CONVECTIVE FORECAST
VALID 06Z SAT 21/05 - 06Z SUN 22/05 2005
ISSUED: 20/05 20:02Z
FORECASTER: DAHL
There is a slight risk of severe thunderstorms forecast across France ... W Switzerland ... Germany.
General thunderstorms are forecast across the British Isles ... SW and central Europe.
General thunderstorms are forecast across E and SE Europe and portions of W Russia.
SYNOPSIS
High-amplitude upper flow pattern persisting across Europe ... with a large-scale ... slowly E/SEWD spreading upper trough over the E Atlantic/SW Europe ... and another long-wave trough ... developing into a cut-off low ... over the Aegean region. Between these features ... upper ridging is spreading across the W/central Mediterranean and E-central Europe. At the SFC ... extensive low is present over the E Atlantic ... with main cold frontal boundary curving from the North Sea across NW France into N Spain on Friday evening. Wave cyclones are expected to travel NEWD along this boundary ... with the lead wave expected over NRN Germany by the beginning of the period ... and another wave ... progged to develop over W Germany late in the FCST period ahead of peripheral vort max.
DISCUSSION
...Germany ... France...
Main problem with the current setup seems to be rather meager/shallow moisture ... SFC dewpoints in the pre-frontal air mass across central Europe will likely be in the 10 to 14°C range after BL mixing ... initially possibly being somewhat higher due to overnight's precip. Though lapse rated may steepen some as mixed boundary layer from Iberia is spreading across central Europe. However ... this EML should be rather shallow ... and MLCAPEs are expected to remain fairly weak ... possibly not exceeding 1000 J/kg.
Weak MCS/convective debris associated with the first wave cyclone may be present over NRN Germany at the beginning of the period. It appears that the main front will extend from N-central Germany straight SWWD across France into N Spain by Saturday 12Z ... with mesoscale wind-shift lines present ahead of the front. These ... and the convergence associated with the cold front should be the main foci for convective initiation during the day. Uncertainty exists on the extent of cloudiness/stratiform rain along and ahead of the boundary ... but confidence exists that insolation will at least locally be sufficient to destabilize the air mass. Also ... GFS advertises DAVA overspreading Germany in the wake of the wave cyclone ... which may help to clear the sky by the early afternoon hours.
Current thinking is that scattered TSTMS will form in the late afternoon hours along and ahead of the cold front across Germany ... France and Spain. 500 hPa flow of 20 to 25 m/s should support some severe TSTMS ... with primary threats being damaging winds and large hail with mainly linear convective modes. Especially over ERN portions of Germany ... SFC wind may be backed ahead of the developing wave cyclone late in the day ... and chance for a few mesocyclones may be enhanced.
Latest GFS and NMM and MM5 runs simulate another frontal wave to develop over SW Germany late on Saturday ... tracking across NE Germany during the night. This will likely prove to be another focus for widespread convective development ... with the possibility for an MCS late in the evening/night hours once BL decoupling occurs. Shear should be strong enough to support a few severe TSTMS ... main threats again being severe straight-line winds and hail.
...Iberian Peninsula...
TSTMS along the cold front over Spain will likely be rather high-based given deep/dry CBL ... with attendant threat for cold-pool formation and bowing line segments ... mainly posing threat for severe wind gusts. Models do not suggest much convective precip ... and confidence in widespread convective development is rather lim,ited ATTM.
...S Ukraine ... E Bulgaria ... E Romania...
TSTMS will likely develop in weakly capped moderately unstable air mass over E Europe/portions of W Russia. Forcing for upward motion seems to be rather weak ... as will be the large-scale vertical shear. However ... favorable interaction of cells with outflow boundaries or similar mesoscale features may locally augment vertical shear ... increasing severe TSTM potential. Friday's Kryvyi Rih (Ukraine) 12Z ascent shows no CINH and much low-level CAPE ... hinting at possibility for an isolated tornado or two. Allover threat is too low for a SLGT however.
...Turkey...
Similar thermodynamic setup as in Spain expected ... in moderately strong shear regime and rather strong large- and mesoscale forcing for ascent. TSTMS should form in the early afternoon hours ... capable of producing severe wind gusts and hail. Instability seems to be quite weak though ... and extent of convective development may be somewhat limited.
...British Isles...
In the deep polar air ... SFC heating will likely promote scattered TSTMS across the British Isles. Shear across SE England is expected to be moderately strong ... and a shallow mesocyclone or two are not excluded ... capable of producing marginally severe winds/hail and maybe a brief tornado. Coverage of severe should be too low for a categorical risk.
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