Storm Forecast
Valid: Tue 24 Oct 2006 06:00 to Wed 25 Oct 2006 06:00 UTC
Issued: Tue 24 Oct 2006 03:28
Forecaster: VAN DER VELDE

SYNOPSIS

The vigorous low pressure area that affected northern France and the Benelux most yesterday is weaker today and most associated active weather moves from the Benelux to northern Poland during the period.
At 12Z the GFS lays the cold front from southern Denmark to Czech Republic and northwestern Italy. A back-bent occlusion will trail over The Netherlands and northern Germany. The warm front touches southern Sweden and later Finland, where already a stationary sharp frontal boundary is present, with a neutral to unstable airmass.

The sharpest temperature/moisture contrasts will be found around the southern Alps as the cold front takes fairly long to remove the warm airmass at the lee side. Some instability will be present and locally released.

A renewed surge of warm air around Portugal will keep convection around.


DISCUSSION

...Benelux, northern Germany...

Although GFS model doesn't indicate a decent amount of CAPE, the calculated EL heights show the possibility of convection, in coastal areas mostly (back-bent occlusion), per GFS convective precip, but also some over land. 00Z soundings of Essen, Herstmonceux, Brest and Trappes show neutral to unstable profiles that are at least not going to stabilize. Low-level shear in those soundings is respectable and supporting a tornado threat, staying in the range of 10-15 m/s, while deep-layer shear will retreat to central Germany. Still moderate values of 10-20 m/s should remain through the area, but mostly unidirectional in areas where instability is largest. Brief mesocyclones remain possible, hence also a few tornadoes, marginally large hail and severe gust events, on an isolated basis.

...southeastern Finland, northwestern Russia...

Along a very sharp, stationary frontal boundary favourable conditions exist for severe weather. 00Z soundings of Tallinn and St. Petersburg show strongly veering winds, increasing with height, with unstable but near neutral stratification. Strong convergence at the front is expected to set off convection which may produce a few tornadoes and some marginally severe hail. To the cold side of the front convection would be elevated and not profiting from veering winds.

...Northern Italy, Slovenia...

00Z soundings in this area have shown a very saturated, slightly unstable airmass with huge veering with height (Milano). As the cold front moves around the Alps, convergence will increase over nothern Italy and produce some storms. Steep midlevel lapse rates are indicated by GFS to border this region at the south side. SREH and DLS up to 25 m/s can generate storm rotation. Marginally large hail or an isolated tornado are not ruled out. An MCS or persistent cluster may form but severe gusts are expected to be rare.

Higher LCL heights during the night over Hungary may suggest thunderstorms may either cease there or be elevated in nature (midlevel instability indices and convective precipitation from GFS model do give signals). Shear will also be less... hence the lower potential for severe weather there.

...Portugal, southwestern Spain...

A similar situation as we have seen the past week. A few hundred J/kg MLCAPE can enter the land and interact with a very strong shear environment, LLS of more than 15 m/s, DLS more than 30 m/s, and 0-3 km SREH up to 400 m2/s2 are certainly dangerous combinations for violent supercells (large hail, significant tornadoes, severe gusts).... LCLs are low and LFC is fairly high in the GFS model so there is some capping or maybe rather elevated convection, but this seems to be caused by very saturated, neutral lower levels - not an obstacle to initiation and rather a catalyst for tornadogenesis ... Reason for not issuing a level 2 at this time is fairly low instability and lack of a more dynamical trigger.
Most activity is anticipated to start during the late evening hours, with low-level convergence near the Spain-Portugal border. Like last week, significant amounts of precipitation are forecast which can lead to local flash flooding.