Storm Forecast
Valid: Fri 09 Nov 2018 06:00 to Sat 10 Nov 2018 06:00 UTC
Issued: Thu 08 Nov 2018 23:38
Forecaster: PISTOTNIK
A level 1 is issued for S France mainly for flash floods.
A level 1 is issued for NW Italy for flash floods and (non-supercellular) tornadoes.
A level 1 is issued for NE Algeria and NW Tunisia mainly for flash floods.
A level 1 is issuedn for W Ireland for severe convective wind gusts and to a lesser degree for tornadoes.
SYNOPSIS
A long-wave trough at mid-levels stretches from England to E Spain at Fri 06 UTC. Its S part cuts off into the Mediterranean basin. Its N part is reinvigorated by a new short-wave that arrives along with a powerful frontal system from the Atlantic Ocean.
Further small cut-off lows at mid-levels are located over the Ukraine, Turkey and off the Libyan coast. Otherwise, the entire E half of Europe is under anticyclonic influence with mild and quiescent conditions.
DISCUSSION
... W Mediterranean region ...
A zone of strong synoptic lift spreads SE-ward ahead of the mid-level low and steepens the lapse rates. Along with seasonably rich low-level moisture, CAPE on the order of 400 to 800 J/kg will build and allow scattered thunderstorms. Vertical wind shear decreases to values around 10 m/s across the lowest 3 km due to the gradual filling of the low and convective overturning.
At the N flank of the low, two foci of interest emerge from the morning to early afternoon:
(1) Onshore and upslope flow along the SE slopes of the Massif Central may support some hours of heavy rain with embedded thunderstorms and a risk of flooding.
(2) Fine-meshed models also agree on a small area of stationary thunderstorms at the Genoa coast, where a land breeze front may establish in an otherwise weak wind field. In addition to localized heavy rain, one or two waterspouts are possible.
Further south, storms will move eastward more progressively. Isolated severe wind gusts and marginally large hail are not ruled out in case multicells form, but the risk appears too low for a level 1. The strongest synoptic lift overspreads NE Algeria in the evening and Tunisia overnight and, along with converging onshore and upslope flow, will likely keep storms active into the night with a risk of flash floods.
... E Mediterranean region ...
Some hundred J/kg of CAPE in weakly sheared, warm air allow scattered, disorganized convection across a wide area, though synoptic subsidence between the various upper-level lows may limit its coverage and depth. A few waterspouts are possible along various leftover convergence zones.
... British Isles ...
The sharp cold front of the Atlantic cyclone will cross Ireland in the 11 to 18 UTC time frame. Forecast models agree on marginal CAPE and a very strong wind field with up to 20 m/s shear and up to 400 m^2/s^2 storm-relative helicity across the lowest kilometer, respectively. Supported by strong lift ahead of the new Atlantic short-wave trough, a cold frontal rainband could form. Severe wind gusts are possible, and one or two tornadoes are not ruled out in case discrete stronger updrafts form within such a line. It is questionable if convection will be deep enough to produce lightning, though, hence only a level 1 is issued.
The cold front will encounter more stable air along its further path over England and Scotland and any remaining convection will gradually weaken. Scattered to widespread, non-convective severe wind gusts are still possible in the warm sector and along the cold front, but they are not reflected by the ESTOFEX threat level scheme.
Scattered postfrontal showers will gradually grow deeper and might produce a little lightning over the Celtic Sea towards the end of the forecast period, but by then the wind field should relax enough to keep the wind gusts below 25 m/s.
