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 15 April 2009 MDT Risk in South France

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Posts : 91
Join date : 2008-09-20
Age : 46
Location : NDS/Germany

PostSubject: 15 April 2009 MDT Risk in South France   Wed Apr 15, 2009 6:01 am


A ridge of high pressure at mid and upper levels of troposphere is stretching from the Mediterranean into Central Europe. To the west a broad cyclonic vortex will influence the weather development. Its center will be located close to the Spanish NW coast with several impulses rotating around it in strong mid level flow, having windspeeds over 30 m/s. Another mid-level low of smaller scale with center over Belarus / Ukraine border will slowly move southwards on the forward flank of the ridge. Generally, eastern Europe will be placed under a large, but shallow trough. At the surface, low pressure systems will affect W and SE Europe, whereas a high will become well-established over Scandinavia. The main frontal zone will stretch across W Europe with warm front crossing southern England by Wednesday morning and cold front should slowly progress across France and Spain to the east.


... S France / NE Spain....

In the left-exit region of the mid-level jet and under one of the embedded impulses rotating around the low, an upward motion is simulated by the numerical models. At the same time, a trough will cross the region, resulting in the strenghtened flow at lower levels of troposphere, reaching up to 20 m/s at 850 hPa level. Behind the region of stratiform precipitation, ongoing surface heating should lead to the destabilization, with several hundreds J/kg of MLCAPE possible across southern France, locally up to 1000 J/kg. GFS keeps also simulating deep unstable layers, so that integrated CAPE values reach quite high values. Deep level shear will be very strong under the mid-level jet, with values well over 30 m/s. However, the strongest flow will not overlap with the region of instability very well, so that around 15 - 20 m/s of DLS is expected in the region with the best instability release. Veering wind profiles associated with the trough will lead to the increase of SREH, so that more than 200 J/kg are possible. Again however, the region of instability will not correlate very well with the enhanced SREH values.

The storms will be easily initiated with the aid of synoptic scale setup and the first storms will develop already by Wednesday mornings around the Pyrenees region. The coverage will quickly increase by noon and due to the favourable forcing, a cluster of storms ( or even MCS) is quite possible, involving well organized multicells and some brief supercells as well, due to the enhanced SREH values and moderate shear. Stronger cells will have the potential to produce isolated large hail, especially in the case if supercells manage to form. This risk will probably quickly subside after the sunset with the dimnishing instability.

In the region of southern France, belt of strong low level shear is simulated by GFS, exceeding 10 m/s ( GFS 18Z run suggests even more than 15 m/s of LLS combined with surface based instability ). Also, SREH0-1 km shows locally enhanced values over 200 J/kg, overlapping with the eastern border of the unstable airmass. With quite moist air at low levels, low LCLs are expected and therefore, tornadoes are possible, if storms can tap this strong low level shear. This threat might persist into the night hours across SE France, as GFS is simulating some boundary layer instability in the region of strong LLS there and very strong forcing.

Level 2 is issued for the region, where the best overlap of shear and instability will exist and the greatest severe weather coverage is expected.
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