Storm Forecast
Valid: Fri 03 Nov 2017 06:00 to Sat 04 Nov 2017 06:00 UTC
Issued: Thu 02 Nov 2017 20:29
Forecaster: PISTOTNIK

A level 1 is issued for Portugal for excessive convective precipitation and (non-supercellular) tornadoes.

A level 1 is issued for SW Spain mainly for excessive convective precipitation and to a lesser extent for tornadoes, severe wind gusts and large hail.

SYNOPSIS

A mid-level trough with a positive tilt stretches from the Ukraine to Austria at Fri 06 UTC and amplifies towards Greece and Turkey. Another mid-level trough approaches the British Isles from the Atlantic Ocean and gradually catches a cut-off low near the Portuguese coast. In-between, mid-level ridging extends from the central Mediterranean region towards southern Scandinavia.
Under weak pressure gradients or even anticyclonic influence, much of Europe is covered by cool and dry air which precludes any deep convection. Richer low-level moisture is present over the Mediterranean Sea and over parts of Iberia. In the latter region, low- to mid-tropospheric lapse rates are also steep enough to allow a little CAPE.

DISCUSSION

... SW Europe ...

In the vicinity of the cut-off low, moist and cool air on top of mild sea water will create a few hundred J/kg of CAPE under weak vertical wind shear. Scattered to widespread thundery showers are expected throughout the forecast period over offshore and coastal areas, and a level 1 is issued for a risk of a few flash floods and one or two non-supercellular tornadoes.
Two special foci for convection exist:
(1) Persistent SW-erly flow advects rich moisture some distance inland into SW Spain (Extremadura and W Andalusia). 0-3 km shear is still enhanced to 15 m/s beneath a departing mid-level jet, which could suffice for better organized storms. In addition to the mentioned main risk of excessive rain and isolated tornadoes, (marginally) large hail and severe wind events are not ruled out in case supercells can form. However, moist profiles and limited instability keeps these risks comparably low. The activity will peak in the afternoon, but increasing synoptic lift may keep it going all night long.
(2) A strong mid-level vorticity maximum will swing around the cut-off low and enter central and S Portugal in the evening and overnight. A high thunderstorm coverage with an enhanced flash flood risk is anticipated, but model forecasts diverge too much with respect to the placement of convergence zones and rainfall peaks to issue a level 2.

CAPE will also gradually build over the W Mediterranean Sea, as an elevated mixed layer spreads over it. Steep lapse rates and strong vertical wind shear would easily support organized storms with a hail and wind risk. However, the capping inversion is very strong and will delay convective initiation to the late hours of this forecast period or even beyond. Current thinking is that first elevated convection may develop towards morning between the Gulf of Valencia and the Lion Gulf, but surface-based storms are not yet expected until the end of this forecast period.