Storm Forecast
Valid: Wed 31 Oct 2018 06:00 to Thu 01 Nov 2018 06:00 UTC
Issued: Tue 30 Oct 2018 20:14
Forecaster: TUSCHY
A level 2 was issued across S France and far NW Italy mainly for excessive rain.
A level 2 was issued across Corsica, Sardegna, parts of Sicily and the Tyrrhenian Sea mainly for large hail, severe wind gusts and excessive rain. A few tornadoes are also possible.
A level 1 was issued across far NW Morocco mainly for excessive rain.
SYNOPSIS
And yet another deep upper-level trough is entrenched over far W Europe (e.g. France to Portugal/Spain). Upstream broad/strong ridge amplification (partially enforced by hurricane OSCAR's outflow) drives a potent supergeostrophic jet towards Portugal and supports further amplification of the trough. This trough interacts with the residual cool/comparable dry air mass over far W Mediterranean. Substantial weaker temperature gradients (compared to the last event) keep the LL response in form of a broad LL cyclone rather diffuse and weak. Numerous low-amplitude waves circle the main trough an impact Portugal and the W Mediterranean. A deep southerly flow regime evolves over the W/CNTRL Mediterranean ahead of the main upper trough and results in improving BL moisture between Sardegna and Sicily.
Elsewhere no thunderstorm risk exists due to lack of essential ingredients.
DISCUSSION
... S France, NW Italy, Corsica, Sardegna, W-CNTRL Italy and Sicily ...
A missing EML atop the mentioned enhanced BL moisture (CNTRL Mediterranean) and steeper mid-level lapse rates atop meager BL moisture (W-Mediterranean) keep instability on the low to moderate side (mainly in the 400-800 J/kg range with peaks aoa 1 kJ/kg between Sicily and Sardegna). A deep southerly low- to mid-level flow regime results in a prolonged period of warm/moist (hence unstable) onshore flow along the coasts of S France and NW Italy. Orographic support causes heavy to excessive rain. The attendant MUCAPE plume advects a bit inland, so we expect embedded thunderstorms in this otherwise orographically driven rainfall event and hence a level 2 area was issued. Despite moderate CAPE values, the perseverative onshore flow assists in excessive rainfall amounts. An isolated coastal tornado event is possible especially along any persistent convergence zone.
Further southeast (Sardegna and Sicily), the conditions for organized DMC improve during the evening and night. Both a leisurely eastbound drifting and consolidating cold front (just west of Corsica and Sardegna) and a passing mid/upper-level impulse support scattered CI. DLS in the 15-25 m/s range and augmented LL shear support a supercell and multicell risk with all kind of severe, including large hail, severe wind gusts and heavy rain. In addition, LCLs below 1000 m and enhanced LL directional and speed shear add a tornado risk to the general threat. Hence a broad level 2 was issued. Improving upper divergence during the night also enhances the risk for upscale growing clusters over the Tyrrhenian Sea with excessive rain and isolated severe wind gusts/large hail the dominant risk.
... Balearic Islands ...
Repeated thunderstorm development during the day increases the risk for local heavy rain. Meager moisture content preclude an upgrade although isolated flash flooding is possible in case numerous thunderstorms (aligned along mesoscale convergence zones) cross the same area. Stronger storms could also bring isolated hail/graupel partially due to rather low WBZ values.
... NW Morocco ...
Excessive rain is possible as a short-wave passes by during the overnight hours. Flash flood is likely and this event could end up in the level 1/2 borderline range, but potential limited amount of CAPE (due to the subtropical air mass) preclude a level 2 upgrade for now.
