Storm Forecast
Valid: Tue 17 Nov 2015 06:00 to Wed 18 Nov 2015 06:00 UTC
Issued: Mon 16 Nov 2015 22:35
Forecaster: GROENEMEIJER

A level 2 was issued for the Benelux countries, a large part of Germany, the NW Czech Republic and W Poland mainly for severe wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for and area surrounding the level 1, including S Ireland, the S United Kingdom, larger parts of Poland and the Czech Republic, all of Germany and parts of Denmark, France, Switzerland, and Austria, mainly for severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

During the forecast period, a strong mid-level jetstreak will rapidly enter western Europe from the Atlantic Ocean. On the southern flank of an associated eastward moving area of low pressure, a strong wind field develops and moves across South Ireland, the southern UK, Benelux, Germany and later Western Poland. Within the seasonably mild and humid air-mass organized severe convective storms are expected.

DISCUSSION

S Ireland, S UK, North Sea, Bneelux, N France, Germany, Poland, Czech Republic, W Poland, N Austria...

Upon approach of a strong mid-level jet, upward vertical motion will ovespread the mild and humid warm sector (with dew point temperatures of 11-14 C) first across southern Ireland, then the southern UK and subsequently further eastward. In response, surface-based convection coverage will increase and probably organize linearly. These storms will be embedded in a strong wind field and gusts that they produce will likely attain severe levels (> 25 m/s).

According to the GFS model, the strongest upward motion should be in the left exit region of the jet streak that becomes particularly focused during the evening. This area of upward motion will move from west to east across the indicated level 2 area, i.e from the Benelux Countries to W Poland in the 21 - 06 UTC time frame. It appears likely that well-organized linear convection with a particularly high risk of severe wind gusts will develop there as the system moves eastward. This will happen in an environment that - per GFS - will boast 200-400 J/kg CAPE, which is quite substantial for the time of year.

The environment of the convection is characterized by strong wind shear (~ 30 m/s 0-6 km) particularly in low levels (~ 15 - 18 m/s 0-1 km). If mor or less isolated convective storms are able to form, a high tornado threat would be present, but it appears more likely that a line will be the dominant mode given the strongly localized forcing and absent CIN.

As it presently seems unlikely that a large wind shift will occur across the line, the risk of tornadoes wih the line seems to be redduced, in accrodance with the results of Clark and Parker. Nevertheless, a few tornadoes cannot be ruled out given the otherwise favourable circumstances including high wind shear, very low LCL and the strong forcing for upward motion that may locally steepen lapse rates considerably.

Behind the area of greatest convective activity, strong non-convective winds are expected occur with similar or even higher gusts.