Storm Forecast
Valid: Fri 25 Apr 2014 06:00 to Sat 26 Apr 2014 06:00 UTC
Issued: Thu 24 Apr 2014 22:07
Forecaster: TUSCHY

A level 1 was issued for Belgium, the Netherlands and NW Germany mainly for an isolated large hail event. A local excessive rain event is forecast.

A level 1 was issued for Hungary, SE Austria, Slovenia and Croatia mainly for excessive rain.

A level 1 was issued for NW-Turkey mainly for large hail and a few severe downburst events.

A level 1 was issued for parts of the W-Mediterranean mainly for excessive rain.

SYNOPSIS

Split flow / blocking pattern continues over Europe. A belt of strong westerlies extends from the Iberian Peninsula to North Africa to Turkey. Numerous impulses are embedded in this flow and affect the Mediterranean. The strongest feature will be a trough, which chokes itself off over the W/CNTRL Mediterranean and drifts slowly to the east. Central Europe still resides beneath weak mid-level geopotential height gradients and another round of widespread thunderstorm development is forecast.

DISCUSSION

... France to the Alps to the W-Balkan States ...

Once again, diurnal driven thunderstorms evolve. Weak shear and widespread initiation favors rapid upscale growth into numerous and disorganized clusters with erratic storm motions. Main risk during the initiation stage will be isolated large hail, but this hazard switches more to excessive rain.

Ingredients favor a somewhat higher hail threat over Benelux and NW Germany, where steeper mid-layer lapse rates push MLCAPE to more than 800 J/kg. Weak shear however keeps the hail risk in check. Still a marginal level 1 was issued.

A plume of effective PWs in excess of 20 mm, BL mixing ratios in excess of 8 g/kg and storm motion vectors of less than 10 kt overlap over Hungary, SE-Austria, Slovenia and Croatia. Excessive rain with flash flooding could become a distinct possibility with slow moving/quasi-stationary clusters.

Storms gradually subside until midnight.

... SE-UK and the SW North Sea ...

Complex forecast in store, as area of interest resides between a SE-ward moving upper low and a rapidly strengthening depression SW of Ireland. Strongest synoptic lift is forecast before noon. That's the time, where some models push the relicts of dying clusters from the previous night towards SE-UK. Rain with embedded thunderstorms is forecast. As synoptic lift exits to the N around noon, some clearing can take place over SE-UK, where BL dewpoints exceed 10 °C beneath 7 K/km mid-layer lapse rates. This should assist in MLCAPE of 500-800 J/kg.

Initiation probably awaits a northward moving surface trough, which pushes a weak front to the NE towards Belgium and the E-UK. CI is forecast along a line Le Havre - Reims (France) and to the north around noon. 10-15 m/s DLS may support an isolated large hail threat over SE UK with more discrete storms. Storms tend to grow upscale betimes and a cluster of thunderstorms spreads to the N/NW. Strong wind gusts and heavy rain will be the main hazard. After sunset, the activity becomes more elevated with a decreasing severe risk over the W-North Sea and CNTRL-UK.

... W-Turkey ...

Overlap of 15 m/s deep layer shear and 400-800 J/kg MLCAPE next to an eastwad moving mid-level impulse cause scattered organized multicell storms. Large hail and an isolated severe downburst event will be possible. This activity decays after sunset.

... W-Mediterranean ...

Cold mid-levels of the upper low overspread the warm Mediterranean and cause up to 800 J/kg weakly capped CAPE build-up. The left exit region of a 30 m/s mid-level jet approaches the CAPE plume and strong upper divergence also covers the area of interest. From the previous day ongoing thunderstorms over the Balearic Island continue to spread east and a cluster of showers/thunderstorms approaches Sardegna during the afternoon and evening hours. Slow moving storms cause excessive rainfall amounts with local flash flooding problems. An isolated waterspout event will be possible, especially beneath the depression's core, where background shear relaxes. During the night, lessening CAPE assists in a subsiding thunderstorm activity. Still, a broad area with heavy rain and embedded thunderstorms affects Sardegna and supports an ongoing heavy rainfall risk.