Storm Forecast
Valid: Tue 18 Sep 2007 06:00 to Wed 19 Sep 2007 06:00 UTC
Issued: Tue 18 Sep 2007 01:28
Forecaster: VAN DER VELDE

SYNOPSIS

A highly baroclinic situation today over Europe, with a large amplitude upper trough over western Europe and elongated surface low pressure from Finland down into Italy, where a sharply defined cold front will wash away the warm airmass. Fairly strong pressure gradients in the lower troposphere create generally moderate shear conditions over a large area just east of the cold front, to strong shear near the surface, veering with height. In the same area significant CAPE is forecast. Severe thunderstorms are expected to occur over a large area.

GFS 12Z and MM5 18Z runs develop a feature of convective nature along the cold front with a track from the Alpine region, southeastern Germany, Poland into the Baltic countries. As instability is uncertain during the morning, some doubt exists about this part of the track, but given favorable quasi-geostrophic positioning it may become the focus for Polish storms.


DISCUSSION

Italy, W Balkan:

Weakly capped large CAPE in the order of 1000-2500 J/kg is thought to develop in the lee of the Alps and surroundings, which by itself enhances the chances of large hail and severe gusts. In addition, strong forcing and moderate deep layer shear around 20 m/s over 0-6 km are likely to sustain widespread, well-organized storms in the form of strong multicells and mesoscale convective systems.

Storm-relative helicity is sufficient for development of mesocyclones (supercells), and may reach even respectable values over the western Balkan, according to GFS 12Z: >300 m2/s2. Supercells may be the primary mode where forcing is less strong, while squally MCSses are more likely at the cold front.

Given that 0-1 km shear in the GFS model hits alarmingly high values in some places (15 m/s over most of W-Balkan, 20 m/s in Croatia...) while LCLs are not very high, likelyhood of tornadoes rises well beyond 'possible'... not excluding violent tornadoes - but with a side note that deeper layer shear (over 6 and 8 km) is not so great as to be favorable for very long-lived supercells.
MCS storms may more easily develop bow echoes under such strong low-level shear conditions, with enhanced damaging gusts threat.

Farther north, in Hungary and Slovakia, transverse motion of the cold front is more likely to force a squall line with a severe gusts threat after initial multi/supercell large-hail producing storms.

Flash-flooding may occur where storms persist.


Poland region:

Shortwave within the upper trough should pass through during the afternoon. While deep layer shear prefers the colder side of the front, GFS suggests some enhanced 0-3 km SREH and decent low-level shear... enough to yield strong multicell systems with chances of severe gusts and large hail... an isolated brief tornado not ruled out.


E-Spain, Balearic region:

Some hints at instability and mid level ascent remain present in GFS, while shear conditions (25 m/s 0-6 km) suggest isolated split/supercell storm modes. Expect large hail to be the primary threat. Low-level lifting may come from the cold front and moist eastern flow lifting over the Spanish coast.


N-Algeria:

20 m/s deep layer shear, >200 m2/s2 SREH, decent instability and sea/mountain circulations will likely trigger isolated severe storms. Dry boundary layer may enhance severe gusts. Large hail is possible given supercell potential and not too warm profile, and >2000 J/kg CAPE.


Netherlands, Germany, Denmark coasts:

Slight chance of waterspouts exists, based on steep near-surface lapse rates and coastal convergence, but shallow convective cloud depth, not so strong low level CAPE build-up and normal speed low-level winds (not being under mid/upper cold pool) makes chances too low for a categorical threat level.