Storm Forecast
Valid: Tue 07 Jul 2009 06:00 to Wed 08 Jul 2009 06:00 UTC
Issued: Tue 07 Jul 2009 07:15
Forecaster: VAN DER VELDE
A level 2 was issued for N Italy, S Austria, Slovenia, Croatia, W Hungary mainly for excessive precipitation and large hail.
A level 1 was issued for E Spain mainly for large hail.
A level 1 was issued for Poland mainly for large hail.
A level 1 was issued for SE UK for spouts and excessive local precipitation.
A level 1 was issued for the SE North Sea area for spouts.
SYNOPSIS
With a depression over the North Sea, a colder unstable airmass enters western Europe. The cold front is not sharply defined, but a gradient in theta-w and boundary layer moisture will pass over Germany during the afternoon. The most significant instability is situated south of the Alps, the Balkan and along the east coast of Spain. The jet circles the cooler unstable airmass over France and creates moderate deep layer shear conditions over northern Italy to Slovenia. A shortwave trough comes through the Italian/NW Balkan level area during the evening and night hours, while shear increases at the same time.
DISCUSSION
...N Italy, S Austria, Slovenia, Croatia, W Hungary...
Significant CAPE >1000 J/kg has built up and shear improves through the period, first over northwestern Italy (moderate values, around 15-20 m/s, SREH up to 200 mē/sē) then later eastward and with stronger low level shear. The conditions are initially favorable for supercell storms with large hail and isolated severe gusts (moderate delta-theta-e and DCAPE), but storms will likely organize into a line parallel to the Alps and move eastward along it. Low LCL heights and weakened lapse rates then support an episode of intense precipitation of long duration, especially when the shortwave moves through, likely with a large MCS. Motion vectors decrease during the night.
Given the stronger low level shear at night, a tornado is not ruled out.
...eastern Spain...
While subsidence settles over the region, GFS is still supporting instability and strong shear (>20 m/s). Supercells may occur with large hail and/or severe gusts.
...Poland...
Here the combination of moderate deep layer shear (15 m/s) and moderate instability (1000 J/kg) should be sufficient for a number of large hail events. An isolated gust may also occur.
During the night, remaining instability could be released at the passage of the cold front, probably embedded in a larger stratiform precipitation due to large scale ascending airmass. Precipitable water content is quite large, and motion vectors small, so an excessive precipitation event is possible.
...southeastern UK...
Very slow storm motion and low LCL heights favor local flash floods, and significant 0-3 km CAPE with weak winds and convergence lines are likely to yield a number of spouts and funnels.
Spouts can also be expected over the southeastern North Sea through most of the period, but most likely seems Wednesday morning when pressure gradients weaken more.
