CONVECTIVE FORECAST
VALID 06Z SUN 18/07 - 06Z MON 19/07 2004
ISSUED: 17/07 23:54Z
FORECASTER: DAHL
There is a slight risk of severe thunderstorms forecast across France ... S Benelux ... west and central Germany ... N Switzerland.
General thunderstorms are forecast across SW and central Europe ... N Balkan States ... extreme W Russia.
SYNOPSIS
Large-scale upper trough is anchored over the E Atlantic ... with several small-scale vort maxima pivoting about its periphery ... affecting western and west-central Europe and parts of S Scandinavia during the period. Strongest perturbation is progged to cross France late on Sunday night/early Monday morning. SLY SFC flow E of the long-wave trough has advected plume of seasonably warm/moist air into W and central Europe ... with the main wavy low-level front expected to extend from N Spain nearly parallel to the continental-European W coast into S Norway by Sunday 12Z.
DISCUSSION
...Germany ... Benelux ... N Switzerland ... France...
Saturday's soundings from central and SW Europe exhibited MLCAPEs in the 500 to 1000 J/kg range ... with mean low-level mixing ratios on the order of 10 g/kg. GFS has produced CAPE up to 2000 J/kg for Saturday 18Z which appears to be too optimistic based on SFC OBS and on 12Z soundings. NMM offers a more realistic solution with 500 to 1000 J/kg CAPE. This is also what should be expected across central and Sw Europe on Sunday though precip from Saturday's convection may locally enhance CAPE somewhat.
Deep-layer shear will be somewhat marginal with about 15 m/s over N France ... Benelux and N Germany ... decreasing to the SE. Towards late Sunday evening kinematic profiles are expected to strengthen ahead of the short-wave trough approaching from the SW ... with 25 m/s 0-6 km shear over central France late Sunday night and the 20 m/s isotach moving into west-central Germany. GFS advertises 0-1 km shear in excess of 7 m/s in a region extending from N France across Switzerland into SW and central Germany towards late Sunday afternoon ... which is also supported by MM5 wind fields.
Saturday's soundings were weakly capped ... and there appears to be little reason to assume this will change on Sunday once the convective debris has mixed out. Residual outflow boundaries ... orographic features ... the main cold front and a mesoscale pre-frontal convergence line which appears to be tied to the warmest 850 hPa theta-e's will likely be foci for convective initiation.
Severe TSTM threat will likely be greatest over NW France ... Benelux and western portions of Germany where large-scale deep-layer shear will be best. Especially towards the evening ... over S Germany and the W Alpine regions severe TSTMS become increasingly likely as well as low-level shear increases. This activity should be strongly diurnally driven.
Another ... more focused severe TSTM threat may develop amidst strong DCVA-related vertical motion regime overspreading France on Sunday night ... though the bad timing of this feature may be somewhat detrimental to a widespread severe TSTM event. However ... shear will be more than adequate for severe and if storms can be sustained through the night ... and NRN France ... S Benelux and W Germany may see severe TSTMS early on Monday morning. However ... later OBS will be necessary to specify severe threat.
Expect primarily damaging wind gusts and large hail over W Germany ... S Benelux and central/northern France towards late afternoon ... with an increasing large-hail threat over SRN Germany and N Switzerland where apart from the increasing large-scale shear orographically-caused flow perturbations may create sufficiently large low-level SRH values. Also ... a brief tornado or two may occur with this activity.
Uncertainty exists with respect to the nocturnal convection expected to develop over N France. The strong forcing may favor a line of TSTMS ... primarily posing a threat of damaging wind gusts. However ... isolated mesocyclones may be present as well ahead of the line ... or imbedded in the line ... enhancing large hail and tornado threat.
...N Spain...
Weak CAPE will likely develop over N Spain with diabatic SFC heating during the day. Indications are that deep/dry CBL will be present ... promoting strong outflow winds ... some of which may exceed severe levels. Also ... given 500 hPa wind speeds of about 17 m/s ... bow echoes and/or a few mesocyclones may form as well ... especially the latter of which posing an additional threat for large hail. TSTM coverage is expected to be rather low ATTM ... and a SLGT is not necessary.
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