Storm Forecast
Valid: Fri 25 Jul 2014 06:00 to Sat 26 Jul 2014 06:00 UTC
Issued: Thu 24 Jul 2014 20:45
Forecaster: TASZAREK
A level 1 was issued for S Norway mainly for excessive precipitation
A level 1 was issued for S Poland mainly for excessive precipitation
A level 1 was issued for N Italy, parts of Austria and Slovenia mainly for excessive precipitation and large hail
A level 1 was issued for SW France and parts of Spain mainly for large hail
A level 1 was issued for coastal area of Algeria and Tunisia mainly for large and very large hail and severe wind gusts
SYNOPSIS
Weak mid-level flow with low horizontal pressure gradient dominate over most of the European continent and favors homogenous airmass internal thunderstorms. High pressure system located over Scandinavia pushes polar jet stream far to the north and provides low-sheared environment over Europe. Weak mid-level trough over Benelux creates wide cyclonic flow over S Europe with the jet streak located over W and central Mediterranean. Rather weak lapse rates and moderate moisture content accompanied by low sheared environment are expected over most of the forecast area. Equilibrium level temperatures exceeding -40C in central European regions may create electrically active ordinary thunderstorms, although lack of strong wind shear will prevent them from organizing into multicells and supercells. Thunderstorms in the most of the regions will form in the afternoon hours and decay after the sunset, however they may go on during the night in northern Scandinavia and Italy where QG-forcing will be available. Most favorable conditions for the severe weather will be located in the W Meditarrean, although strong convective inhibition is expected to prevent from convective initiation.
DISCUSSION
Southern Norway
Convective initiation is expected in the afternoon hours along the axis of the Norwegian Mountains in the SW part of the anticyclone located over Scandinavia. Mixing ratios up to 10 g/kg and lapse rates over 6.5 K/km will provide CAPE values up to 1500-2000 J/kg. Taking into account weak mid-level flow and PW over 30mm, convective cells may create threat of excessive precipitation. Flash flooding event in the mountainous areas cannot be rulled out.
Southern Poland
In the convergence axis of SE flow between anticyclone over Scandinavia and trough over Benelux thunderstorms will receive low-level forcing. Convective cells are expected to form along convergence line and move NE. Training storms moving in the environment rich in moisture (PW 30-35mm) will pose a risk of producing high amounts of precipitation. Due to weak atmospheric flow, thunderstorms are expected to have slow motion and this will increase risk for flash flooding phenomena, convection should dissipate in the evening.
South-western France
Along the axis of the shortwave associated with trough over Benelux, thunderstorms in the environment of CAPE up to 2000 J/kg, high boundary-layer moisture and moderate wind shear (10-15m/s) will create risk for isolated large hail, especially in the region of Pyreneans mountain range.
Northern Italy, parts of Austria and Slovenia
In the environment of CAPE up to 1500-2000 J/kg resulting from high boundary-layer moisture and diurnal heating, convection is expected to be initiated due to low-level convergence. Similiar as in Norway and Poland weak atmospheric flow and high PW will pose risk of excessive precipitation and especially in Alps, risk of flash flooding phenomena. Thunderstorms capable of producing large amounts of precipitation in N Italy are also expected in the Saturday's morning. Due to CAPE values up to 2000 J/kg in NW Italy, large hail is also not rulled out.
Coastal areas of Algeria and Tunisia
Combination of CAPE up to 2500 J/kg and wind shear of 30m/s pose a risk for very dangerous thunderstorms with supercells capable of producing very large hail and severe wind gusts. Although QG forcing will be available, convective inhibition associated with warm and dry air advection from Sahara Desert will likely prevent from convective initiation.
