Storm Forecast
Valid: Sun 22 Jun 2014 06:00 to Mon 23 Jun 2014 06:00 UTC
Issued: Sat 21 Jun 2014 22:05
Forecaster: GATZEN/BEYER

A level 1 was issued for south-western France mainly for large hail, excessive precipitation, and severe wind gusts.

A level 1 was issued for northern Spain mainly for large hail and severe wind gusts.

A level 1 was issued for north-eastern Spain mainly for large hail and excessive precipitation.

A level 1 was issued for the southern Alpine region (Maritime Alps) for large hail and locally excessive precipitation

SYNOPSIS

A large and wide upper level trough is influencing most parts of eastern Europe and western Russia. Further to the west a ridge can be found that shows a negatively tilted axis reaching from the central Mediterranean seat to the British Isles. Therefore most parts of Central Europe are flooded with a cool air of polar origin.

Southwestern Europe, then, is affected by a substantial upper level low situated to the west of the Iberian Peninsula. On its forward flank, a warm and well mixed air mass is advected into parts of Spain and the southern half of France. In the boundary-layer, moisture increase is expected especially over eastern Spain and southern France, leading to CAPE. In addition, short wave features are forecasted to travel from Spain into southern France during the evening and night hours of the forecast period.


DISCUSSION

Northern and north-eastern Spain, south-western to central France

An elevated mixed-layer has spread across eastern Iberia, the west Mediterranean, and southern France. Low-level moisture has started to increase especially over south-western France, but latest soundings do not indicate rich low-level moisture yet.

On Sunday, thermal low over Iberia will allow for persistent low-level convergence over northern Spain and southern France. Models agree on the moisture pooling and indicate a 30 hPa mixed layer mixing ratio of about 12 g/kg north of the Pyrenees and along the sea-breeze convergence over eastern Iberia in the afternoon. CAPE up to 1500 J/kg is likely due to strong diurnal heating.

Main focus of initiation will be the mountains over northern and north-eastern Iberia. Storm organization will increase in the western portions, where deep layer vertical wind shear of 15 m/s is forecast. Multicells and supercells are not ruled out, capable of producing large hail. Further east, weaker storm organization limits the threat of large hail, but slow moving storms may lead to excessive precipitation locally.

Later in the period, some QG forcing due to passing short-wave troughs may allow for an upscale growth of storms. Expected region will be the convergence zone north of the Pyrenees, where a cluster of storms may develop in the evening and night hours. Large hail and excessive precipitation are possible, as well as severe wind gusts. The cluster is forecast to move north-east into the Massif Central, where excessive rain is expected to be the main threat in the night hours. Given the rather weak vertical wind shear and some uncertainties about the amount of moisture, a level 1 is issued. If moisture will evolve as good as models predict, severe reports may match the level 2 criteria.

Alpine region, northerly Apennines

Steep lapse rates overlap with mixing ratios of 10 to 12 g/kg which results in 1000 to 2000 J/kg of CAPE. Due to the lack of synoptic scale lift, convection that develops preferentially during the afternoon hours is strongly coupled to the topography. DLS values are mostly around or slightly above 10 m/s. Thus pulsating storms that influence each other and give rise to new developments along outflow boundaries are most likely. Given the high CAPE values, locally large hail events are possible especially in the early stage of a thunderstorm development. Due to topographical effects, also an excessive precipitation event cannot be ruled out. A LVL1 area was indicated in the regions were conditions are most likely for a severe weather event.

Later in the afternoon storms may grow into the forelands in north-western Italy.

Northern and eastern lightning areas

Lapse rates are forecasted to steepen due to the combination of cool upper air masses and warming of lower levels due to diurnal heating, with best lapse rates near the trough centre over the Baltic Sea region and at a short-wave trough axis that crosses Belarus eastward. Low-level moisture is poorly developed with a maximum over Belarus and western Russia, where CAPE can be expected to exceed 500 J/kg.

Diurnally driven storms are forecast, especially over southern Scandinavia and from Belarus into western Russia. Given a rather strong north-westerly flow, these storms may organize and small bowing storms are not ruled out, capable of producing severe wind gusts. Marginally hail is not ruled out. Storms will rapidly weaken after sunset.