Storm Forecast

Storm Forecast
Valid: Tue 03 Nov 2015 06:00 to Wed 04 Nov 2015 06:00 UTC
Issued: Mon 02 Nov 2015 20:33
Forecaster: PISTOTNIK

A level 1 was issued for S France and adjacent sea areas mainly for excessive precipitation, tornadoes and to a lesser extent for severe wind gusts.


A broad upper-level ridge into central and E Europe continues to be the dominant feature on the weather maps. It is surrounded by a strong zonal flow from Iceland across Scandinavia into Russia, a cut-off low that moves from the Bay of Biscay towards England, and rather low geopotential over the E Mediterranean Sea. Mild air masses prevail, apart from near-surface layers over the continent which tend to cool down under anticyclonic influence at this time of the year.
Most of the convective activity will occur on the forward flank of the cut-off low and will be related to a short-wave trough which shifts NE-ward over France and the W Mediterranean Sea.


... W Mediterranean into S France ...

A plume of very warm air with steep vertical temperature gradients, originating from Algeria, wraps northward and overspreads a seasonably moist boundary layer over the Mediterranean Sea. This combination results in CAPE on the order of some hundred to possibly 1000 J/kg, which is strongly capped.
Large-scale lift ahead of the trough axis will slowly erode the capping inversion. The best chances for surface-based convection exist near the French coastline, where lift is maximized by still strong onshore/upslope flow in the morning and the following passage of a wind shift line. It is likely that the current forecast period starts with an active MCS just offshore which works its way eastward in the 06 to 12 UTC time frame. With 0-3 km vertical wind shear between 10 and 15 m/s, an embedded supercell is possible at the southern flank of such a system, which could spawn one or two tornadoes or produce marginally severe wind gusts in offshore and coastal areas. Onshore areas will mostly see some hours of heavy rain with a yet unclear convective contribution. Due to a limited duration of the strongest rainfall and resulting lower precipitation totals than on Monday, a level 1 seems sufficient this time (NWP models predict peak accumulations around 50 mm).
After 12 UTC, the convergence line continues to shift eastward until it will become stationary over the Gulf of Genova overnight. Under subsiding CAPE and shear, thunderstorms become less likely but scattered showers are still possible and pose the risk of a few waterspouts.

Current thinking is that the capping inversion will otherwise hold over the W Mediterranean Sea, though there are chances for elevated convection throughout the forecast period. Lightning activity will be limited and severe weather is not expected.

... inland and W France ...

The short-wave trough will also cross France from SW to NE during daytime. Its lift will partly be compensated by simultaneous cold air advection, but especially in case of some insolation, a few hundred J/kg of CAPE may temporarily be realized.
Isolated storms may develop which could consume just enough vertical wind shear for multicellular organization before the wind field relaxes. Marginally large hail or severe wind gusts are not ruled out. However, the time frame for surface-based storms is limited to the noon and afternoon hours, and the possibility of severe weather appears too low for a level 1.

... Spain, Portugal ...

Well-mixed maritime air is advected into Portugal and Spain from the SW, and showers may become deep enough to produce a little thunder in the first half of the forecast period. Little CAPE and weak vertical wind shear make severe weather unlikely. The arrival of a new frontal wave will quench further convective activity in the evening.

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