelling fast NE, as denoted by the straight-line forecast hodographs. Severe wind gusts and large hail will be the main threats. This system will be also able to produce excessive precipitation as the LL flow ahead of the cold front, in the warm sector, will be parallel to the mean flow, and training convection will increase the flash flood threat. Bow-shaped segments may also increase the probability of tornadoes, but so far models do not show very promising ingredients for tornadogenesis.
In NE Spain models forecast a strong low-level convergence zone, along with the passage of the mid-level vorticity lobe in the afternoon, where very moist air masses overlap with very steep-lapse rates. The result may be more than 2000 J/kg of MLCAPE, weakly capped, where DLS will be no less than 25 m/s. One or two supercells are well expected, able to produce severe to damaging wind gusts and large to very large hail.
.... N Spain, France, Switzerland, Germany, Austria, S Belgium, Luxemburg and Czech Republic .....
Diurnally driven convection and local convergence zones will develop storms in the afternoon hours that will be able to produce locally severe wind gusts and large hail. Global models calculate 1-2 kJ/kg MLCAPE after 15z with 10-15 m/s DLS over a large part of the level 1 area. Storms will be able to cluster in MCSs where high delta-thetae values suggest strong downbursts as the main threat. Within these storms, a heavy rain causing local excessive precipitation and flash floods cannot be ruled out, especially given rather weak storm motion and PW exceeding 30 mm. Storm activity will continue during the night of Tuesday, but the only threat is expected to be excessive precipitation in parts of France and Germany.
.... N Italy, Slovenia, Croatia and Bosnia-Herzegovina ....
Elevated convection from the previous night will become surface-based by 12z in N Italy. New discrete storm cells should appear on the Alps and due to weak mid-level flow, excessive precipitation events may verify in NE Italy and W Austria. Later in the afternoon, DLS increases and storms may cluster in MCS propagating to the east. At the leading edge of a warm front that reaches Czech Republic by noon, enhanced low level flow will increase the threat of excessive precipitation mostly on elevated areas. Rather steep mid-level lapse rates create up to 1 kJ/kg MLCAPE and large hail events are possible.
.... E Ukraine and SW Russia ....
The LL convergence zone will continue to provide lift to unstable air masses during Tuesday and the early morning of Wednesday. MLCAPE is forecast to exceed 2 kJ/kg and most storms should remain discrete, but an overlap of high CAPE and 15 m/s suggest that 1-2 MCS may form. Given the weak storm motion and high PW, excessive precipitation events may lead to flash floods inside the level 1 area. Delta-thetae values also cannot rule out wet microbursts.
