CONVECTIVE FORECAST - UPDATE
VALID 14Z TUE 22/07 - 06Z WED 23/07 2003
ISSUED: 22/07 14:07Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across south France

There is a slight risk of severe thunderstorms forecast across east Germany and W Chech Republic.

General thunderstorms are forecast across north ... central ... east and southwest Europe.

SYNOPSIS

Broad southwesterly flow is covering western ... central and northern portions of Europe ahead of large-scale upper trough ... with a vort max ATTM stretching across W Germany and the North Sea ... lifting northeastwards during the FCST period. At low levels ... plume of moist/unstable subtropical air is extending northeastwards ahead of the upper long-wave trough. This feature is progged to cross SE Germany ... Austria ... the Chech republic ... Slovakia ... Hungaria ... Poland and the Baltic States during the FCST period. Synoptically quiescent conditions are prevailing over the central and western Mediterranean ... where impressive elevated mixed layer from the Atlas Mountains is present. Main Low-level Cold front E of the theta-e tongue is stretching from W Scandinavia across central Germany into central France.

DISCUSSION

...S France...
...S France ... Based on 00 Z and 06 Z radiosonde ascents ... on 850 hPa theta-e fields ... and on latest SFC/SAT observations ... afternoon MLCAPEs in excess of 1500 J/kg are expected across south France where unimpeded insolation will be possible throughout the day and where ample moisture is present with dewpoints of around 20°C. However ... problem appears to be weak forcing for upward motion across this region and the presence of a relatively strong cap ... and deep convection may fail to initiate. Mesoscale features such as the sea breeze front or orography/upslope flow will be the main foci for possible TSTM development. Also ... weak DCVA assumed by GFS late in the period at the northern edge of the theta-e tongue/along the cold front may provide additional source of lift. Deep-layer shear is expected to increase to 40 to 50 knots late in the day/evening over S France.

Is storms initiate ... thermodynamic and kinematic setup will be more that adequate for organized severe thunderstorms ... likely supercells. Expect the whole spectrum of severe weather ... very large hail ... damaging wind gusts and an isolated tornado or two ... especially late in the day/evening when LCL heights are low.

However ... confidence that TSTMS initiate remains uncertain ATTM ... but given several potential foci for initiation and degree of severe weather expected in case TSTMS do initiate ... a SLGT is warranted.

...Central and east-central Europe...
00 Z and 06 Z Soundings are affected by low/mid-level cloudiness and debris of yesterday's convective activity. However ... indications are that the high SFC dewpoints are quite shallow ... and that mixed-layer dewpoints will only be in the 12 to 15°C range. Further ... steep low/mid-level lapse rates are revealed by the soundings. With diurnal heating ... shallow low-level moisture should be mixed out ... and a deeply mixed BL is expected to develop ... with rather low CAPE walues. This in conjunction with about 30 knots deep-layer shear ... suggests a few severe wind gusts are possible again ... cold-pool formation may briefly support bow-echo type storms but convection is not expected to be particularly long-lived and organized. Decaying elevated MCS over N Germany is leaving a MCV in its wake ... which may be a possible focus for initiation. Also ... numerous outflow boundaries and the cold front ATTM over central Germany ... supported by weak DVCA will likely be focus for TSTMS.

Given that isolated severe wind gusts are the primary threat ... while large-hail and tornado threats are minimal ... and slightly weakening shear ... a SLGT is not necessary. If thermodynamic setup proves to be stronger towards the afternoon than currently expected ... an upgrade may be needed. AMENDMENT DISCUSSION

Though 12 Z soundings across E and S Germany reveal very meager instability ... radar imagery indicates that several storms across Saxony and Saxony-Anhalt in E Germany exibit strongly deviate motion vectors ... suggesting that the inflow into these cells may be quite helical. More isolated TSTMS over S Germany showed similar behavior ... one of which produced 3 cm large hail already. These storms are likely mesocyclonic ... and in addition to the severe-wind threat, a large-hail threat is present now as supercellular mode of the TSTMS is much more prominent than anticipated earlier. Hail and damaging wind threat will persist primarily across E-central Germany as well as over the Chech Republic where winds are also liable to be modified by orography to yield locally high SRHs.

#