Storm Forecast
Valid: Wed 17 Oct 2018 06:00 to Thu 18 Oct 2018 06:00 UTC
Issued: Wed 17 Oct 2018 06:08
Forecaster: PUCIK

A level 2 was issued across E Algeria and Tunisia mainly for excessive rainfall, large to very large hail and tornadoes.

A level 1 was issued across C to W Mediterranean mainly for excessive rainfall.

SYNOPSIS

Much of continental Europe remains under dry and stable conditions with an extensive high pressure system at the surface, centered over NE Ukraine, now slowly weakening. This weakening is partly due to the cut-off cyclonic vortex centered over the Czech Republic, slowly moving N-ward. Active weather pattern persists over Scandinavia under the influence of a strong zonal flow with low-amplitude wave crossing the W part during the overnight hours. This short-wave will have detached from a longer amplitude wave, its base separating into a separate vortex and heading towards SW Iberia together with a cold front. Isolated to scattered thunderstorms are forecast along the S coastlines. Surface cyclogenesis is also forecast over E Algeria and Tunis, with WAA and E to SE-ly surface flow observed over the Tyrrhenian Sea and further W towards the Balearic Islands. Most of the DMC is forecast in this area.

DISCUSSION

... E Algeria, Tunisia ...

Widespread thunderstorms are already ongoing over E Algeria in moist easterly flow, slowly backbuilding towards Tunisia. Continuous inflow of moist airmass from CAPE-rich reservoir over the sea (where steep lapse rates from Sahara overlap with abundant moisture) will likely further feed the ongoing convective storms and sustain them for hours. High threat of excessive rainfall and of flash flooding is forecast for the region in this setup. E-ly surface flow changing into SW-ly with height will likely result in quasistationary convective systems in some places, exacerbating the heavy rainfall risk. This risk is forecast to increase overnight as the lower tropospheric flow increases from E, yielding more powerful inflow to the storms.

Apart from heavy rainfall risk, threat of large to very large hail and also from tornadoes is forecast over E and SE part of the Level 2, where curved hodographs with high SRH would likely result in several strongly rotating supercells. Tornado threat will be enhanced with high LLS and also with low LCLs along the coastlines. What remains questionable is the degree of CIN and its effect on initiation in the high CAPE and strong shear regime of SE and E part of the Lvl 2 area.


... Tyrrhenian Sea, E coastlines of Sardegna and Corsica, N Algeria, parts of the Balearic Sea ...

Sounding observations from 00 UTC suggest a continuing threat of heavy rainfall within thunderstorms in this area as skinny CAPE profiles combine with low LCLs and high RH throughout the lower and mid troposphere. Furthermore, vertical wind profile suggest slow moving or outright quasistationary convection. Scattered storms are already ongoing in the area and the coverage is only expected to increase during the forecast period. Other threats, compared to rainfall, will be much less pronounced.