Storm Forecast

Storm Forecast
Valid: Fri 20 Oct 2023 06:00 to Sat 21 Oct 2023 06:00 UTC
Issued: Thu 19 Oct 2023 22:15

A level 2 was issued across parts of N Italy mainly for heavy rainfall, tornadoes and damaging wind gusts.

A level 1 was issued across central to S Italy and the E Adriatic coastline mainly for heavy rainfall, tornadoes and large hail.

A level 1 was issued across SE France mainly for heavy rainfall.

A level 1 was issued across SW France mainly for severe wind gusts and tornadoes and to the lesser degree for heavy rainfall.


... SW France ...

A deep low is forecast to arrive over France from the Bay of Biscay. To the south of it, a strong windfield is forecast in the lower troposphere with 850 hPa windspeeds of up to 30 m/s. Hodographs suggest that majority of the shear will be confined in the bottom 1 km with large amounts of streamwise vorticity available for the potential right-moving storms. Low-topped supercells and short linear segments are forecast to develop over the sea and move inland. Severe wind gusts and tornadoes are forecast with the more intense storms. Besides that, given numerous rounds of storms possible and majority of CAPE located in the warm part of the cloud, heavy rainfall may occur as well. The highest threat is forecast between 09 and 15 UTC.

... N/NE Italy ...

Models agree on the presence of very strong vertical wind shear throughout much of the troposphere. 0-3 km bulk shear is forecast to exceed 35 m/s and 0-1 km bulk shear to locally reach 20 m/s. At the same time, strongly curved hodographs are forecast with 0-500 m SRH exceeding 250 m2/s2. The kinematic environment will be thus be conducive for intense supercells with strong low-level rotation. The wind field is forecast to get less pronounced after 15 UTC.

Less agreement among the models concerns the development of surface-based CAPE over the Po valley. While ICON-D2 EPS simulates swaths of intense low-level vorticity tracks, suggestive of surface based supercells, ECMWF and GFS forecast soundings suggest elevated storms over this area. Higher certainty concerning the surface based storms will exist over the coastline of NW Italy. Storms will be initiated primarily through the orographic lift and will spread from Liguria towards the Po valley and the Alps in the first half of the day.

If surface-based supercells develop, tornadoes and damaging wind gusts will be likely. Given the intensity of the low-level shear, tornadoes may be strong. Heavy rainfall will also occur along the southern rim of the Alps and the southwestern rim of the Apennines.

... S to Central Italy ...

Storms are forecast to spread south during the day in the environment of strong vertical wind shear. Across the Tyrhennian Sea and Sicily, the storms are only forecast in the early morning hours of Saturday, coincident with the arrival of the cold front. Besides heavy rainfall and isolated tornadoes, large hail threat will increase towards the south, as the mid-tropospheric lapse rates and abundance of CAPE below -10 degrees isotherm increase. If isolated supercells develop over Sicily in the morning hours, very large hail will be possible as well. Differences in the models concerning the coverage of storms precluded the Lvl 2 issuance.

... E Adriatic Sea coastline ...

High CIN values are forecast across the area throughout much of the day. Gradual synoptic-scale lift is forecast to remove the CIN towards the late evening and night hours, which should lead to the increase in the convective coverage. Heavy rainfall will be the primary threat with the storms, but tornadoes will be possible too along the coastline given the strong low-level shear and high values of SBCAPE in the bottom 3 km. Large hail may occur as well with supercells. There is some disagreement in the models concerning the southward extent of storms with ICON-EU being the most aggresive.

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