Storm Forecast

Storm Forecast
Valid: Sun 06 Jul 2014 06:00 to Mon 07 Jul 2014 06:00 UTC
Issued: Sun 06 Jul 2014 07:39
Forecaster: VAN DER VELDE

A level 2 was issued for northern Spain and southern France mainly for large hail.
A level 2 was issued for northeastern France to western Germany mainly for severe wind gusts, a chance of an isolated tornadoes and large hail.
A level 1 was issued for northern Italy and SW Germany mainly for large hail.


A cold front belonging to a low pressure system over the British Isles and a large amplitude Atlantic mid level trough move into western Europe. Before the front the low-level flow has a strong southerly component advecting warm air which is also rather saturated with mixing ratios of 12 g/kg at 05Z at the surface across parts of France and Benelux countries. In southern France the values even range into 14 g/kg. Resulting MLCAPE values (after mixing the boundary layer) will vary between only 300 J/kg in the north and over 1000 J/kg in the south, respectively, at 12Z. The UK and Ireland will be in the cool airmass with also sufficiently cold ELs to allow thunder, mainly over the UK which lies at the advancing edge of the PV intrusion.


...central France to Netherlands and Germany...

The capping will be weak, but insolation seems to be limited by the broken cloud cover from western France to Netherlands already, so initiation may be questionable there, and GFS model indicates a zone with poor mid level lapse rates in the late morning. On the other hand, a maximum of low/mid level winds and strong low-level shear passes that area in the late morning. Chances for tornadoes and severe gusts would be enhanced, but the confidence is quite low that storms develop in that corner. This may instead happen further to the east in the afternoon. Still the CAPE should be better to the south (eastern France) and also a patch of subsiding motion in mid levels may cross Benelux.
Further to the east and south, there are no strong and well-defined convergence zones found in the GFS model. This may be a good thing in that a dragging mess of convective debris in the only initiation zone may not develop, which allows a number of discrete storms to form over a wider weakly capped region helped by some orographic lifting. However, problematic may become the system that has already developed over southern France and is on its way to northeastern France with extensive cloud cover. Behind it, conditions will likely recover as a sufficiently long period of sun will be available. Ahead of it, too little warming may limit the severe weather potential.
Vertical shear will be sufficent, in principle, for supercells (15 m/s to over 20 m/s) with only slightly enhanced SREH for both left and rightmoving cells. In fact this illustrates a delicate balance between warm air and cold air advection, with the latter environment losing instability. Over northeastern France and northward, low-level shear dominates (10-15 m/s at 12Z, then decreasing). So the threat can be summarized as large hail from supercells, mainly those further to the east where cloud bases can remain higher (including isolated cases in SW Germany), and tornado/bow echo severe gust chances especially from northern France, Belgium, eastern Netherlands and western Germany. Although the limited CAPE and evolution of the early morning convection reduce the confidence in widespread severe weather, a marginal level 2 is issued.
At the west side of the Alps storms may stick along the cold front during the night with some potential for local excessive convective rain.

...southern France...

Given the slowness with which the cold front will invade this area and the presence of already high moisture content (13-14 g/kg), pooling of moisture in the wake low north of the Pyrenees will benefit growth of higher MLCAPE values to 1000-1500 J/kg and this will be combined with 20 m/s 0-6 km shear, sufficient reason to be confident of several supercells with large to very large hail.

...northern Spain...

While there is just 10-15 m/s 0-6 km shear over this area, the front passes only late at night with significant dynamics as the upper trough/PV intrusion curls up and creates a deepening surface low at the Mediterranean coast. The 00Z Zaragoza sounding shows a very steep lapse rate environment with just enough moisture to make CAPE with some lifting of mid levels. The high cloud bases (up to 2000-2500m) and moderate shear support large-hail-producing multicell and isolated supercell storms. SREH values will be low as there is little directional shear, but this chances along the east coast after 00/03Z, together with a convergence zone perpendicular to the Catalonian coast. Large hail should remain the most significant threat in the early morning hours.

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