Storm Forecast
Valid: Sun 08 Jun 2014 06:00 to Mon 09 Jun 2014 06:00 UTC
Issued: Sun 08 Jun 2014 07:21
Forecaster: VAN DER VELDE
A level 2 was issued for France, Belgium, Germany mainly for severe convective wind gusts and large hail.
A level 1 was issued for parts of Sweden and northern Russia mainly for excessive convective rain.
A level 1 was issued for Latvia, Estonia, western Russia mainly for a chance of tornadoes and excessive rain.
A level 1 was issued for Bulgaria and Greece for a chance of large hail and severe wind gusts.
SYNOPSIS
A mid level ridge stretches from Africa to central Europe. An Atlantic depression produces strong mid level height gradients with a maximum of over 30 m/s 500 hPa winds from northwestern Spain to the English Channel. West of the ridge a plume of very steep lapse rates is present, advected from northern Africa. It overspreads a narrower region (France, Belgium, N Germany) where low-level mixing ratios are forecast to be 10-14 g/kg, leading to predicted MLCAPE values of 1000-2500 J/kg. The origin of the water vapour appears to be from local evapotranspiration rather than advection and tends to be overestimated by models, particularly GFS. On the other hand, yesterday's measured absolute moisture at the surface was 11-12 g/kg (morning) and 12-14 g/kg (afternoon, Belgium), in line with predicted values. Currently (04Z) central France shows an area with values over 12 g/kg and overnight storms. 00Z soundings do show that drier air will mix out some of the moisture. Unlike yesterday, A source of deep lift (e.g. trough) is lacking.
Eastern Europe also has quite steep lapse rates and may see over 1000 J/kg MLCAPE locally, but is generally dry. More humid unstable conditions prevail over Turkey, northern Scandinavia and Ireland. A mid level cold low is situated over the Baltic countries with lift produced over the cold front pushing into Russia.
DISCUSSION
...France, Belgium, Germany, Poland corridor...
An existing overnight mesoscale convective system will likely continue in some form into Germany. New convection is thought to initiate later during the afternoon over France. An issue for surface-based convection may be the mixing of dry air (see 00Z soundings) with the moist surface layer. There is also a lack of small and large scale forcing which can limit initiation. However, the few storms that initiate may quickly grow larger in an environment with large CAPE, strong vertical windshear, and strong cold pools, and travel large distances. The shear includes 15-25 m/s DLS and over 200 mē/sē of 0-3 km SREH. GFS storm-relative hodographs are large and circular over a large area, although not so much over northern Germany. Multicell clusters and supercells are likely, with the main threats of widespread severe wind gusts and large to very large hail. The gusts are extra supported by large delta-theta-e signifying steep lapse rates and mid level dry air entrainment enhancing cold downdrafts.
Around 21Z low levels in the hodographs show a significant improvement in low level curvature in the Belgium/Netherlands area and a tornado would not be ruled out.
...Baltic/Russian area...
The cold mid level low advances to the east with upward motions overlapping with unstable airmass. An isolated excessive rain event cannot be excluded near the center of the low where storm motion is slower and strong low-level buoyancy supports spout-type tornadoes. Hail will be likely but small in size.
...Sweden, northern Russia...
In these two areas very weak storm motion vectors, high precipitable water and relative humidity, some CAPE and convergence combine which can lead to excessive convective rain events. Large low-level CAPE can help spin up spout-type tornadoes.
...Bulgaria and Greece...
After 18Z moderate low-level wind shear develops in this region while weak CAPE environment may still allow some thunderstorms. These may get gusty and an isolated tornado is not ruled out. But the chance exists that storms form only earlier when shear is mostly negligible but CAPE over 1000 J/kg. Isolated large hail may occur. There is no clear mesoscale form of lift.
