Storm Forecast
Valid: Sat 28 Apr 2018 06:00 to Sun 29 Apr 2018 06:00 UTC
Issued: Sat 28 Apr 2018 07:01
Forecaster: TASZAREK

A level 1 was issued for NE Spain, S France, N Italy and parts of Switzerland mainly for the large hail, severe wind gusts and in lesser extent for excessive precipitation.

A level 1 was issued for a corridor from W Slovakia to N Serbia mainly for the large hail, severe wind gusts and in lesser extent for excessive precipitation.

SYNOPSIS and DISCUSSION

Broad high dominates over SE Europe while a long-wave with a strong jet enters W and SW Europe. Jet streak and frontal boundary extends from S Spain, through S France, Benelux, N Germany and Baltic Countries. Rich boundary layer's moisture exceeding 10 g/kg (mostly as a result of the strong diurnal heating and evapotranspiration) overspreads NE Spain, S France, S Germany, N Italy, Czech Republic, Slovakia, Hungary and Balkan Peninsula. Lapse rates are steepening within approaching long-wave on its eastern side. Small to moderate instability develops within the aforementioned area of rich boundary layer's moisture. Peak values of ML CAPE (1500-2000 J/kg) are expected in the belt from N Serbia to W Hungary. Small instability is also expected within steep lapse rates in the cold polar air mass (on the cool side of the frontal boundary) over W Russia, Finland, N Sweden and British Isles. Low-topped convection developing in these areas provide small chances for lightning. Storm should develop mainly during afternoon hours as a result of daytime heating, and fade in the late afternoon. Much better conditions for organized DMC fall on NE Spain and S France where under the support of strong vertical wind shear and synoptic-scale lift, thunderstorms may organize into multicell clusters and supercells capable of producing large hail (given steep lapse rates), severe wind gusts (convective mode), and in lesser extent excessive precipitation in a possible training storm modes. Storms within this area should maintain until nighttime hours. Although less QG-lift will be available in N Italy and Alpine area, CI should take place within the diurnal heating, local convergence zones and orographic lift. These storms will have access to weaker shear (~ 15 m/s), but given steep lapse rates and wind field interacting with orography, a few local events of a large hail and severe wind cannot be ruled out. Excessive precipitation is also involved as a threat, especially within slowly moving cells and complex orography. Storms in this area should drop their activity in the evening hours. NWP data for Balkan Peninsula and Hungary looks the most challenging. Although decent instability and moderate shear is available in the afternoon hours, which may promote large hail, severe wind and heavy rain, there is a strong uncertainty related to CI. Lack of sufficient QG-lift, presence of CIN and very limited model signal indicate rather low probability. Current understanding is that storms will be unlikely to form and therefore 15% lightning probability is issued for this area. However, if DMC occurs, thunderstorms with a potential of producing severe weather will be possible. Some model scenarios point to a very late CI around late afternoon / evening hours and lightning activity during nighttime. A level 1 is issued for this possibility.