Storm Forecast
Valid: Fri 22 Aug 2014 06:00 to Sat 23 Aug 2014 06:00 UTC
Issued: Thu 21 Aug 2014 20:48
Forecaster: BEYER

A Level 2 was issued for Eastern Spain mainly for large hail as well as locally excessive precipitation and severe wind gusts.

A Level 1 was issued for Bosnia and Herzegovina, southern Romania as well as parts of Croatia, Macedonia (FYROM), Albania and Bulgaria mainly for large hail and to a lesser extent for excessive precipitation and severe wind gusts

A Level 1 was issued for the coastal areas as well as islands in the Baltic Ocean and the eastern part of the North Sea.

SYNOPSIS

An extensive long wave trough is influencing most parts of Europe with rather cool and unstable airmasses. The main center of the trough complex will be situated over the North Sea between Scotland and the Norwegian coast. Several short wave troughs can be found on its southern flank that travel eastward / northeastward during the forecast time. The most prominent ones can be found over eastern Europa and Great Britain.

On the forward flank of the former one a surface low can be found. A cold front that is associated with this low is moving eastward influencing parts of Eastern Europe and Western Russia. The short wave trough over the British Isles is connected to a surface low over the North Sea. Its partly occluded frontal system is moving from Great Britain east/southeastward influencing Northern France, BeNeLux, Denmark, Northwestern Germany and southern Scandinavia during the forecast time.

On the other hand most parts of Southern Europe and the Balkans are under the influence of warm and partly humid airmasses

DISCUSSION

Eastern Spain...

...a coastal boundary is present that separates quite humid airmasses originating from the Mediterranean and dry but potential unstable airmasses from the Spanish inland. The humid airmasses are characterized by mixing ratios of 10 to 14 g/kg while the unstable airmasses show lapse rates exceeding a vertical temperature gradient of 8 K/km. In the regions were both airmasses can partly overlap CAPE values between 1000 and 1500 J/kg are forecasted. Best conditions can be found in the coastal regions were CAPE values even can reach 2000 J/kg.

Shear values are forecasted to be rather high having a deep layer shear (0  6 km) of 15 to 20 kn, in the evening hours locally even 25 kn. In addition 0 - 3 km shear is forecasted to have values up to 15 kn. Having those high values of shear, supercells are quite likely to form. High CAPE values and combination with high shear can produce large or even very large hail. In addition severe wind gusts are locally possible. In a later stage also excessive precipitation cannot be ruled out having ppws between 30 to 35 mm.

Main problem, during most parts of the day is the lack of lift. This will change in the late afternoon and in the evening when a short wave trough (nicely visible by an IPV maximum) enters the area from the west.

Bosnia and Herzegovina, southern Romania as well as parts of Croatia, Macedonia (FYROM), Albania and Bulgaria...

...mixing ratios are forecasted to lie between 11 and 13 g/kg. Due to diurnal heating steeper lapse rates can develop in the afternoon and overlap with those humid airmasses. As a result CAPE values between 500 and 1000 J/kg can develop. At the same time rather high DLS values (0  6 km) of 20 to locally even 25 kn are forecasted. This enhances the chance for organized convection that may be supercellular in an early stage and can be become multicellular later on.

Large hail is the most prominent threat while severe wind gusts are connected to the strongest storms since the background wind field is only weak (winds in 850 hPa: 5 to 10 kn) . An excessive precipitation event that can occur in a later stage of the convection can also not be ruled out but should be of minor importance since ppws are only around 30 mm.

While thunderstorm are most active in the afternoon hours this activity is forecasted to decrease quite rapidly in the evening hours due to the lack of insolation and the lack of a pronounced lift mechanism.

Coastal areas as well as islands in the Baltic Ocean and the eastern part of the North Sea...

...having cool airmasses in upper levels (T500: between -20 and -25°C) and really warm water surfaces (North Sea: mostly 16 to 18°C, Baltic Ocean: mostly 16 to 19°C) rather high instability is forecasted especially over the ocean and the coastal areas. Although mixing ratios are only between 6 and 8 g/kg, a few hundreds of J/kg of CAPE are forecasted. In the northernmost part of the Baltic ocean values even exceed 500 J/kg.

Therefore convection is forecasted in an environment of low vertical shear. Since upper level winds are rather slow and LCLs are around 1000 m (and lower), low level convergence can lead to the development of water spouts. Having a westerly flow this is mainly true for the westerly exposed coastal areas and islands. If a line of convection develops that is moving over one and the same area the accumulation of larger amounts of precipitation is possible. However, LAMs do not highlight this threat as it was the case during the last weekend. So this threat is quite local and of only minor importance.

The activity is independent of the time of day. Activity is connected to the coolest mid-level airmasses and thus linked to short wave troughs or short wave features.

Rest of the forecast area...

...over Finland steep lapse rates are forecasted to overlap with mixing ratios of up to 8 g/kg. Thus convection is forecasted to be more widespread. Having only little vertical shear values this convection should have a pulsating character. Marginal hail and local stronger wind gust are possible with the strongest storms.

The partly occluded front is forecasted to enter BeNeLux and Northern France during the afternoon hours. Higher values of mixing ratio surrounding this frontal boundary lead to CAPE values of 200 to 400 J/kg. Thunderstorms that develop should stay non-severe having rather weak vertical wind shear.

The cold front over Eastern Europe/Western Russia is characterized by a narrow tongue of enhanced humidity (mixing ratios between 10 and 12 g/kg). The highest values can be found around the Black Sea (e.g. Crimea: 13 g/kg). Thus, the highest CAPE values are also available in this region. However vertical shear is forecasted to stay mostly below 15 kn. To the west/southwest of the Black Sea shear is higher but instead initiation is more questionable. However, if storms manage to form large hail is possible. Since this is too uncertain, no level area was drawn.