Storm Forecast
Valid: Sat 04 Jun 2016 06:00 to Sun 05 Jun 2016 06:00 UTC
Issued: Sat 04 Jun 2016 05:36
Forecaster: TASZAREK / TIJSSEN

A level 2 was issued for S Belgium, E France, SW Germany, Switzerland, W Austria and N Italy mainly for excessive precipitation.

A level 1 was issued for parts of W, CNTRL and E Europe mainly for excessive precipitation and in lesser extent for large hail.

A level 1 was issued for parts of W Russia and Belarus mainly for large hail, excessive precipitation and severe wind gusts.

A level 1 was issued for N Tunisia and N Algeria mainly for large hail and severe wind gusts.

SYNOPSIS

Omega shape pattern covers N and NW Europe with high centred over Norwegian sea. The rest of the Europe remains under weak horizontal pressure gradients with values stretching from 1010 to 1015 hPa. A belt of a warm and moist (PW > 30mm) airmass extends from France through CNTRL Europe up to Balkan Peninsula. Due to weakly sheared environment and thus slow storm motion along with high coverage of the storms and rich moisture profile a main weather threat falls on excessive precipitation and local flash flooding. Threat concern particularly mountain slopes and areas surrounding a meso-low centred over E France and SW Germany. Severe thunderstorms with hail potential are possible in a warm sector of the trough over W Russia. A subtropical jet streak is placed over N Africa. Within a passage of the shortwave trough, severe thunderstorms are possible over N Tunisia and N Aleria where a good overlap of instability and shear will take place.

DISCUSSION

...CNTRL Europe and surrounding areas covered with level 1...

A weakly sheared warm and moist air mass stretches from France, through CNTRL Europe, up to Balkan Peninnsula. Mixing ratios of around 10-11 g/kg and lapse rates amounting 6.5 - 7.0 K/km along with diurnal heating create CAPE of around 1000-1500 J/kg. Evapotranspiration and weak airflow remaining over CNTRL Europe for the last days, contributes to a development of a moist vertical profile with PW exceeding 30mm. Thanks to a meso-low with circulating PV "waves" centred over E France/SW Germany, a persistent convective precipitation overlapping with a large-scale precipitation is expected over area indicated by a level 2. Mesoscale NWP models indicate local accumulated precipitation values exceeding 40-60mm over E France, S Germany, W Austria, Switzerland and N Italy. Large coverage of storm with weak motion, local convergence zones and moisture advection taking place on the orographic lift zones may however contribute to a local precipitation amounts exceeding 80-100mm and thus create risk of flash flooding. A broad level 2 is also justified by the fact that in previous days these areas received already a high amounts of precipitation and a moist soil has a lower capabilities of adopting to another large precipitation amounts. Although SW France, Czech Republic, Slovakia, Hungary Balkan Peninsula will remain under similar airflow and moisture profile environment, a lack of local meso-low with good large-scale lift, provides a lower threat for excessive precipitation than in level 2 area. However, due to orographic lift and moisture converging zones, a local flash flooding cannot be ruled out. This concerns also NE Spain that although has a lower moisture amount in a vertical profile, due to orographic lift interactions also lies under a threat of excessive precipitation. Thanks to almost a not existing vertical wind shear, a threat for severe wind gusts and large hail is strongly limited. However, due to a local CAPE zones exceeding 1500 J/kg, a hail up to 2-3cm cannot be ruled out. A peak precipitation intensity in level 1 area will fall on the late afternoon hours while duration of precipitation in CNTRL Europe may cover the whole forecat period.

...Belarus, W Russia...

Thanks to a good overlap of mixing ratios ~ 10 g/kg with lapse rates over 7 K/km and strong diurnal heating in a warm sector ahead of the cold front approaching from N, a CAPE up to 2000-2500 will develop in the afternoon hours. Although vertical wind shear will be marginal, a strong thermodynamic instability clustering into multicells will be capable of producing severe convective weather phenomena involving in particular large hail, severe wind gusts and excessive precipitation. Increasing LLS in the late afternoon hours may pose a threat for tornadoes, however due to unfavourable undirectional and weak shear, a threat is marginal.

...N Tunisia, N Algeria...

A low pressure area is expected to move NE over Algeria towards the coast. On the north side of the low, north-easterly surface winds will advect moist air from the Mediterranean Sea inland. With strong lapse rates being advected from the Sahara, models agree on a small area of 1000 J/kg CAPE build-up over the level 1 area. Strong capping affects the area delaying initiation until late afternoon. Strong westerly winds associated with a jet streak results in a 20-30 m/s DLS. In addition to that, the easterly component in the surface winds leads to 200-400 m2/s2 0-3 km SRH. In this environment, initiating storms may quickly turn supercellular, and long lasting rotating updrafts are possible. This creates a threat for large hail and severe wind gusts. During the evening, convection is expected to move offshore, gradually diminishing the threat.