Storm Forecast

Storm Forecast
Valid: Mon 20 Aug 2012 06:00 to Tue 21 Aug 2012 06:00 UTC
Issued: Sun 19 Aug 2012 23:47
Forecaster: PUCIK

A level 1 was issued for France, Germany and Northern Poland mainly for large hail and severe wind gusts.

A level 1 was issued for Northwestern Italy mainly for excessive preciptiation and large hail.

SYNOPSIS

A primary macrosynoptic feature at mid and upper levels of troposphere will remain in the form of a ridge stretching from the southwestern Mediterranean into Central Europe and also Scandinavia. This feature is expected to de-amplify with time and shift towards east. Slight height falls are forecast over the western and northern flanks of this ridge and enhanced flow is simulated here as well. A well-defined short-wave trough will travel towards southeast along the eastern flank of the ridge, affecting mostly extreme western Russia. A small-scale short-wave should travel over Northwestern France, BENELUX and Northern Germany during the afternoon/evening hours.

An EML with a very hot airmass will spread towards northeast during the day, along the western flank of the ridge, covering much of Iberia, France and western half of Central Europe. A cold front will progress slowly towards east during the day, across France and Germany. An ill-defined surface pressure field is forecast, with high developing over Scandinavia and an extensive high over Southern half of Europe with the most significant low over Russia during the day. Here, a significant cold front will surge to the southeast.


DISCUSSION

... France to Germany and Northern Poland ...

A plume of steep mid-level lapse rates, in conjunction with falling mid-level heights and abundant low-level moisture (dew points around 20C) should contribute to the moderate to strong destabilisation with MLCAPE values reaching mostly between 1000 and 2000 J/kg, locally possibly even higher. As in the previous days, capping should be the primary issue, especially regarding the southern/eastern extents of the Level 1. Most of Sunday 12 UTC soundings reveal strong capping underneath the EML, limiting the convection only to the elevated nature. Regarding Monday, falling heights and cooling mid-level temperatures might help in the cap weakening, especially in the northern extents of Lvl 1. Moreover, a small scale short-wave trough might affect France and Germany during the afternoon / evening hours, so that some further lift might be available for CIN removal. Moderate wind shear is forecast, with the highest values (around 20 m/s) of DLS simulated closer to the cool side of the frontal boundary, i.e. western and northern extents of Lvl 1. Locally enhanced wind shear might come from the low level wind field backing ahead of the pre-frontal trough.

Models diverge on the degree of storm coverage and the crucial point in the unfolding of the convective scenario will be the removal of cap in the afternoon hours. The best chance for this will exist over Northeastern France and Central/Northeastern Germany. Generally speaking, one of the primary threats should be large (or even very large) hail with steep lapse rates and high values of CAPE, combined with moderate wind shear, allowing for some well organised multicells or even brief supercells. Steep lapse rates and high temperature vs dew point spread also suggests the possibility of severe wind gusts in downbursts, which should be another primary threat.. A large Lvl 1 was introduced for the region, where conditions will be favorable for these. Questionable initiation prevents the issuance of Lvl 2 attm, but an upgrade might become necessary later on, especially for Germany.

... Northwestern Italy ...

Mountains might help in CI over this region, with moderate CAPE and weak wind shear regime allowing only for pulse type updrafts. Large hail and excessive precipitation might occur with slowly moving and stronger multicells within this region.

... Western Russia ...

As the active frontal boundary moves across the region, models simulate some CAPE values in the warm sector of the low. Strong short-wave trough is forecast to move in the wake of the cold front, possibly aiding in the release of the CAPE in the warm sector, especially close to the cold front. A big cluster of storms, possibly an MCS, is well possible in this setup. Wind shear should stay rather weak in the zone of the highest CAPE and some of the models, e.g. ECMWF, produce only rather marginal CAPE (few hundreds J/kg of MLCAPE). Some marginally severe winds/hail are not ruled out, but the coverage might be too low to warrant a Lvl 1.

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