Storm Forecast
Valid: Mon 12 May 2014 06:00 to Tue 13 May 2014 06:00 UTC
Issued: Sun 11 May 2014 23:04
Forecaster: PISTOTNIK

A level 1 was issued for parts of Romania, Bulgaria and SE Serbia mainly for large hail and severe wind gusts.

A level 1 was issued for parts of the Netherlands and NW Germany mainly for tornadoes.

SYNOPSIS

A broad long-wave trough covers Western and Central Europe. Shorter waves round its base and translate from East-Central Europe into the Baltics and Belarus and from the Celtic Sea into France. A fairly strong jet stream, often reaching more than 25 m/s at 500 hPa, is present along the Southern and Eastern flank of the long-wave trough from Spain across Italy and the Balkans into Northeastern Europe.
At the surface, the parent cyclone is situated over Southern Sweden, and a long and wavy cold front curls southward across Belarus and the Western Ukraine into Romania and then westward along the Alps into Northern Spain. In its wake, cool maritime air flows into Western and Central Europe, while warm and relatively moist air is present in the Mediterranean region and advected deeply into the Eastern Ukraine and Russia.
Various regions along the frontal boundary become a possible focus for deep convection again on Monday, though the overlap of shear and instability relaxes considerably compared to Sunday. Further convection will develop in the postfrontal air mass.

DISCUSSION

... Belarus, Ukraine, Romania, Bulgaria ...

Ahead of the short-wave trough aloft, a frontal wave is forecast to form over the Western Ukraine and to move into Belarus. Low to moderate CAPE will build in response to diurnal heating in its moist warm sector. Showers and thunderstorms will form from the late morning hours along and ahead of the cold front. Overlap of instability with deep-layer shear around 20 m/s is most robust in Romania and Bulgaria, where strong multicells and a couple of supercells with a primary threat of large hail and severe wind gusts are expected.
Further North, the magnitude and depth of the instability decreases, and its overlap with enhanced shear becomes more fragile. Convection will be more strongly fostered along the cold front or a forerunning convergence line (especially WRF shows such a structure), and rapid clustering is expected. The unfavourable direction of the shear vector parallel to the boundary makes a better organization too unlikely to extend the level 1 northward. Cold pool formation may result in some gusty winds, though, and enhanced low-level shear and veering might result in an isolated tornado near the tip of the wave in Belarus, even though confidence in electrified convection is limited that far North.

... Balkans, N Adriatic, S Alpine region ...

A particularly complex scenario evolves in the wake of the Alps. Sunday's widespread and severe storm activity over Northern Italy, Slovenia and Hungary will push an organized outflow front southward overnight, which will likely nest somewhere across Southern Serbia, Bosnia and the Dinaric Mountains where it becomes exposed to daytime heating again. Moisture enrichment will take place along this boundary, while favourably veering wind profiles may regionally be present to its immediate North. However, the forecast models agree on a rather sluggish buildup of CAPE, which is plausible by the lot of convective debris that will likely linger over the region. Subsidence behind the first short-wave trough in the first half of the day will further act to delay or even suppress the onset of new convection.
Current thinking is that only a few showers and thunderstorms will form in this area, preferably near the remnants of the boundary or over orographic features of the Balkans. Further West, a renewed shallow cyclogenesis over Northern Italy will lead to a return of moister and slightly warmer air from the Adriatic region to the Southern Alps, where confidence in convective initiation is highest over Southeastern Austria.
The delicate issue is the quite strong deep-layer shear with values between 20 and 35 m/s, increasing from North to South. Any storm that manages to grow deep and persistent enough to consume this shear may turn quite well-organized and would pose a primary threat of large hail. However, all in all this scenario seems too conditional for a level 1 at the moment.
Forcing for upward motion will increase in the evening and night hours ahead of the second short-wave trough, so isolated storms may initiate all night long. Their focus will likely shift to the Northern Adriatic Sea and Slovenia overnight.

... W and Central Europe, S Scandinavia ...

Widespread afternoon showers and scattered thunderstorms will form as soon as the postfrontal air becomes exposed to daytime heating. Low cloud bases and strong low-level buoyancy may result in a few funnel clouds and isolated spout-type tornadoes.
The highest coverage of convection is expected over the Netherlands and NW Germany, where a surface trough will slowly move southeastward, intensified by frictional convergence along the coast. Enhanced low-level shear (>10 m/s) and favourable veering are expected ahead of this line, and a low-end level 1 is issued for a slightly enhanced threat of brief tornadoes.
Near the center of the parent low-pressure system over Southern Sweden, slow-moving thundery showers may result in localized high rainfall accumulations. However, due to the limited moisture content of the air mass, a level 1 does not seem to be necessary here.