Storm Forecast
Valid: Thu 07 May 2009 06:00 to Fri 08 May 2009 06:00 UTC
Issued: Wed 06 May 2009 22:36
Forecaster: GATZEN

A level 1 was issued for southern France and north-eastern Iberian Peninsula mainly for severe hail.

A level 1 was issued for western Black Sea region mainly for severe hail.

SYNOPSIS

The geopotential will further rise across south-western Europe in the range of a broad subtropical ridge. As a new Atlantic trough amplifies into north-western Europe, weak upper disturbances will affect portions of west Europe during the period. To the east, an quite intense short-wave trough cuts off over the Baltic Sea region and moves to the Black Sea during the period. A strong mid-level jet will be present along its western flank.

DISCUSSION

North-eastern Iberian Peninsula, southern France

At the southern flank of the main westerly jet, a relatively weak upper trough migrates eastward during the day reaching the west Mediterranean Sea in the evening hours. Rather cool mid-level air mass is present in the range of the trough and spreads across the Iberian Peninsula and southern France. While cool low-level air mass is expected to spread into the western portions of the Iberian Peninsula, the approach of the mid-level trough will likely be associated with destabilization further east as diurnal heating will be in place over the Iberian plateau, and steep lapse rates will likely spread across eastern Iberian Peninsula and southern France.

Main uncertainty concerns the content of low-level moisture. Latest observations indicate relatively poor mixing ratios below 10 g/kg over most places with the highest values across southern France and north-eastern Iberian Peninsula. A significant increase in low-level moisture is not expected on Thursday, and current thinking is that diurnal heating will lead to dry inverted-v profiles. With decreasing mid-level temperatures, CAPE in the order of 500-1000 J/kg seems to be likely in the afternoon hours, though.

Given a weak thermal low across the Iberian Peninsula, slightly veering winds are expected across southern France, and the development of weak convergence lines is likely north of the Pyrenees. Upslope flow along the eastern coasts of the Iberian Peninsula will also cause some low-level forcing. A few storms are likely in the afternoon and evening hours that will have a potential of severe hail with the stronger storms. Especially in the southern portions, a severe wind event is not ruled out given locally large downdraft CAPE.

Storms will likely cluster given only weak vertical wind shear and cool downdrafts in the afternoon hours. In the evening, models indicate increasing 700 to 500 hPa westerly winds at the southern flank of another trough across northern France. Over most places, this is not expected to increase the chance of severe weather significantly due to decreasing low-level convergence and weak QG forcing, although vertical veering profiles may slightly improve. However, latest models indicate low-level convergence north of the Pyrenees to continue in the evening hours, leading to low-level moisture flux convergence as well as increasingly favourable veering profiles. Supercells are not ruled out along and east of this boundary when it really develops and severe hail may be a threat in the evening hours as well. Although a tornado is not completely ruled out, lack of rich low-level moisture and instability will be a strong limiting factor. Over the Iberian Peninsula, weak forcing will likely suppress convection in the evening hours.

Central France to western Alpine region, western Germany

On Thursday, a short-wave trough will move into France in the evening hours and will reach western Germany on Friday morning. At low levels, rather warm air masses originating from southern France will spread north-eastwards. Showers and thunderstorms are likely given rather steep mid-level lapse rates and QG forcing. Limiting factor will be the cool and dry boundary-layer, and storms will likely be elevated. The best potential for surface-based storms exists over central France, where low-level moisture will be sufficient in the evening hours. Given rather strong westerly winds in the 700 hPa level, some storms may organize, but threat of severe hail seems to be too weak for a level 1.

Eastern Ukraine

As the cut-off low moves southward into the Black Sea region, warm air masses spread northward ahead of a cold front reaching the eastern Ukraine in the late morning hours. Warm air mass will likely be characterized by rather moist low levels and moderate mid-level lapse rates, and CAPE is expected to develop in response to some diurnal heating. Given strong forcing along the cold front, showers and thunderstorms will likely develop. As vertical wind shear is moderate, organized storms are not ruled out, capable of producing large hail. The chance seems to be too marginal for a level 1, though.

Western Black Sea region

Ahead of the approaching cut-off low, a strong mid-level jet streak moves southward into the western Black Sea region in the afternoon hours, providing QG forcing. At lower levels, models indicate that a tongue of relatively warm and moist air mass remains ahead of the surface cold front reaching the Black Sea in the evening. Ahead of this front, veering profiles are forecast to overlap with strong deep layer vertical wind shear in the afternoon hours. Current thinking is that showers and embedded thunderstorms will likely form along the cold front, and the stronger storms may pose a weak threat of large hail. Limiting factor is the weak instability, and the level 1 may be too optimistic. Storms will likely weaken when moving over the rather cold Black Sea.