Storm Forecast
Valid: Wed 08 Sep 2010 06:00 to Thu 09 Sep 2010 06:00 UTC
Issued: Wed 08 Sep 2010 06:07
Forecaster: PUCIK

A level 1 was issued mainly for Central Mediterranean for excessive precipitation and severe hail, less for severe wind gusts or tornadoes.

SYNOPSIS

The most important macrosynoptic feature at mid and upper levels of troposphere will be a cyclonic vortex centered over Central France at 05 UTC per satellite observations. It is a part of a low-geopotential system stretching from Atlantic to the Western Mediterranean. The movement of this vortex will be only slight to the east/northeast while the southern flank of its trough will experience some amplification towards south, especially towards Thursday. The whole system is especially on its southern and southwestern flank surrounded by a belt of strong mid and upper tropospheric flow. To the northeast, a region of high geopotentials is observed over Scandinavia and it will persist also during this forecast period. A shallow cut off low is situated to the south, centered over Ukraine and moving to the east.

At the surface a complex low pressure system will cover most of Western Europe and will stretch to Northern and Central Italy. To the east, a high pressure system will stall over the portions of Scandinavia and Eastern Europe, centered over Southern Finland.

DISCUSSION

... Central Mediterranean ...

Ahead of the approaching trough, a plume of higher lapse rates is advected from Northern Africa towards northeast. At the same time, with mid levels cooling, weak upward motions and most importantly, high SST and surface dew points, moderate destabilisation is forecast especially over the Mediterranean Sea. Degree of destabilisation will be smaller over the coastal and inland areas with higher CAPEs observed along the western portions of coast and probably over Northern Italy. Values of CAPE will increase to the south, but along with increasing CIN and less favorable, if any, synoptic-scale forcing.

Enhanced mid and upper level tropospheric flow should result in moderate to strong wind shear, especially in 0-6 km layer. Values of bulk shear in this layer might reach up to 25 m/s over Central Italy (morning hours) and coastal areas of Serbia, Albania and Bosnia and Herzegovina (late evening hours). Forcing will be most favorable in the northern part of region and the highest thunderstorm coverage will be most probably seen over Northern Italy and Northern Adriatics.

The most distinct threat will probably be an excessive precipitation. At the meantime (05 UTC) a parallel-stratiform MCS is moving from Southern France into Northern Italy with system propagation towards south. This coincides with the favorable moist enhanced southerly flow at lower levels of troposphere. This flow will slowly translate to the east during the day, as should MCS as well. During the evening hours, excessive precipitation threat might shift into the Northern Adriatics region. This threat will be less pronounced towards the southern regions, but also western coastal areas of Italy and Balkans might see some "cell train-effect".

With moderate values of CAPE and moderate to strong values of wind shear, environment will be supportive of well organised convection, possibly including few supercellular storms. Especially with supercells, but also with other strong multicellular storms, threat of severe hail will be present, along with severe wind gusts. Severe wind gust threat will be more pronounced in the western parts of Level 1 and later in the day, as drier airmass advects over the region. Tornadoes are not completely ruled out, especially over western Balkans, where enhanced low level shear will reach values over 10 m/s, along with low LCLs.