CONVECTIVE FORECAST - UPDATE
VALID 15Z WED 16/07 - 06Z THU 17/07 2003
ISSUED: 16/07 14:39Z
FORECASTER: GROENEMEIJER / DAHL

There is a slight risk of severe thunderstorms forecast across eastern France, northern Italy and western Germany as well as parts of the Benelux countries.

General thunderstorms are forecast across

SYNOPSIS

An upper ridge over cnetral Europe moves gradually eastward. An upper low over the Bay of Biscay moves towards the Channel region. On its southeastern flank a shortwave from central to southeastern France will move northeastward. A cold front occlusion/cold front can be analysed from near Lands End to London to Dunkirk to near Lyon at 12Z. A convergence line is located from near Antwerp to the Ruhrgebiet.

DISCUSSION

...southeastern France, northern Italy...
Quite significant northward moisture return has taken place during the last 12 hours west of the Alps, where surface dew points have risen strongly. This has resulted in MLCAPE50 from near 1500 J/kg in Nimes (per LFME sounding) to a few 100s of J/kg further northward and eastward into southern Germany.

Over the lower Rhone Valley and the western Alpine flanks, convection is taking place. Given quite strongly backed surface winds, especially near the Mediterranean coast, that may yield a quite helical storm inflow and strong deep layer shear....a couple of supercells are possible with this activity. Expect large hail and damaging gusts in places. Locally, high amounts of precipitation may fall, since the storms will be strongly backbuiliding. There will also be a small threat of tornadoes. As upper trough moves eastward, it will induce upward vertical motions over northern Italy that will likely be suffieciently strong to initiate a few storms. Here, there will also be ample latent instability (~ 1000 - 1500 J/kg MLCAPE50) and deep layer shear for the formation of supercells. These storms may produce Large hail and damaging winds. Tornado threat is not particularly high, but interaction of mesoscale boundaries such as outflow boundaries and sea-breeze fronts may lead to the formation of one or two tornadoes.

......Switzerland ... W Germany ... BeNeLux... ...
Shortwave trough ATTM over France ... will lift northwards ... with its DCVA-related upward vertical motion field expected to spread into BeNeLux and central Germany Wednesday evening/night. Occluding frontal system is present ahead of the trough ... as indicated by 850 hPa theta-e analyses.

Ample moisture is present in the narrowing warm-sector airmass. 00Z Mallorca sounding is showing the quality of this airmass ... CAPE in excess of 2000 J/kg, rich BL moisture and an EML present atop the BL. Though it is not believed that thermodynamic profiles are such strong in the northern portions of the warm sector ... primarily owing to lower SFC dewpoints ... substantial CAPE ... possibly on the order of 1500 J/kg should be present.

Deep-layer shear is expected to remain quite weak ... possibly on the order of 20 knots. Strong low-level veering is present though ahead of the theta-e plume over western portions of Germany and BeNeLux.

Weak elevated thunderstorms have already formed across central and western Germany amidst the theta-e advection regime ... and over France ahead of the vort max along the cold front. Indications are that TSTM activity ahead of the vort max will increase over the next hours ... and formation of a linear severe MCS is anticipated ... which will move across Switzerland ... SW Germany and BeNeLux late in the day/evening. However ... shear profiles are expected to remain quite weak across these regions ... and severe threat should decrease substantially as the MCS moves into Germany. Also ... as frontal system occludes ... storms will likely become elevated towards late evening. Main severe threat appears to be marginally severe wind gusts with this activity ... and possibly some marginally severe hail.

Of concern are strongly veering winds over BeNeLux and SW Germany ... low-level airmass is rather dry across central Germany though ... and if TSTMS manage to root down into the boundary layer ... CAPE of the inflow would likely be rather meager ... and chance of severe-hail producing supercells in this regime appears to be remote. Farther west though ... SFC-based moisture is stronger ... and severe-hail threat appears to be rather high if a storm manages to tap helical boundary-layer air. Isolated TSTMS may also form within the warm sector ... possibly along outflow boundaries or hills/mountains ... This activity will also have some potential of becoming severe ... especially if locally high SRH is ingested ... which is rather likely over the complex terrain of Switzerland and S Germany. Large hail and severe wind gusts may occur with this activity.

Today's/tonight's scenario is somewhat conditional ... basically with respect to SFC-based supercells ahead of the MCS expected later tonight ... and with respect to the MCS itself which appears to lose much of its strength as it moves into weak-shear environment. However ... a few severe-thunderstorm events will likely occur today ... and a SLGT is marginally warranted.

#