Storm Forecast
Valid: Sat 21 Jun 2008 06:00 to Sun 22 Jun 2008 06:00 UTC
Issued: Fri 20 Jun 2008 23:20
Forecaster: DAHL

SYNOPSIS

Deep but strongly positively-tilted upper longwave trough is establishing over the eastern Atlantic, supporting strong warm advection over the western parts of Europe. Associated main low-level baroclinic zone will be stretching from the central UK eastwards across Germany and S Poland into Belarus and NW Russia by Saturday afternoon per latest GFS output. Rather deep polar air will be present underneath an eastward-progressing upper trough over Scandinavia ... while most of S Europe will remain under quiescent synoptic conditions.

DISCUSSION

... France ...

Over France, CAPE should undergo some increase during the day as steep mid-level lapse rates are advected across the region from the Iberian peninsula, and up 1000 to 1500 J/kg MLCAPE may well develop late in the afternoon. GFS simulates 20 m/s DLS over N France in the late afternoon/evening hours, sufficient for well-organized storms ... if initiation occurs.

There should be some large-scale forcing for upward motion owing to widespread LL warm advection and slight DCVA contributions late in the evening/night. However, cap strength should increase as a result of low/mid-level warm advection, so that widespread convective development is not anticipated at the moment. However, model suite does consistently come up with at least weak precip over France/Benelux, though timing and location of convective evolution remains quite uncertain.

The currently supported scenario seems to be that quite isolated storms will form over central/southern France during the afternoon hours when/where shear should be too weak to support a significant severe threat. Late in the evening/night, some possibly elevated - and apparently rather weak - storms may form over western France, presumably tied to low-level WAA. If these storms managed to tap more unstable air from the boundary layer, upscale growth into an MCS and a threat for large hail and damaging wind gusts would exist. However, the current simulations do not support this scenario.

At the moment, a level one threat does not seem to be warranted, although an upgrade may be required if SFC-based storms manage to form in favorably sheared environment over N or W France in the late afternoon/evening hours.

... N Poland ... Baltic States ...

Comparatively shallow convection will likely develop over Scandinavia and the Baltic States with daytime heating. Over the Baltic States and N Poland ... strong forcing and about 20 m/s DLS will be provided by an upper vort max which crosses the area during the period. Expect line segments and isolated mesocyclones with this activity, capable of marginally severe hail/wind. Low LCL heights, little capping and 5 to10 m/s LLS suggest that an isolated tornado or two cannot be discounted, either.

... Iberian Peninsula ...

Very isolated late-afternoon high-based thunderstorms may develop over Iberia. Expect very gusty winds with this activity, which may briefly attain severe levels, given deep/dry sub-cloud layers. However, activity will be too isolated for a categorical risk.
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