Storm Forecast
Valid: Tue 01 Jul 2014 06:00 to Wed 02 Jul 2014 06:00 UTC
Issued: Mon 30 Jun 2014 22:06
Forecaster: BEYER/GATZEN

A level 2 was issued for north-eastern Spain and South-western France mainly due to large hail and severe wind gusts.

A level 2 was issued for north-western parts of Russia mainly due to large hail and severe wind gusts as well as for tornadoes.

A level 1 was issued for central Ukraine mainly due to severe wind gusts and large hail.

A level 1 was issued for the northern and eastern Alps mainly for large hail.

SYNOPSIS

A broad trough is covering most parts of Europe. One branch of the trough is influencing Eastern Europe. Its axis is orientated from the Baltic States all the way to the western parts of the Black Sea. With a westerly component rather cool air masses are advected into Central, and Eastern as well as Northern Europe. Downstream of the trough axis, hot and humid air masses are advected with a southerly flow into the easternmost parts of Europe as well as Western Russia.

The second branch of the broad trough is influencing Western Europe. During the forecast period the southern part of the positively tilted trough cuts of near the Bay of Biscay. On the downstream axis of the cut of low warm and humid air masses are advected to the north, influencing most parts of Southern Europe (Eastern Spain, Southern France, Italy and the Balkan Peninsula). On the other hand, over Portugal, Western Spain and the British Isles rather cool air masses are active. Both air masses are separated by a quasi-stationary surface cold front.

The ridge between both branches of the broad trough is quite shallow showing only slight anticylonical curvature.

DISCUSSION

Eastern Spain and southern France

During the day instability develops due to the advection of steep lapse rates from the inland of Spain as well as by diurnal heating of the lower layers. This instability overlaps with enhanced low-level moisture characterized by mixing ratios between 10 to 12 g/kg. This results into CAPE values of several hundred up to 1500 J/kg as indicated by WRF model.

Downstream of the upper level trough a surface low is strengthening over the eastern parts of Spain. In addition, due to the southerly flow in mid-levels, lee cyclogenesis takes place north of the Pyrenees. Both results in converging winds and thus lift that can support the development of convection. In addition, embedded in the low pressure area over Spain, a surface cold front that is more or less stationary throughout the day, creates a veritable horizontal temperature gradient (850 hPa: 10 degree over Western Spain and up to 20 degree over eastern parts of Spain). This in combination with more cloudiness in the area of influence of the cold front and insolation to the east of it should support the development of a secondary circulation during the day. That will also support the lift component.

Current thinking is that first thunderstorms can develop in the early afternoon hours over North-eastern Spain. Having rather high values of DLS (0-6km: 15 to 20 m/s) and veering wind profiles organized convection in form of supercells is possible. Large hail and isolated severe wind gusts are the main threat with those cells. Given rather high mid-level winds (i.e. 500 hPa: about 30 kn) and a storm motion vector that is more or less perpendicular to the surface front, excessive precipitation events are rather unlikely.

In the later afternoon hours several Local Area Models indicate that initiation will take place to the north of the Pyrenees. Those thunderstorm can cluster into a multicellular or even mesoscale convective system that then travel further eastward throughout the evening and night hours. Large hail is the main thread at the beginning of this development, whereas severe wind gusts and locally excessive precipitation come into play when a larger system manages to develop. The former is most likely with bowing segments. Due to the low-level advection of humid air masses from the Mediterranean, LCLs a rather low (around and lower than 1000 m). In addition models show enhanced values of LL shear (0-1 km: around 7 m/s) and storm relative helicity. Thus a tornado cannot be ruled out, especially when the low level wind profile is modified by the development of a larger system.

Ukraine and North-western Russia

The cold front of a low pressure that has its centre over north-eastern Europe travels eastward. Downstream warm and humid air masses are advected northward. Highest mixing ratios (9 to 12 g/kg) concentrate along a narrow tongue. The resulting CAPE values of 800 to locally 2000 J/kg can partly overlap with shear values of 15 to 20 m/s (0-6 km). Highest values of shear can be found in the southern-most part of the area (Ukraine). As visible for instance in the IPV forecast or by looking in the 850 hPa isohypses, a pronounced mid-level short wave trough travels eastward that together with the surface front should provide the needed lift.

Organized convection should develop during the day in front of the cold front. Due to the high shear values the development of a few supercells is possible. They can bring large hail and severe wind gusts. Later on single cells may organize into a closed line. This will enhance the threat for severe wind gusts when bowing segments can develop.

In the region of the warm front (north-western Russia) also strong to severe convection is possible. Having high values of low-level shear and low LCLs the development tornadoes cannot be ruled out. Given additionally high values of SRH1km such a tornado may also become strong.


Alpine region

Through-out the day the overlap of instability and low-level humidity results in the development of a few hundreds J/kg of CAPE. DLS is between 15 to 20 m/s and better organized thunderstorms are possible. Supercells may produce large hail and severe wind gusts. Since a synoptic scale lift mechanism is not available, storms are strongly tight to topography, and storm motion vectors to the south-east will disable storms to move away from the Alps. Storms develop especially in the afternoon hours and dissipate rapidly in the evening.

Bretagne

This region is influenced by the upper level low over the Bay of Biscay. Mixing ratios of 9 to 10 g/kg and developing instability due to the combination of cool mid-levels and diurnal heating of lower layer result in CAPE values of up to 800 J/kg. Since shear is only weak (0-6 km: < 10 m/s) pulsating and short living storms are most likely. Marginal hail and a few strong wind gusts are most likely.

Northern Europe

A pronounced short wave trough is travelling eastward during the day. It is accompanied by a modest mid- and upper-level jet. Steep lapse rates overlap with only small values of mixing ratio (6-7 g/kg). Nevertheless, a few hundred up to locally 1000 J/kg are possible in the afternoon hours. Shear is weak in most parts of the affected area. Only to the south values up to 15 m/s are possible. Several thunderstorms that are mostly short-lived should develop throughout the day. Marginal hail and strong wind gusts are most likely. In addition in the centre of the trough one or two tornados may develop in the southern parts of the Baltic Ocean (type-II-tornados). Nevertheless since only little severe events are expected no LVL area was indicated.