Flash flood in Northern Italy

Foehn winds in the Alpine area are definitely not a unconvenient situation during autumn. But the current synoptic setup results has some extreme effects. On the southern part of the Alpine arc the accumulated rain exceeded several 100mm while in Germany temperatures up to 24 °C were observed.

Let’s have a look at the current synoptic setup:

A cut of low in the Mediterranian area advects very warm air on his eastern flank northwards. Over the still warm water of the Mediterranean Sea (with SST above 20°C) can soak up moisture. This moist low level flow pattern with ThetaE greater than 40°C is blocked by the Alps and results in heavy rain. The rain rate is further enhanced by convective storm developments. Some models prognosted more than 600mm accumulated over the whole rain event. Reason is the very stable situation with the blocking high pressure system over eastern Europe.

Geneva was already hit by a flash flood. The following videoclip gives an impression of the devastating local effects:


Some weather forecast models show the possibility that this low pressure system evolves under low shear conditions into a system that can be compared to tropical stroms. These typically develop in the equatorial area when SST exceed 27°C. Contrary to the low pressure systems in middle latitudes shear is not favorable for development. Therefore the initiation process of the forecasted “subtropical storm” over the Mediterranean Sea next week is different. But the appearence with a warm core, and high wind values surrounding a wind-minimum in the centre is very similar. And in this form a very rare event.


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