Storm Forecast
Valid: Sun 17 Aug 2008 06:00 to Mon 18 Aug 2008 06:00 UTC
Issued: Sat 16 Aug 2008 21:13
Forecaster: SCHLENCZEK
SYNOPSIS
An intense low pressure system at upper and lower levels south of Iceland is forecast to move eastward, reaching the British Isles on early Monday morning while occluding. It will lead to strong WAA at lower levels over western Europe in the next few days. The low pressure system over Poland that caused a major severe weather outbreak on Friday moves northward while weakening. Nevertheless, severe thunderstorms should be expected in northeastern Europe in the vicinity of a 35 m/s upper jet streak. The well-defined cold front over Poland should move northeastward. Weak pressure gradients and low geopotential height are present over parts of western and central Europe. Ahead of the steering low west of Ireland, a shortwave trough is forecast to translate eastward from southeastern England towards northern Germany.
DISCUSSION
... Estonia, SE Finland...
Latest GFS runs suggest that the jet streak over Poland substantially weakens during the night with attendant decreasing deep layer shear over the region discussed here. Some slight overlaps of 20 m/s deep layer shear and more than 1 kJ/kg MLCAPE are most likely over the northern Baltic States. The degree of instability is somewhat uncertain as remnants of today's convection as well as frontal mid-level clouds will hinder insolation. Deep layer shear between 15 and 20 m/s and about 200 J/kg SRH3 will support rotating updrafts. Any discrete cell that develops in such an environment has a great chance to become supercellular, capable of producing isolated large hail and damaging gusts. Enhanced 10 - 15 m/s LLS as well as low LCL heights may allow an isolated tornado. Only in close vicinity of the frontal boundary, CIN seems to be low enough for convective initiation. As the upper level flow is almost parallel to the front, storms will tend to merge into one or two large systems that may produce severe straight-line winds and torrential rainfall. Flash floods may occur where storms persist.
...Parts of NW Russia...
About 1 kJ/kg MLCAPE is forecast in the vicinity of the warm front that stretches from southern Finland towards northwestern Russia. Strong 20 m/s DLS and about 15 m/s LLS as well as enhanced SRH may allow isolated supercells with large hail, severe gusts and perhaps a tornado. As a huge cap is present in that region and forcing is expected to be rather weak, widespread storm initiation is unlikely which leads to a marginal level-one threat.
...S England, N France, N Belgium, Netherlands...
A shortwave trough will overspread the region during the evening hours when a few hundred J/kg of CAPE should be available in the region. Strong QG forcing may initiate linearly organized storms or isolated multicells with gusty winds and perhaps some small hail. Severe weather is not expected as DLS and CAPE are rather weak.
Some multicell storms with a marginal hail threat may also develop over Italy where some hundred J/kg of CAPE and about 10 m/s DLS should be available during the afternoon.
