Storm Forecast

Storm Forecast
Valid: Sun 18 Aug 2013 06:00 to Mon 19 Aug 2013 06:00 UTC
Issued: Sun 18 Aug 2013 06:20
Forecaster: VAN DER VELDE

A level 1 was issued for eastern Spain mainly for marginally large hail.
A level 1 was issued for southern Norway for a chance of tornadoes.
A level 1 was issued for northern Algeria and Tunesia mainly for severe wind gusts and large hail.

SYNOPSIS

An cold front is entering from the Atlantic into western Europe. Weak elevated CAPE with low convective cloud tops moves over Belgium, Netherlands and northern Germany. Over southern Scandinavia more SBCAPE is mixed in, and the flow is faster which causes stronger shear conditions. The frontal system will move all the way to northern Scandinavia and the Baltic countries before 06Z Monday. Slightly higher CAPE will be present in a band across the Alps and Czech Republic, with a marginal 10 m/s deep layer shear, as well as a patch over eastern Spain. Over all the areas, it appears that low level buoyancy is larger than mid level buoyancy, which can be unfavorable for large hail.
More CAPE is predicted over northern Algeria and southern Black Sea.
During the night, an interesting dynamic trough enters N/E France where slight instability, low LCL and somewhat enhanced shear come together.

DISCUSSION

...eastern Spain...

Model hodographs show slowly SSE-ward storm motion with curvature in the lowest levels, corresponding to somehwat enhanced (100-150 m/s) 0-3 km SREH. Despite the relatively weak mid level buoyancy, a cell may develop a rotating updraft and produce large hail. Delta-theta-e is larger than 20 over this area, and downbursts with strong/severe gusts may be stronger than usual as a result. But there is no organized mesoscale forcing except local terrain, so only an isolated storm or two are expected.

...Algeria/Tunesia...

A cut-off upper low over the western Mediterranean causes some destabilization and deep layer shear enhanced over 15 m/s, allowing supercells and multicells. High cloud bases of 2000-2500m are favorable for the wind gust and hail threats.

...Norway, Sweden...

Unstable air combines with low LCL heights and 15-20 m/s flow at 1-3 km altitude. GFS hodographs are large and clockwise turning in the lowest kilometers. In values, 12-15 m/s 0-1 km shear and around 20 m/s 0-6 km shear are predicted. While elevated parcels produce CAPE over a large area, parcels rooted near the surface are found according to GFS in more confined area over southeastern Norway during the afternoon. It then seems to disappear but another instability and shear optimum would be possible over northern Sweden later, early Monday morning.

...Austria to Czechia...

Good 0-2 km deep convergence and weak capping suggests good chances of storms, which likely organize into multicells. In the south, however, hodographs seem rather small and mid level buoyancy lacking, so large hail may be marginal and very isolated at best. Shear is a bit stronger to the north, but EL heights taper off to only the -15C level. Low level buoyancy in the GFS model shows unusual signal at the north flank of the Alps. If this proves real, it causes enhanced potential for non-mesocyclonic tornadoes (spouts).

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