Mesoscale Discussion

Mesoscale Discussion
Valid: Wed 14 Jun 2017 14:00 to Wed 14 Jun 2017 18:00 UTC
Issued: Wed 14 Jun 2017 14:21
Forecaster: PISTOTNIK

... N Italy ...

Latest surface observations indicate 2m dewpoints between 19 and 24C in NW Italy (Piedmonte region). Increasing westerly mid-level winds advect steep lapse rates from the Western Alps over the Po Valley. This combination yields CAPE well in excess of 1000 J/kg, possibly up to 2000 J/kg (see 12z Cuneo sounding).
First explosive storms have recently formed at the dryline (the boundary between moist easterly upvalley winds from the foreland and dry westerly winds mixed down from the mountains). Thanks to 0-6 km shear on the order of 20 m/s, they will quickly organize into supercells and move onto the forelands. Large hail may occur and even exceed 5 cm in size locally. Since vertical wind shear across lower levels is rather weak, the wind risk is lower, though cold-pool-driven severe wind gusts as well as flash floods may become a secondary risk in case a larger cluster forms later on.

Further east, locally lower 2m dewpoints indicate that low-level moisture is more shallow and more prone to being mixed out - e.g., CAPE according to the 12z Milano sounding is only 200 J/kg. However, storms are expected to initiate and become severe as well towards the end of this nowcast period, when increasing easterly winds (resulting from Adriatic sea breezes as well as from an outflow produced by an ongoing storm cluster over Friuli) resupply moisture and a zone of strong convergence is expected along its leading edge.

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