Storm Forecast
Valid: Fri 09 Jul 2010 06:00 to Sat 10 Jul 2010 06:00 UTC
Issued: Thu 08 Jul 2010 15:35
Forecaster: TUSCHY

A level 2 was issued for the western Black Sea, E-Moldova, E-Romania and NE-Bulgaria mainly for excessive rainfall.

A level 1 surrounds the level 2 mainly for excessive rainfall.

A level 1 was issued from N-Belarus all the way to the White Sea mainly for excessive rainfall in the southern part and large hail / severe wind gusts for the northern part.

A level 1 was issued for SW/W-France mainly for excessive rainfall.

SYNOPSIS

An unseasonably large cyclonic vortex over the N-Atlantic assists in the advection of hot desert air from SW-Europe to the northeast. Large-scale ridging over central Europe will be the result. A weakening upper trough over SE-Europe still causes unsettled conditions. The same for N-Europe, which is placed beneath active westerlies.

DISCUSSION

... France and Benelux ...

A short-wave over the Bay of Biscay at 06Z fragments into two pieces, one moving rapidly to the NE over N-France and Benelux (e.g. crossing Benelux at 12-15 Z) and the second piece, which stalls somewhere over S-France, as WAA and attendant geopotential height increase overspread and break up that disturbance. Hence, N-France/Benelux may see some weak forcing, whereas S-France experiences decaying thermal print at mid-levels, all more or less supportive for initiation. Descending into the low-levels, any forcing mechanism becomes diffuse to say at least. A weak frontal signature runs from NW France to the E/ENE and may serve for thunderstorm initiation, whereas further to the south, mesoscale moisture pools/convergence zones/decaying outflow boundaries and topography determine, where initiation may occur.

Once again, complaints about the boundary layer in some models has to be expressed with unreal dewpoint jumps despite ongoing diurnal mixing and limited soil moisture content. Ongoing moisture advection from the western Mediterranean my be forced in a confined layer beneath stout capping, which persists over S-France until the morning hours (9th July), so moist layer may be deep enough to prevent diurnal mixing at least over S-France. Dewpoints in the lower twenties are therefore forecast over SW-France, decreasing to the north and east and reaching again lower twenties along the boundary over N-France due to some weak moisture pooling. However we exclude dewpoints in mid twenties (GFS) and hence tweak CAPE magnitudes down to roughly 1500 J/kg MLCAPE.

Shear at all levels is weak to modest with some better anvil layer shear over N-France and Benelux. Storm motion will be slow over S-France, increasing somewhat to the north, where some multicells may occur. However, main storm mode will be rapidly clustering thunderstorms with numerous small clusters affecting some parts of the highlighted area.

Models diverge in respect of QPF forecasts. All models break out some precipitation along the boundary over N-France, taking that convection into Benelux with a weakening trend as CAPE wanes. No real forcing crosses N-France and Benelux (despite the weak piece from the upper wave), so isolated thunderstorms are forecast with marginal hail and strong wind gusts but we neither see enough UVV nor shear to issue a level 1 for now. Over SW/W France, initiation becomes even more questionable with QPF model spread being significant. Thermal trough at mid-levels, northeastward spreading EML, a very moist BL and good insolation may create an environment, supportive for explosive thunderstorm development, however initiation remains uncertain and probably bound to mesoscale features. Nevertheless, given cooling above 800 hPa, cap nearly vanishes until late afternoon, so it is hard to believe that no thunderstorm may form beneath cyclonic upper flow. Any developing thunderstorm will be in an environment favorable for producing torrential rainfall amounts next to isolated large hail. A level 1 was introduced and this could be an high-end level 1 in respect of the rainfall risk. Thunderstorms may last well into the night / morning hours.

... Moldova, E-Romania and NE-Bulgaria ...

A favorable set-up evolves for an high-end convective rainfall risk. Quasi-stationary upper low remains centered over the west-central Black Sea, inducing a moist easterly onshore flow for a prolonged period, especially as cyclonic vortex spins down to lower-levels, inducing a strengthening vortex down to 850 hPa. The main question remains how far west DMC will move. All available model data indicate excessive 12h/24h QPF amounts with values of 80-150 l/12h and 100-250 l/24h just offshore with GFS being the most aggressive one, taking the precipitation well inland. That far out, it remains too uncertain to exclude the eastern parts of Moldova, Romania and Blugaria, so we issued a broad level 2 for excessive rainfall. Thunderstorms keep going until the morning hours (10th July).

... White Sea and surrounding areas ...

Overlap of 500 - 1000 J/kg MLCAPE and 15-20 m/s DLS may support organized multicells/isolated supercells with a large hail and severe wind gust risk. Thunderstorms are bound to the passage of an upper wave, so overall risk gradually moves to the E/NE with a decreasing trend in coverage and intensity after sunset.

The level was expanded well to the south, where non existent shear but abundant CAPE cause slow moving storms with an isolated excessive rainfall risk over far W-Russia. A gradual eastward shift is forecast with ongoing weakening after sunset.