Storm Forecast
Valid: Sat 30 Aug 2014 06:00 to Sun 31 Aug 2014 06:00 UTC
Issued: Sat 30 Aug 2014 02:46
Forecaster: TUSCHY

A level 1 was issued for S-Norway and the Kattegat mainly for excessive rain and an isolated tornado event.

A level 1 was issued for E-CNTRL Spain mainly for an isolated large hail event.

A level 1 was issued for NW Italy mainly for isolated large hail.

SYNOPSIS

Ex-hurricane's (CRISTOBAL) WAA induces rapid mid-layer thickness increase west of Ireland (e.g. 500 hPa height increase in excess of 10 gpdm/12h). This causes a sharpening thermal gradient over Ireland and UK between the ridge and a stationary vortex over N-Scotland. Both, the northward building ridge and intensifying mid/upper jet winds along the SW fringe of that vortex cause the vortex's slow SE-ward motion during the overnight hours ... approaching Benelux from the NW during the end of the forecast. Advection of moist marine air ahead of that low result in marginal thunderstorm conditions for this forecast period.

Positive tilted trough over NE Europe features cold mid-levels, but no distinct moisture source is available to feed any storms. Hence scarce thunderstorm probabilities exist.

Numerous broad mid/upper waves, cool mid-levels and decreasing mid-layer thickness affects S-Europe including the Mediterranean. As 700 hPa temperatures remain aoa 6°C, the air mass remains capped for most parts ... excluding mountainous regions.

DISCUSSION

Isolated thunderstorms occur beneath the SE-ward drifting vortex and affect a broad area, including the North Sea, N/CNTRL Germany, Denmark and SW-Norway. Slim CAPE and shear preclude organized DMC although heavy rain is possible over Denmark and SW-Norway, where a tongue of 30 mm ( ) PWs and strongest LL convergence interfere with each other. Organized bands of showers/thunderstorms could also produce an isolated waterspout event.

EZ/GFS also show an augmented risk for convectively enhanced heavy rainfall for E/NE Germany during the night, as plume of 30 mm ( ) PWs overspreads the area from the W. We don't see enough LL forcing and CAPE to go with a level area. Storms should remain progressive, too, although during the second half of the night, GFS points to local training over far E/NE Germany. The surface trough's geometry dictates that risk and ongoing model discrepancies of that trough lessen confidence in any level issuance.

For the rest of Europe's activity, daytime driven mountain convection is forecast over Albania/Greece and Italy. Marginal hail and gusty winds will be the main hazard. Two exceptions exist:

a) Thermal low over CNTRL Spain ensures onshore flow along the E-coast of Spain with 500 - 800 J/kg MLCAPE build-up as BL moisture sneaks beneath plume of steeper mid-level lapse rates over CNTRL Spain. Winds at 500 hPa won't be so strong with a mean wind of 25 kt from the west, but good turning exists through the low/mid-levels. A few temporarily better organized storms could produce an isolated large hail event with downbursts well possible due to a well mixed/dry and deep subcloud layer.

b) The Alps see backing mid-level winds ahead of an approaching weak upper trough. This allows moist/unstable air to affect the Alps. Low tropospheric moisture peaks over NW Italy but moisture plume fans far to the NE towards the Czech Republic. Focus for organized convection remains N-Italy and the Alps, where 40 kt (300 hPa) westerly flow should assist in an isolated longer lived storm. A localized large hail event would be the main risk. Heavy rain is possible over S-Austria with repeatedly crossing showers/thunderstorms, but no model offers QPF amounts equal to a level 1.

Overall rapid decay of storms is forecast after sunset, excluding the Alps, where a few storms continue well into the night. Also the S-North Sea experiences an upswing of thunderstorm activity as cooler mid-levels move atop a coastal fetch of enhanced SSTs.