Storm Forecast

Storm Forecast
Valid: Mon 21 Apr 2014 07:00 to Tue 22 Apr 2014 06:00 UTC
Issued: Mon 21 Apr 2014 07:20
Forecaster: SCHLENCZEK

A level 1 was issued for NW Germany and the Netherlands mainly for excessive rainfall and large hail and to a lesser extent for tornadoes.

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

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

A level 1 was issued for the eastern Adriatic region (incl. Croatia and Bosnia) mainly for large hail, severe wind gusts and excessive rainfall.

SYNOPSIS

An upper cut-off low over eastern Germany / Czech Republic moves westwards, merging with another cut-off low southwest of the British Isles. In the vicinity of these low pressure systems, a zone with 500 hPa temperatures at or below -20C is found from the E Atlantic via W / C Europe towards the Black Sea. The southern part of this area is delimited by a WSWerly jet streak, stretching from Morocco via S Italy towards the Middle East. The best chances for deep moist convection are found ahead of the cut-off low over Germany towards Benelux, and in the intersection of the cold upper airmass and the jet streak from the N Maghreb States via Iberia and the SW Mediterranean towards Italy and the Adriatic coast. Except for the very southern part of the SW Mediterranean, CIN should not be too much for convective initiation. The greatest chance for organized severe convection should exist along the SE boundary of the upper low, which is S / SE Iberia via Balearic Islands and Corsica / Sardinia towards Italy and the Balkans.

DISCUSSION

...NW Germany and Netherlands...

Advection of a humid airmass with mixing ratios of order 6 - 7 g/kg from east to west increases the amount of MLCAPE to 300 - 500 J/kg with very little CIN. Another point is the quite strong background flow with 10 - 15 m/s at 850 hPa. This could lead to multicellular storms with marginally severe hail and a threat of local flooding. Convection is expected to initiate in the late morning hours and will likely persist until the late evening. Due to low wind speeds near the ground and the possibility that some narrow surface convergence regions may form, an isolated funnel / weak tornado is not ruled out either.

...NE Spain and Balearic Islands...

An airmass with some hundred J/kg of MLCAPE is found ahead of an upper trough which will slowly move eastwards. In this environment, some 15 - 20 m/s of 0-6 km bulk shear and locally enhanced LL shear are present which increases the chance that an organized severe storm with large hail and marginally severe wind gusts may form. Due to the locally augmented LL shear, there is also a slight chance of a tornado. Convection is expected to start in the morning hours and will likely persist until the evening.

...Italy...

An area of some 200 - 500 J/kg MLCAPE overlaps with strong 15 - 20 m/s deep layer shear and convection which occurs in this part of Italy will likely turn into multicells or a few supercells. The main threat should be large hail and severe wind gusts but an isolated tornado is not ruled out either (due to locally > 10 m/s LL shear). Maximum activity should be expected in the early afternoon hours but convection will likely go on until the late evening.

...E Adriatic coast...

A local maximum of instability, shear and SRH is found along the coastline of the Adriatic, predominantly over Croatia and Bosnia. Storms which initiate in this region are likely to become severe with large hail and severe wind gusts but due to orography, there is also a risk of flash floods. It is likely that convection starts in the early morning and persists until Tuesday.

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