Storm Forecast
Valid: Sat 09 May 2015 06:00 to Sun 10 May 2015 06:00 UTC
Issued: Fri 08 May 2015 21:37
Forecaster: DAFIS

A level 1 was issued for much of Central Europe, such as Germany, Austria, Switzerland, N Italy, Slovenia and Croatia mainly for large hail, isolated tornado and damaging wind gusts.

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

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

SYNOPSIS

High geopotential heights prevail over west and east Europe whilst a low pressure system moves at the base of a major shortwave trough in the southern parts North Sea during the start of the forecast and approaches Norway and Sweden from the SW. Early morning storms are expected in Netherlands and Denmark, dissipating during the day as cold air masses approach from NW. In addition, a mid and upper-level trough exists in Balkans where the diurnal heat will initiate storm development after noon hours. Isolated to scattered pulsating storms are also forecast at several parts of the continent, like south Italy but the rapid clustering and numerous disorganized storm clusters during the late afternoon/evening hours will vanish until midnight. Some multicells though may continue during the night in Adriatic Sea.

DISCUSSION

... Central Europe ...

As the aforementioned vortex is about to slide to the E-NE, mid-level height gradients temporarily increase with a strengthening gradient wind field being the net result. Besides that, some forcing accompanies that trough, which overspreads a wavy W-E aligned surface boundary. BL moisture is best just along the immediate boundary and hence a belt of 500-700 J/kg MLCAPE is forecast to evolve during the day in the highlighted area. DLS up to 20 m/s will be enough for multicell storms to evolve during the afternoon and evening time frame. Shear vectors are aligned nearly parallel to the front, which may result in rapid clustering of storms, but at least during the initiation stage a few large hail and strong wind gust reports are expected. Some high resolution models show an overlap of 500-1000 J/kg MLCAPE, 25 m/s DLS, up to 200 SRH-1 and LCLs at or below 600 m that would support an isolated tornado risk, mainly in Germany. Towards the night, southern parts of Slovenia and Croatia may see convective initiation, with another possibility of MCS travelling south towards Adriatic Sea, where waterspouts are not ruled out. Overnight / Sunday morning convection may propagate storms to come onshore in NW Balkans.

... Greece and Italy ...

Southerly to southwesterly flow advect hot airmass characterised by steep mid-level lapse rates from Ionian Sea towards Greece. Here, a moderately steep lapse rates, along with dew points between 14 and 17°C, locally even higher, will contribute to the marginal to moderate destabilization during the daytime heating. Most of the area should be under a very weak vertical wind shear, with most of the cells staying quasi-stationary. Especially over Peloponnesus area, MUCAPE is forecast to exceed 1000J/kg and with DLS of 15m/s or more marginal hail events may occur and even very isolated downbursts are not ruled out with stronger cells. The GFS model produces lots of convective rain across Greece, during afternoon/evening and night, but the high-res models are more conservative. This is where MLCAPE values in GFS are higher than 1000 J/kg, while it barely exceeds 500 J/kg elsewhere. For Italy the main risk will be large hail events and severe downbursts, even though in NW parts Convective Initiation might have troubles where the best conditions regarding CAPE-shear overlap will exist. MLCAPE is forecast to reach 1200J/kg and mixing ratios will exceed 10g/kg in a wide area, while GFS predicts some tens of millimeters to fall within a few hours. Wind shear should be weak to moderate, perhaps limiting the supercell threat (and very large hail risk) .