Storm Forecast
Valid: Fri 27 Jul 2012 06:00 to Sat 28 Jul 2012 06:00 UTC
Issued: Thu 26 Jul 2012 21:59
Forecaster: TUSCHY
A level 1 was issued for N-Spain, France, Benelux and NW Italy mainly for large hail (a very large hail event is possible), strong to severe wind gusts and heavy rain.
A level 1 was issued for parts of Germany mainly for strong to severe wind gusts, an isolated large hail event and heavy rain.
A level 1 was issued for E-Poland, W-Romania/Bulgaria, Serbia, parts of Hungary and Slovakia mainly for excessive rainfall, a few funnel/tornado events, isolated large hail and strong wind gusts.
SYNOPSIS
A deep trough is placed over Scotland and moves a bit to the south. A weakening upper level low over E-Europe moves slowly to the east but still assists in an area with deep convergent flow over Poland/Slovakia and Hungary.
Ridging over N-Africa builds to the north and enhances the geopotential height gradient over France and also the mid/upper level wind field mainly over the W-Mediterranean.
Cool mid-levels and abundant BL moisture are present over most parts of Europe and hence an active convective day is anticipated.
DISCUSSION
... France, Benelux, Germany and NW Italy ...
Isolatd to scattered convective initiation (CI) is forecast during the day with scattered to widespread coverage after sunset. Despite lowering mid-level geopotential heights, strong thermal ridge is still placed over most of France/Germany. However, strong diabatic heating is likely, so convective temperatures will exceed the required threshold during the day. At upper levels, weak waves traverse the area from SW to NE (becoming stronger betimes due to stronger cyclonic curvature of the flow) and numerous PV streamers also affect those areas, so CI can occur everywhere. The most likely CI during the daytime hours probably exists along the fringe of the thermal ridge, namely NW/N France and Benelux into NW Germany. Further to the south over C/S-France and S-Germany, CI will be bound to topography (at least until sunset).
During the evening hours onwards, a pronounced short wave crosses France and parts of Germany and provides adequate lift for numerous overnight thunderstorm clusters, which then also affect C-France/Germany.
Shear will be on the moderate side with DLS in the order of 10-15 m/s, increasing to 20 m/s over far N-France and Benelux and far S-France. Shear in the lowest 3 km remains marginal during the day but increases to 15 m/s as the short wave approaches during the night. A rather pronounced MLCAPE axis of 1500 J/kg evolves from W-France all the way towards Benelux. This amount of CAPE and aforementioned shear magnitude assist in organized multicell thunderstorms and isolated supercells during the day, the latter risk mainly over Benelux and far NW Germany, where the passage of a weak 'wave' increases SRH. Large hail, heavy rain and a few strong downburst events are forecast although we do not want to rule out an isolated very large hail event with any longer-lived storm. Thunderstorms may already grow upscale into a first MCS event mainly over Benelux during the afternoon hours. In that case, a concentrated swath of strong to severe wind gusts would be a distinct possibility. However, weak forcing and marginal wind field beneath 3 km next to mixed model signals preclude a level 2 upgrade for now. As a side note, in the 18Z model runs, there are more signs of an environment, conducive for enhanced supercell probabilities over Benelux (stronger 3km shear, enhanced SRH-3). If this trend continues, an upgrade may be needed tomorrow during the day.
Isolated storms over Germany will be slow moving with good mid-tropospheric moisture present, so heavy rain accompanies that activity, next to isolated large hail.
During the evening hours and the night, numerous storm clusters (probably organized into numerous MCS events) bring strong to severe wind gusts, isolated large hail and heavy rain to France/parts of Germany and Benelux. Again, weakening CAPE and moderate wind field next to the magnitude of the expected forcing do not inidcate any area with a risk of a concentrated swath of severe wind gusts for now and hence a broad level 1 remains in place.
We expanded the level 1 towards NW Italy and parts of Switzerland as a few severe mountain storms are expected with an overlap of 20 m/s DLS and up to 1kJ/kg MLCAPE. Large hail, strong wind gusts and heavy rain will be the main hazard, also affecting SW/S-Switzerland before gradually weakening during the night.
... N-Spain and SW/S-France ...
Placed ahead of a gradually approaching deep trough, another round of partly daytime driven, partly forced DMC is forecast. Models diverge in coverage of thunderstorms during the night, but most models show scattered CI over N-Portugal and N-Spain. This activity might evolve into a storm cluster during the evening hours onwards bevore moving to the NE into SW/S France. 15-20 m/s DLS and 500-1000 J/kg MLCAPE overlap, so well organized multicells/isolated supercells are forecast during the initiation stage and a couple of hours thereafter (before clustering). Large to very large hail and a few strong to severe downbursts accompany those storms. Shear only increases towards the Mediterranean, but CAPE magnitude decreases at the same time. Also QPF signals are pretty marginal over NE Spain. However we can't rule out an isolated storm with similar risks and hence we expanded the level 1 far to the east.
A cluster of storms affects SW France during the night with heavy rain and strong wind gusts being the main hazard.
... E-Poland, Slovakia, W-Romania/Bulgaria and Serbia ...
Meridionally elongated and quasi-stationary convergence zone remains the focus for scattered thunderstorm activity. Rich BL and moderate mid-level lapse rates result in a concentrated swath of 1000 - 1500 J/kg MLCAPE. Weak shear preclude well organized storm structures, so pulsating storms with isolated large hail, strong wind gusts and excessive rain are forecast. Despite lower LL CAPE values, a few funnel/spout-type tornadoes are again possible.
... Scotland ...
Placed beneath the cold-core low, some LL CAPE/vorticity may assist in a few cold-core funnel/short tornado events mainly along the NW/N coast of Scotland. Due to the limited thunderstorm activity and low-end chance for severe events, no level 1 was added.
