Storm Forecast
Valid: Wed 19 Sep 2018 06:00 to Thu 20 Sep 2018 06:00 UTC
Issued: Tue 18 Sep 2018 20:03
Forecaster: PUCIK

A level 1 was issued across Sardegna, Corsica, Tunisia, Italy and the Tyrrhenian Sea mainly for excessive rainfall.

SYNOPSIS and DISCUSSION

Europe is generally divided into two synoptic-scale regimes. Northwestern and Northern Europe is under influence of an active weather pattern with series of short-wave troughs rapidly moving in SW-ly to W-ly mid to upper tropospheric jet-stream. During the forecast period, one of the short-waves will cross British Isles into the Norwegian Sea, associated with a strong synoptic-scale lift and rapidly deepening low pressure system. Scattered DMC may form along and behind the advancing cold front, though organised severe thunderstorm risk is not foreseen. Particularly over Scotland, severe non-convective wind gusts can be expected as 850 hPa flow will exceed 40 m/s atop steep lapse rates.

To the south, flow gradually weakens. A weakly cyclonic disturbance progresses E-ward slowly and will reside over the Tyrrhennian Sea during Wednesday. Tuesday 12 UTC soundings from Decimomannu, Pratica Di Mare and Trapani reveal very moist, unstable airmass with PW values between 40 and 50 mm, combined with high RH and low LCLs/LFCs. Scattered to widespread DMC is forecast to initiate in these conditions with threat of very heavy rainfall. With the large amounts of latent heat release in the convective storms, pressure is forecast to drop within the low to mid-troposphere. Towards the night and morning hours, it is possible that strengthening low-level wind field on the flank of the deepening low will result in an organised MCS with very high excessive rainfall threat. The exact location is hard to determine, differs in individual NWP runs and will strongly depend on antecendent development. Current thinking is that such MCS will most likely form E of Sardegna. Lvl 2 was considered for E part of coastline, but was discarded as most of precipitation is simulated offshore.