Storm Forecast
Valid: Sat 29 Nov 2014 06:00 to Sun 30 Nov 2014 06:00 UTC
Issued: Fri 28 Nov 2014 18:48
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for E Spain into S France mainly for excessive precipitation.

A level 1 was issued for the Balearic Islands and surroundings for severe wind gusts, tornadoes and excessive precipitation.

A level 1 was issued for N Algeria mainly for severe wind gusts.

A level 1 was issued for N Morocco and coastal areas of S Spain for waterspouts and excessive precipitation.

A level 1 was issued for parts of Scotland for severe wind gusts and to a lesser degree for tornadoes.

SYNOPSIS

A blocked flow pattern covers the European sector. Two upper-level lows sit over Ukraine-Belarus and over southern Spain, respectively. They are separated by an upper-level ridge from the central Mediterranean into France, central Europe and Scandinavia.
The situation at the surface is more straightforward: cold, dry air and high pressure (up to 1040 hPa) over northeastern Europe are opposed by mild, humid air and low pressure (down to 990 hPa) over southwestern Europe. In-between, a remarkable temperature inversion (cf. Fri 12 UTC Bergen sounding: -5°C at 900m and 9°C at 1200m above sea level) has established in a broad belt across central Europe, as a shallow wedge of cold continental air from the east constantly undercuts warm air advection aloft. Further suppressed by large-scale subsidence, this frontal plane remains inactive precipitation-wise (apart from Hungary and Romania).

DISCUSSION

... Mediterranean region ...

Most of Saturday's convection will occur in vicinity or on the forward flank of the Spanish low pressure system.
Warm air advection overspreads much of the Mediterranean Sea and creates a possibly favorable setup for organized storms. Orographic flow modifications, variations of low-level moisture and a complex interplay between lift and a capping inversion make details very tricky. Four areas are discussed in more detail below:

(1) Adriatic Sea:
Still downstream of the main upper-level ridge and ahead of the warm air advection regime, an old short-wave trough crosses the Adriatic Sea on early Saturday morning. A few hundred J/kg of CAPE will be present under rather weak vertical wind shear. Scattered thundery showers may still be active at the beginning of this forecast period, mostly along the coastlines of Croatia, Bosnia-Hercegovina and Montenegro. Subsidence and a strengthening cap should quench further convective activity by noon.

(2) Ionian Sea, Tyrrhenian Sea, western Mediterranean Sea:
Warm air advection creates a wide area of favorable wind profiles for organized storms with deep-layer shear between 15 and 25 m/s and 0-3 km storm relative helicity between 100 and 400 m^2/s^2, generally increasing throughout the forecast period and locally peaking at even higher values. A broad band of stratiform cloudiness and scattered rain will move northeastward during daytime. It will be associated with pronounced warming especially between 850 and 700 hPa as an Africa air mass is ingested. Low-level moisture will be adequate to create CAPE on the order of a few hundred to up to 1000 J/kg over sea surfaces, but the strong cap will mostly suppress storms in the warm sector. Convective initiation will be confined to the vicinity of the eastward moving cold front of the Spanish cyclone, and will await the arrival of vorticity maxima in the left exits of two consecutive jet streaks which round the upper-level low center.
The first vorticity maximum crosses the Baleares and surrounding waters from the southwest in the first half of the forecast period. Its rising motion will likely overlap with the western fringes of the strong shear regime. A couple of well-organized storms are expected, including the possibility of supercells and/or an MCS with a risk of severe wind gusts, tornadoes and heavy precipitation.
In the wake of this first vorticity lobe, the cold front is forecast to become stationary from the Baleares into central Algeria, before it again accelerates eastward with the arrival of the second vorticity maximum overnight. Closer to the Algerian coast, quality of low-level moisture becomes more questionable: strong southwesterly flow across the Atlas mountains (25 to 35 m/s at 850 hPa!) may well push down to the sea in form of particularly warm and dry downslope windstorms, as already indicated by many observations from Friday afternoon. Forecast models still produce patches of CAPE on Saturday and into the following night, though. If thunderstorms form in this environment, strong evaporative cooling as well as orographic channeling along the Tell Atlas may promote severe wind gusts, accompanied by dust storms in inland areas. The level 1 and the low probability thunder areas were extended eastward towards Tunisia to cover such a possibility late in the forecast period.

(3) Coastal areas of E Spain into S France:
On the foreward flank of the Spanish cyclone, strong southeasterly flow extends across the depth of the troposphere. Wind speeds will peak above 25 m/s at 850 hPa. With constant onshore transport of Mediterranean moisture, forecast models agree on precipitation accumulations in excess of 100 mm over coastal parts of Catalonia and in the eastern Pyrenees. Much of this precipitation will be stratiform, but chances of embedded thunderstorms increase in the afternoon and evening with strengthening large-scale lift. Hence a level 2 was issued for a risk of quite numerous flash floods and mudslides.
Precipitation will start to ease overnight and will move more deeply into southern France, where high rainfall accumulations are possible as well but an involvement of thunderstorms is more doubtful.

(4) westernmost Mediterranean Sea, S Spain, N Morocco:
Near the center of the upper-level low, scattered to widespread thunderstorms will form over water and more isolated ones over land. A few waterspouts may spin up along convergence zones, and isolated heavy precipitation events are possible. (Side note: further flash floods become more likely outside of our forecast domain in southern Morocco, where sadly more than 30 people were already killed by floodwaters in the past few days.)

... Scotland ...

An occluding frontal system crosses Scotland from the northwest after 18 UTC while it is overrun by a sharp vorticity maximum in the left exit region of a small jet streak. Strong lift will create neutral or slightly unstable profiles up to 500 hPa. There are limited chances that a little CAPE will partly overlap with strong vertical wind shear and helicity beneath the jet streak.
In case a convective line forms, lightning and wind gusts slightly above 25 m/s are possible, and an isolated tornado is not ruled out. A low-end level 1 was issued to account for such a scenario.