Storm Forecast
Valid: Sat 10 May 2014 06:00 to Sun 11 May 2014 06:00 UTC
Issued: Fri 09 May 2014 20:42
Forecaster: TUSCHY

A level 1 was issued for E-France, Belgium, the Netherlands, Luxembourg, NW Switzerland and parts of Germany mainly for severe wind gusts, an isolated large hail risk and an isolated tornado event.

A level 1 was issued for Estonia and areas to the east mainly for an isolated large hail and tornado event.

A level 1 was issued for parts of the Ukraine mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Ireland/UK mainly for an isolated tornado event.

A level 1 was issued for E-Poland mainly for an isolated large hail risk.

SYNOPSIS

A strong anticyclone over the Azores keeps its strength through most of the forecast, before lowering thickness values mark the begin of the anticyclone's weakening and westward retreating phase during the overnight hours. The Azores' high faces a pronounced vortex north of Ireland, which moves towards the N-North Sea during the night. Both features cause an active frontal zone with intense mid/upper jet streaks (70 m/s at 300 hPa and 45 m/s at 500 hPa). Both jets work their way towards France/Germany with a gradual weakening trend, while strongest winds lag behind and remain offshore over the Atlantic. Downstream ridge shifts east and covers parts of the Balkan States, whereas a long-lived upper trough over SE-Europe drifts slowly to the east towards the far W-Black Sea and CNTRL Turkey.
A sharp upper trough lifts from Poland to the NE. Phasing of the polar front jet and a branch of the subtropical jet cause a broadening and intensifying mid/upper jet over far W-Russia.

Aforementioned cyclonic vortex over Ireland/North Sea also has a pronounced LL vortex, which crosses Ireland/Scotland during the day and enters the N-North Sea during the night. This feature steers warm and cold fronts to the east, at which the cold front will be the focus for enhanced convection. This front is placed over the W-English Channel during the start of the forecast, affects Benelux and E-France during the afternoon hours and crosses Germany during the night. Interaction of that front with the Alps forces the front to slow down with a wavy appearance during the end of the forecast.

DISCUSSION

... E-France, Benelux, the Netherlands and Germany ...

A complex forecast ahead. Placement of the mid/upper jets remains unfavorable as its right exit covers eastbound moving cold front over N-France. Some changes occur from noon onwards, as the upper jets drifts a bit to the south and a coupled upper jet configuration evolves along with the right entrance of another jet streak over Germany. Hence, strong upper divergence occurs over Benelux and E-France just as the cold front moves in from the west.
However, a few negative items remain: Upper divergence shifts rapidly E/NE and leaves the front behind during the afternoon hours. Surface front remains anchored beneath the anticyclonically sheared fringe of the mid/upper jets for most the time. Strongest synoptic forcing is displaced to the north (crosses the S-North Sea from W to E), but it also affects that area around noon. Cold front moves into a diffluent regime over Germany during the night with weakening forcing noted in model forecasts. Despite those inhibiting points, we still think a broad level 1 for a forced convective line is needed.

Setup won't favor widespread DMC, but a forced line of mixed shallow/isolated deep convection probably evolves along that cold front. Most probable CI/enhancement of convection is forecast over NE France with peak activity thereafter between S-Belgium, Luxembourg and CNTRL Germany during the afternoon and evening. Dependent on how deep updrafts evolve, either 0-6 km shear in excess of 30 m/s or 0-3 km shear up to 20 m/s both favor organized convection with embedded supercells. An isolated large hail event can't be ruled out, but the main hazard will be strong to severe wind gusts as 25 m/s winds at 850 hPa mix down to the surface. With bowing line segments forecast, concentrated swaths of severe wind gusts are possible. In addition, LLCAPE of 50-100 J/kg, LL speed shear in excess of 15 m/s and SRH-1 of 150 m^2/s^2 and more point to an augmented tornado risk both with more discrete cells but also with embedded (shallow) supercells/bow echoes. We expanded the level 1 to the Netherlands and NW-Germany, where stronger synoptic forcing exists. However concerns exists with approaching dry slot, which could preclude convection. This is reflected in local models. Still, at least a few organized storms can't be ruled out (e.g. along dry slot/front interface).

Both, diminishing forcing and decaying CAPE support a weakening of that convective line over CNTRL Germany, but we would not be surprised if enhanced convection pushes as far east as E-Germany with an ongoing strong to severe wind gust and isolated tornado threat. The southern part of the front drops south/southeast over Germany and becomes aligned more parallel to the background flow. This sometimes causes a breakup of a solid convective line with ongoing weakening but more discrete cells. Strong LL shear and DLS continue to point to an isolated severe wind gust and tornado risk. A similar risk continues over extreme E-France and NW Switzerland. That's why the level 1 was expanded far south.

... Alps and N-Italy ...

Not much change to yesterday's setup is forecast, although EZ and GFS both show at least isolated CI over N-Italy and the southern part of the Alps. A reason seems to be a somewhat stronger LL cyclonic vortex over extreme NW Italy and therefore stronger upslope flow along the S-Alps. BL dewpoints in the lower tens should support isolated CI with 300-600 J/kg MLCAPE. DLS of 15 m/s could support a temporarily more organized thunderstorm with an isolated large hail threat. This activity diminishes after sunset.

... Estonia and adjacent regions ...

EZ and GFS both agree that a low pressure trough lifts to the NE and won't intensify into a closed LL depression. Still, a small warm sector is forecast to evolve over Estonia towards Saint Petersburg. Final LL shear depends on how strong backing/ageostrophic deflection of the BL wind will be, but latest data keeps the shear on the weak side (e.g. 10 m/s speed shear but SRH-1 increasing to 100-150 m^2/s^2). Overlap of 800 J/kg MLCAPE and 15-20 m/s DLS indicate an isolated large hail risk next to one or two tornadoes. The threat diminishes after sunset due to loss of insolation and CAPE.

... Parts of Belarus, E-Poland, W-Ukraine and Romania ...

Leisurely eastbound moving cold front sparks numerous thunderstorms during peak heating with a rapid decay around sunset. Strongest mid/upper flow remains displaced to the west from CAPE tongue. Despite an isolated large hail risk with a more intense pulating storm, rapid clustering of convection and MLCAPE below 500 J/kg keep the severe risk below a level 1. E-Poland was upgraded however as 25m/s DLS and 300-600 J/kg MLCAPE overlap between 9-12 Z. An isolated large hail event is possible.

... CNTRL/E-Ukraine ...

Moisture advection from the W-Black Sea pushes BL dewspoints in the mid tens with mixing ratios exceeding 9g/kg in a few places. Lapse rates of 7K/km atop that air mass cause 500-1000 J/kg MLCAPE in 15-20 m/s southerly deep layer shear. Initiation awaits a NE ward lifting vorticity lobe and scattered organized thunderstorms with large hail and severe wind gusts are forecast to move E/NE. Although a gradual weakening trend during the night is expected, isolated thunderstorms continue well into the night, fueled by the anomalous moist BL and steep lapse rates.

... Ireland and UK ...

Occlusion wraps around the vertically stacked vortex and undercuts cold mid-levels. Weakly capped 400-800 J/kg SBCAPE evolve due to mid-level lapse rates of 7K/km and aforementioned well mixed marine BL. Scattered thunderstorms evolve with peak activity between noon and sunset. Weak shear at mid/upper levels precludes organized updrafts, so marginal hail and gusty winds will be the main hazard. Attention turns to the lowest 3 km with enhanced LLCAPE and SRH-1 aoa 100 m^2/s^2 especially from parts of N-Ireland to CNTRL UK. A few funnel/short-lived tornado events are forecast. Thunderstorms and severe risk diminishes until midnight.