Storm Forecast
Valid: Sun 22 May 2016 06:00 to Mon 23 May 2016 06:00 UTC
Issued: Sat 21 May 2016 22:44
Forecaster: TUSCHY

A level 1 was issued for NE Spain mainly for large hail and severe wind gusts.

A level 1 was issued for W-Turkey mainly for large hail.

A level 1 was issued for NW Switzerland and parts of Germany mainly for an isolated large hail and severe wind gust risk.
In addition, a level 1 was issued over NW Germany for an isolated tornado event beside the wind and hail risk.

SYNOPSIS

Amplified pattern remains in store over Europe. Deep trough over W-Europe acquires a neutral tilt during the forecast and transformation into a cut-off over the Alps will be in progress. Downstream ridge axis extends from the Adriatic to the Baltic Sea with another trough over E-Europe showing no significant shift to the east.

At lower levels, an extensive frontal boundary bisects Europe and reaches from Norway to Germany to the W-Mediterranean and arches back towards S-Portugal. Uncertainties exist where surface cyclones evolve along that boundary, but lee cyclogenesis over NW Italy during the night is a reasonable forecast. Another vortex seems to evolve over Denmark with a potential third and very weak one over SE Germany. This scenario would result in faster frontal motion over S-Germany and the W-Mediterranean.
A diffuse/weak depression over Turkey gradually exits the forecast area to the east.

DISCUSSION

The air mass ahead of that frontal boundary features only modest BL moisture. Combined with no substanial EML atop, CAPE will be an ingredient which limits overall severe risk during this forecast. Nevertheless, a few regions could see organized DMC on an isolated to scattered scale.

... NE Spain ...

Latest synop data reveals surface dewpoints in the lower tens and no substantial moisture advection is forecast ahead of the approaching front. However, strong deep-layer lift with the progressive trough and modest mid-level lapse rates should support 400-800 J/kg MLCAPE along/S of the Pyrenees. CI will occur before noon and convection will move to the east before leaving NE Spain around sunset. 25 m/s DLS assists in a few rotating updrafts and isolated large hail next to strong/isolated severe wind gusts are possible.

... Switzerland and Germany ...

Large model spread in mesoscale pattern continues especially for the Alps. Most models agree in foehn conditions along the W-/N-Alps, which also impacts far S-Germany. Interesting to see some models offering an increase of surface dewpoints by 5-10 K as surface winds back from S to E just N of the Alps. Not sure where significant BL moisture increase in some models comes from, but with saturated soils and good insolation, at least a modest increase of BL moisture could occur during the day. In addition, moisture pooling along numerous wind shift zones may add another focus for better BL moisture. Despite better moisture, warm temperatures at the top of the BL keep air mass mostly capped.

Looking up towards the mid-levels, only weak forcing can be expected despite the passage of subtle short-waves. The main QG forcing arrives during the night as the big trough approaches.

Hence, confidence in isolated CI exists mainly along the W-fringe of the cap during the afternoon- e.g. over NW Switzerland. Forecast soundings show mid-level CAPE of 800 J/kg, combined with decent DLS (20 m/s), so the main risk will be large hail. A well mixed/deep and dry subcloud layer also supports a few strong/ isolated severe downburst events with strongest storms. This convection moves NE during the evening and overnight hours, finally affecting SW/CNTRL Germany during the night. Weakening DLS and low CAPE should lower the severe risk betimes.
Initiation further east over far SE-Germany seems unlikely right now.

Further north, models diverge with CI either along the cold front (W/NW Germany) or the prefrontal convergence zone (CNTRL Germany) or both. Overall, DLS decreases during the day as mid/upper geopotential heights start to increase in response to a westward building ridge. Daytime forcing will occur either due to passing weak short-waves or the front/convergence zone, whereas during the night, only the prefrontal convergence zone should play a role in CI.
Combining those signals results in two regions with enhanced thunderstorm probabilities:

One over W/NW Germany along eastward pushing cold front. Forecast soundings show thin/elongated CAPE profiles with a moist BL and enhanced 1-4 km flow. Transient bowing lines with strong/severe wind gusts and marginal/isolated large hail will be the main hazard, whereat the hail risk increases a bit towards NW Germany. With LCLs below 1 km and some backing BL wind ahead of the cold front, an isolated tornado event will be possible. This activity vanishes after sunset.

A second focus will be CI over CNTRL Germany.Thicker mid-level CAPE profiles and 15-20 m/s DLS should support a few well organized multicells with large hail and strong to severe wind gusts (the latter risk due to a deep and dry subcloud layer). Upscale growth into a temporarily better organized line is possible, which could affect the W-Baltic Sea during the night. This line would be cold pool driven as no synoptic support is expected.

A broad level 1 was issued to include ongoing uncertainties but to also reflect numerous foci for enhanced probabilities for organized DMC.

... France, UK, Ireland and Scotland ...

Scattered to widespread diurnal driven DMC occurs beneath the base of the upper trough. 400-800 J/kg SBCAPE and 10-15 m/s DLS (especially along the periphery of the trough) support strong thunderstorms with marginal hail and gusty winds. An isolated large hail event is possible and also an isolated tornado/funnel event due to augmented LLCAPE. A general decrease in coverage and strength of thunderstorm activity is forecast during the night although isolated thunderstorms may occur over coastal or offshore areas.

... W-Turkey ...

A similar risk (compared to W/NW Europe) exists over W-Turkey. 15 m/s DLS and local MLCAPE near 1000 J/kg may result in a more pronounced large hail risk and a level 1 was added. Thunderstorms decay after sunset.

... Far W-Russia ...

Scattered thunderstorms evolve around noon, mature during the afternoon hours and decay until midnight. DLS of less than 10 m/s but 500-800 J/kg MLCAPE assists in pulsating storms with isolated large hail and gusty winds.