Storm Forecast
Valid: Mon 27 Jun 2011 06:00 to Tue 28 Jun 2011 06:00 UTC
Issued: Sun 26 Jun 2011 22:34
Forecaster: DAHL

A level two threat has been issued across the N Ukraine for large hail, damaging wind gusts, and tornadoes.

A level one threat has been issued across the SE UK mainly for large hail, damaging wind gusts, and tornadoes.

A level one threat has been issued across N France and the Benelux countries mainly for large hail and damaging wind gusts.

SYNOPSIS

A high-amplitude upper flow pattern exists over Europe, with a longwave trough setting up over the eastern Atlantic. Another long-wave trough is present over eastern Europe and will close off into a cut-off cyclone on Monday. At low levels ... the eastern Atlantic trough supports advection of warm/moist air into western and central parts of Europe, as well as the northward expansion of an EML plume atop this warm/moist air mass.
The SE European trough is accompanied by a rather intense SFC low, but elsewhere rather quiescent synoptic-scale surface conditions prevail.

DISCUSSION

... SE United Kingdom ... N France ... Benelux ...

It seems that some instability will develop in the pre-frontal air mass across the southern British Isles and western continental Europe as the mid-level lapse rates increase in response to the northward spreading EML. The GFS CAPE values are not to be trusted given suspected problems in the surface-flux scheme that overestimates the diurnal latent-heat flux cycle. ECMWF shows that some 500 - 1000 J/kg of CAPE will develop over the SE British Isles and up to 2000 J/kg over France and the Benelux countries, which seems to be more reasonable than what GFS is advertising.

First storms should develop over the SE British Isles amidst the 850 hPa thermal gradient, probably in the afternoon hours, gradually increasing in coverage as the day/evening progresses. The activity should migrate across the Channel region and expand southwards during the night, when the convection will likely become slightly elevated. It seems that upscale growth into one or more elevated MCSs may occur, affecting extreme N France and the Benelux countries early Tuesday morning.

The severe threat with the convection over the British Isles in the afternoon and evening hours should be dominated by well-organized multicells and supercells given DLS of about 20 m/s (increasing from about 15 m/s over the extreme SE parts of the UK to over 25 m/s over the central portions of the UK) and rather strong LLS (exceeding 10 m/s over the extreme SE UK). However ... CAPE should be weakest where shear is strongest, and vice versa. Also, the LLS maximum is expected where DLS is weakest. Despite these somewhat negative factors, it seems that isolated large hail and damaging winds will occur, and perhaps also a brief tornado or two. At this time, a LVL1 threat seems to be sufficient, though an upgrade to LVL2 may be required if the CAPE and shear fields become aligned more favorably, or if CAPE proves to be higher than currently anticipated.

As the storms progress into the Channel and expand across N France, the severe threat should shift to mainly large hail, though the anticipated elevated nature of the storms may limit the overall severe threat somewhat.

The air mass over the northern portions of France should become rather unstable during the afternoon hours, but it seems that the cap will be strong enough to obstruct widespread convective development. Model guidance suggests isolated convection, and this activity would pose some threat for large hail and damaging winds given some 15 m/s deep shear and MLCAPE of order 2000 J/kg. However, the overall threat should be limited as it seems unlikely that storms can be sustained given strong capping and no focused low-level ascent.

... N Ukraine ...

Another focus for severe weather exists over the N Ukraine where weak to moderate instability will coexist with 20 m/s DLS and 10 m/s LLS. Though capping seems to be weak and storms may be numerous, leading to unfavorable interactions, a few supercells are likely, capable of large hail, damaging winds and also tornadoes. In addition, excessive convective rainfall may occur.