Storm Forecast
Valid: Fri 20 Nov 2015 06:00 to Sat 21 Nov 2015 06:00 UTC
Issued: Thu 19 Nov 2015 23:36
Forecaster: PISTOTNIK

A level 1 was issued for the United Kingdom for severe convective wind gusts.

SYNOPSIS

A strong frontal zone runs from the Celtic Sea eastward to the Ukraine on Friday at 06 UTC. It gets pushes towards the south, as a pronounced long-wave trough starts to amplify SE-ward over Scandinavia, the North Sea and the British Isles.
Near the surface, an outbreak of polar air starts flooding much of Europe, and it further sharpens and activates the frontal boundary. Two frontal waves cross the Ukraine and central Europe, respectively. Within the cold air, various low-pressure cores persist or form anew over the Baltic and North Sea.

DISCUSSION

... France, Switzerland, S Germany, Austria, Hungary ...

The second and stronger one of the mentioned frontal waves will form over the English Channel at the beginning of this forecast period. The model pool is highly discordant with respect to its evolution and track, which makes forecasting particularly challenging this time.
The Thursday 00 UTC runs staked the two most extreme solutions: BOLAM suggested a northerly path across the Czech Republic into the N Ukraine and an explosive deepening from 1010 hPa to 978 hPa until Saturday 06 UTC. On the other hand, WRF with GEM initial conditions showed no frontal wave at all (!) but a slow and steady southward motion of the cold front, resulting only in a benign lee cyclogenesis over N Italy with a minimum pressure still above 990 hPa late in the forecast period.
Later model runs started to converge towards intermediate solutions. Therefore a track of the new cyclone right along the N Alpine rim into Slovakia and the W Ukraine and a deepening to about 985 hPa seem to be the most plausible scenario. The high sensitivity of this development can be explained by a strong feed-forward process with large-scale lift ahead of the deepening trough in case of a northely path, but little or no such interaction in the case of a southerly path.
Fine-meshed forecast models agree on the formation of a narrow convective line along the cold front. It will cross France, Switzerland, and possibly S Germany (in case of a northerly path of the cyclone). The Alps will then destroy its structure before it will likely re-organize over Hungary after midnight. Placed on the anticyclonic flank of the jet streak, this line will probably stay shallow. Only in case the cyclone passes yet further north, the line could grow deep enough over S Germany to produce limited lightning activity and cold-pool-driven wind gusts. However, this scenario appears too unlikely to issue a lightning or level 1 area.
The primary risks of this frontal wave are of non-convective nature: The strong wind field (westerly winds with 25-40 m/s at 850 hPa) and the high moisture load in its warm sector will result in precipitation sums over 50 mm in the French and Swiss Alps and gale- to hurricane force gusts on the mountains. Severe downslope winds and extremely high temperatures may occur in the evening at east-facing slopes of the Italian and, in case of a northerly track, also the Austrian Alps (limited area models forecast 2-meter temperatures up to 24°C at midnight in the Piedmonte region).

... British Isles, Norwegian, North and Baltic Sea plus surrounding coastlines, Latvia, Lithuania, Belarus ...

The stream of polar air over the mild sea surface results in limited CAPE and scattered thundery showers, which will organize near various small low-pressure systems (possibly with warm-core characteristics). Early in the forecast period, the main axis of low pressure will run from the central North Sea along the Danish/German border to the Polish coastlines.
After 15 UTC, a dominant vortex will likely slip southward along the E coast of Scotland and England. Along its NW flank, the models agree on a small but very strong NE-erly wind maximum with 25-35 m/s at 850 m/s, which may easily result in similar gusts at the surface under such well-mixed conditions. Strong storm surges and high waves may cause coastal flooding. Heavy showers are possible as well. Despite expected widespread severe wind gusts, doubts about the presence of lightning (which is the criterion for "convective" events in the ESTOFEX setting) allow only a level 1.

Otherwise, the deep mixing of the polar air will keep the magnitude of both CAPE and vertical wind shear on the low side. Severe weather is unlikely, but one or two waterspouts are not ruled out near persistent convergence zones or small-scale low pressure cores.

Ahead of the most pronounced vorticity maximum, isolated thundery showers may also travel inland into Lithuania and Belarus.