Storm Forecast
Valid: Tue 12 Aug 2014 06:00 to Wed 13 Aug 2014 06:00 UTC
Issued: Mon 11 Aug 2014 22:01
Forecaster: BEYER
A Level 1 was issued for eastern parts Croatia, northern parts of Bosnia and Herzegovina as well as Serbia, eastern parts of Romania and Ukraine, central parts of Belarus and far western parts of Russia mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.
A Level 1 was issued for the Po valley mainly for severe wind gusts, large hail and to a lesser extent for tornados
A Level 1 was issued for Northeastern Spain mainly for large hail and severe wind gusts
A level 1 was issued for Western France mainly for severe wind gusts
SYNOPSIS
A wide long wave trough is situated with its center over the Norwegian Sea. Due to the trough a more or less zonal orientated frontal boundary is present over most parts of Western and Central Europa. It separates warm and humid airmasses over the Mediterranean and cooler airmasses to its north. The jet-streak of a seasonal quite unusual strong upper level jet covers France, Southern Germany and Czech Republic. At the end of the forecast period a pronounced short wave trough enters Western France from the Bay of Biscay.
Far Eastern Europe and Western Russia are influenced by the main frontal system of the low that is connected to the long wave trough described above. The cold front and a prefrontal convergence zone are forecasted to travel eastward during the forecast period.
DISCUSSION
Parts of Eastern/Southeastern Europa as well as Northwestern Russia...
...along the eastward moving cold front a tongue of humid air with mixing ratios between 10 and 12 g/kg is forecasted. Diurnal heating downstream of the front helps to develop instability that in combination with humidity leads to CAPE values of 800 to 1200 J/kg. Although the development of a convergence can be anticipated from surface streamlines, convective activity should be connected to the passage of the cold front since humidity downstream of the front is quite low.
The CAPE along the front can overlap with DLS (0 6 km) of around 15 m/s. Thus, some better organized convection is possible. Storm motion is forecasted to be more or less from the south to the north and thus parallel to the orientation of the cold front. This will limit the risk for severe wind gusts but enhances the threat for excessive precipitation. Having ppws of 40 to 45 mm and Corfidi vectors that show sideward moving storms emphasize the risk of locally large amounts of precipitation within a short time period.
In addition a few large hail events at the beginning of the convective activity are possible. This risk will be limited later on, when a formation of a convective line is quite likely. If bowing structure can develop along such a line, this will lead to a threat of severe wind gusts.
Po valley...
...humid airmasses are present over the area of interest that mainly originate from the neighbored Mediterranean Sea as surface streamlines indicate. Steep lapse rates are forecasted to develop especially over the mountain ranges. In addition a surface low is forecasted to develop right over the Po valley. This on one hand can help to advect steeper lapse rates to lower lands that then can lead to CAPE values of around 1000 J/kg. On the other hand the development of the surface low also strengthens the shear values. This can nicely be seen by veering hodographs especially in the lower layers. The following shear values are forecasted by NWP: 0-6 km: 25 to 30 m/s, 0-3km: around 15 m/s.
Due to the high shear values well organized convection is possible. A few supercells are forecasted to develop. They can bring large hail and severe wind gusts. Since LCLs are quite low (around 1000 m) and LLS shear will be enhanced by the development of the surface low as well as by the interaction with topographic flow one or two tornados are also possible.
Main uncertainty is the overlap of steep lapse rates and humidity. If those lapse rates manage to travel in the Po valley area even a LVL2 may fulfill. Since this is not certain and LMAs also give only a few signals we only issued a LVL1.
Most of the convective activity is forecasted to take place in the afternoon and evening hours.
Northeastern Spain...
...a surface low is forecasted to be present of the eastern half of the Iberian Peninsula. With the help of this low a constant advection of humid airmasses is forecasted for the coastal regions of Eastern Spain. Mixing ratios of up to 16 g/kg are visible in NWP. On the other hand steep lapse rates that developed due to diurnal heating over central Spain are advected eastward. Both can be seen in the surface streamlines and by a strong signal in moisture flux divergence (0-1km).
CAPE values are forecasted to reach values up to 2000 J/kg and overlap with around 15 m/s of deep layer shear (0 6km). Several storms should develop especially in the afternoon hours that can make use of the ingredients. Large or locally even very large hail are possible when supercells develop. In addition severe wind gusts are possible. Having in mind that LCLS are quite low in the coastal region due to the high boundary layer humidity a tornado that develops with the help of locally enhanced LLS (0 1km) cannot be ruled out.
Since coverage of the convection is forecasted to be not that widespread only a LVL1 was issued.
Western France...
...this region is becoming interesting during the night hours, when a pronounced short wave trough reaches the area of interest and leads to enhanced lift. A wave low on the southern flank of the main low pressure complex is forecasted to develop downstream of the trough axis. Its cold front crosses southern France during the night. Strong low level convergence is forecasted with this front.
Thunderstorms that develop along the frontal boundary have the chance of producing severe wind gusts since a mid-level jet can be found in 850 hPa that has wind vectors of up to 40 kn that are almost perpendicular to the front.
Further inland the risk for thunderstorms is limited since CAPE values rapidly decrease.
Northern/Central Europa lightning areas...
...diurnal heating in combination with cool upper level airmasses lead to the development of a few hundred J/kg of CAPE during the day. Seasonal warm water of the Baltic Ocean and the North Sea will further support this. A few short wave features enhance the lift ingredient and thus the development of convection. This is for instance true for Central Germany where a maximum of IPV is travelling eastward during the day.
Most parts of Northern Europe are influenced by only weak shear. Pulsating convection that may bring especially strong wind gusts and marginal hail/graupel is thus the prominent mode. Since several storms and showers travel over the same area the amount of precipitation can reach higher values in accumulation of the whole forecast period. This is mainly true for the western coastal areas of Denmark, Norway and Sweden due to the westerly flow.
The area affecting Central Germany also shows enhanced values of DLS between 0 and 6 km (around 20 m/s). Thus a few better organized thunderstorms are possible that may produce locally severe wind gusts and marginal hail. However, the lack of CAPE prevents a LVL1 area.
