CONVECTIVE FORECAST
VALID 06Z MON 21/06 - 06Z TUE 22/06 2004
ISSUED: 21/06 00:19Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across eastern parts of the Baltic States, western and central Belarus, extreme eastern Poland, extreme southeastern Finland and Russia: Pskovskaya, Leningradskaya, Novgorodskaya adm. regions and southwestern Karelia.

General thunderstorms are forecast across much of northwester, central and eastern Europe, across the northern and eastern Iberian peninsula and Sicily.

SYNOPSIS

Monday at 06Z... zonal flow is expected over much of central Europe on the southern flank of a longwave trough. Downstream of the trough, a southwesterly flow has a speed maximum over the Baltic states at 06, that moves into northern Russia. An area of low surface pressure at the tip of a stable frontal wave, moves northeastward in concert with the right entrance region of this jet. A mid-level trough from the northern North Sea into England is expected to move into the Low Countries during the day.

DISCUSSION

...western and central Belarus, northwestern Ukraine, extreme eastern Poland, eastern Baltic States, parts of west-central Russia, southern Karelia. ...
At 06Z the frontal zone should be located along a line from near Brest(BY) to Vilnius to Novgorod to near Vologda, with the tip of a baroclinic wave between Novgorod and Vilnius, that is expected over southern Karelia by midnight. Some, mostly elevated, convection will likely be ongoing northwest of the frontal zone at the exit of a 20-25 m/s southerly low level jet. On the order of 400-800 J/kg of MLCAPE50 are expected to form east of the front in response to solar heating over much of the slight risk region, with higher values most likely in the south. Because of low capping and strong forcing for upward vertical motion, surface based convection should initiate by late morning across most of the risk area. Strong shear and forcing suggest supercells and linearly organised convection will be the main convective modes. Given that strong (10-18 m/s) low level shear is expected to be place over the northern half of the slight risk are, and strong 150-300 m2/s2 0-1 km storm relative helicity is forecast near the frontal zone while LCL heights should be low, a few tornadoes are likely to occur. Greatest chances will be in an area bounded by a line from Lake Ladoga to Vyborg to Tartu to Pskov to Novgorod to Lake Ladoga, thereby including St. Petersburg. A few tornadoes could also occur further south along the front. Apart frok tornadoes, isolated large hail and strong to severe straight-line winds can be expected with storms.

...southern England, Benelux, northwestern Germany...
Large scale forcing for upward vertical motions are expected across the area ahead of a shortwave trough from the northern North Sea to central england, that slowly moves southeastward. A few 100's of J/kg CAPE are expected to form within the uncapped polar air mass, in which scattered, locally clustering storms will likely form during the late morning and afternoon. As a westerly jet overspreads the area during the day, about 20 m/s deep-layer (0-6 km) shear should be present over extreme southern England the southern Benelux and much of Northrhine-Westphalia. especially in those areas, convection may organise into strong multicells, and perhaps a few supercells, yielding athreat of isolated large hail events and a few strong gusts. At this moment, the combination of rather marginal CAPE, and the fact that a large part of the deep-layer shear is forecast to be located above 700 hPa, create some uncertainty with respect to the coverage of the severe activity, so we refrain from issuing a slight risk at this moment.

...northern and eastern Spain...
Although quite limited CAPE should be present, isolated convection should be able to initiate over the indicated area. Given that ample (20-25 m/s 0-6 km) deep-layer is expected over the area, formation of a few supercells is possible. If supercells form, these should be capable of producing some large hail and strong to severe gusts. The limited amount of instability and anticipated low storm coverage preclude the issuance of a risk category for now.

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