Storm Forecast
Valid: Mon 26 May 2014 06:00 to Tue 27 May 2014 06:00 UTC
Issued: Sun 25 May 2014 21:49
Forecaster: TUSCHY

A level 2 was issued for Belgium, Luxembourg, parts of the Netherlands, parts of Germany and parts of N-Austria mainly for excessive rain.

A level 2 was issued for parts of the Balkan States mainly for excessive rain.

A level 1 surrounds both level 2 areas mainly for heavy to excessive rain, (very) large hail and strong to isolated severe wind gusts. An isolated tornado threat exists for Benelux and W-Germany.

A level 2 was issued for NE Algeria and N-Tunisia mainly for very large hail and severe wind gusts.

A level 1 surrounds the level 2 for a lesser hail risk.

A level 1 was issued for NE Spain and offshore areas mainly for large hail and an isolated tornado risk.

A level 1 was issued for parts of France, Switzerland and NW Italy mainly for excessive rain and an isolated large hail risk.

SYNOPSIS

The forecast starts with a stationary trough over Ireland/Bay of Biscay and an extensive high over E-Europe. Phasing of the polar and subtropical jets cause the southern part of the trough to accelerate east and a NW-SE aligned channel of low geopotential heights develops from France to N-Italy. At the same time, the E-European high weakens, as another trough over Scandinavia moves south.

In the lower troposphere, strengthening convergence marks the onset of a long-lasting surface low pressure channel, which runs from Belgium all the way to Romania.

DISCUSSION

... SE-UK, Belgium, Luxembourg, parts of the Netherlands and parts of Germany ...

EZ, GFS, Euro4 and UKMO all offer a rather tight model solution regarding the placement of a pronounced low-tropospheric convergence zone. It should run from Belgium to CNTRL and SE Germany. WRF (especially with lower resolution) takes the convergence zone a bit more to the north.

Persistent convergence assists in moisture pooling and 850 hPa dewpoints of 8-10 C and BL mixed-layer mixing ratios in excess of 10 g/kg. Effective PWs exceed 20-25 mm during the evening and 25-30 mm during the overnight hours.
At least some modest deep layer shear exists over W-Germany and Benelux, where 0-3/6 km bulk shear of 10-15 m/s is forecast. BL shear strengthens during the late afternoon and evening hours with 10-15 m/s LL shear and 100-200 m^2/s^2 SRH-1 forecast. Shear drops to less than 5-10 m/s to the SE with Corfidi vectors of 5-10 kt or less (also fostered by local forecast soundings).
EZ and GFS both show similar MLCAPE peaks along the convergence zone with 500-1000 J/kg.

Not much capping is present, so first storms probably evolve before noon (or continue from the night). Steady increase of thunderstorm coverage and intensity is expected thereafter and numerous clusters move along that convergence zone from SE to NW. Initial storms pose a large hail risk beside strong wind gusts. Rapid upscale growth of ongoing storms is forecast beneath upper divergence and increasing synoptic forcing. Numerous clusters move along that convergence zone from SE to NW. Excessive rain is a serious hazard with aforementioned parameters and a few places with either training or repeatedly crossing thunderstorm clusters could see rainfall amounts in excess of 50-80 mm/24h. This risk continues all night long!

A limited spatiotemporal overlap exists for a few tornado events over Benelux and W-Germany during the afternoon and evening hours with increasing BL shear and still uncapped/weakly capped conditions.

Overnight clusters also affect SE UK. Decreasing CAPE limits the convection's organization and severity so only marginal hail and gusty winds are forecast. The main hazard remains the heavy to excessive rainfall threat.

... Hungary, Croatia to N-Greece and parts of S-France and S-Switzerland ...

A similar setup to the one described above evolves over parts of the Balkan States. Weak DLS and roughly 1000 J/kg MLCAPE with effective PWs of 20-25 mm and BL mixing ratios in excess of 10 g/kg point to another area with enhanced excessive rainfall probabilities. The main difference to Germany and Benelux is weaker low-tropospheric convergence. Nevertheless, expect early and widespread CI and numerous slow moving storm clusters bring excessive rain to an area, which still suffers from previous flooding. Initial storms also bring large hail, as forecast soundings show fat mid-level CAPE profiles. This risk continues all night long with a decreasing hail risk.

A region with enhanced convergence exists from S-CNTRL France to S-Switzerland and NW-Italy. Training convection could produce excessive rainfall amounts with flash flood problems. Lower CAPE and PWs kept this below a level 2 threshold although we would not be surprised about a few very high rainfall and numerous flash flood reports.

... E/NE Europe...

During the daytime hours, ongoing high pressure with 850 hPa temperatures of 14-16 °C should induce enough capping to suppress convection. An isolated event along mountains however can't be ruled out. Another area with afternoon storms will be far S-Ukraine due to onshore moving sea-breeze fronts. A weakening cap and constantly increasing BL moisture (due to advection and evapotranspiration) result in increasing thunderstorm chances during the evening hours and during the night. DLS below 10 m/s points to pulsating and rapidly clustering storms. MLCAPE of 800-1000 J/kg however assists in numerous severe storms with large hail and strong to severe downburst (high base) events. A few very large hail events are possible. Clustering storms also bring heavy to excessive rain. Complex storm-storm/storm-outflow interaction could locally induce training convection with enhanced flash flooding. No clear focus for level 2 probabilities exists for now and hence a broad level 1 was issued.

... NE Spain ...

The upper trough affects the area during the daytime hours. Surface dewpoints in the mid-tens beneath rapidly cooling mid-levels cause widespread 400-800 J/kg MLCAPE. Strongest synoptic forcing affects the area between 06-15 Z and that's the time frame with the highest chance for organized convection.
Ongoing storms from the night build east and a cluster of storms covers NE Spain before noon. 15-20 m/s DLS and 7.5 K/kg mid-layer lapse rates show a large hail threat with strongest storms. An isolated tornado risk exists along the coast, especially when a closed surface depression just offshore verifies. The thunderstorm risk abates after sunset.

... NE Algeria and N Tunisia ...

Scattered thunderstorms erupt during the afternoon hours. Up to 800 J/kg MLCAPE and 25-30 m/s DLS cause rapid storm organization with supercells forecast. Large to very large hail will be the main hazard and an isolated extreme hail event can't be ruled out. Severe downbursts accompany high-based storms. Thunderstorms move offshore with rapid weakening anticipated due to a strengthening cap.