Storm Forecast

Storm Forecast
Valid: Fri 06 Nov 2015 06:00 to Sat 07 Nov 2015 06:00 UTC
Issued: Thu 05 Nov 2015 22:01
Forecaster: PISTOTNIK

No threat levels were issued.

SYNOPSIS

The formerly dominant upper-level ridge over E Europe gradually flattens, while a new strong ridge with a positive tilt amplifies from Iberia towards central Europe. A weak trough axis in-between extends from the Baltic Sea to Tunisia and slowly moves east. Further upstream, a strong WSW-erly flow starts spreading from the Atlantic Ocean into the NW half of Europe.
Near the surface, high pressure with mild air still prevails over much of the continent, while a strong mature cyclone is located between Scotland and Iceland. Warm and moist Atlantic air is advected into the British Isles, France, BeNeLux, Germany and Denmark.

DISCUSSION

... Ireland, Ulster and Scotland towards S Norway ...

The cold front of the Icelandic cyclone will cross the N half of the British Isles in the 10 to 17 UTC time frame and will reach S Norway late at night. Very mild and moist air will be present in the warm sector with forecast 2m temperatures of 12-14C and 2m dewpoints of 10-12C. As the jetstreak inches southward, large-scale lift on its cyclonic side may suffice to create marginal CAPE with equilibrium levels high enough to allow deep convection with weak lightning activity.
Vertical wind shear would be sufficient to enable the formation of a convective line (0-3 km shear between 15 and 20 m/s and 0-1 km shear around 10 m/s, under mostly unidirectional profiles), though neither the stength of the background wind field nor the amount of lift suggested by the forecast models are convincing. ECMWF shows a particularly benign solution with almost no CAPE, and also GFS has downplayed its comparably more aggressive scenario over the latest forecast runs.
If a convective line should indeed form, a few wind gusts around 25 m/s or an isolated tornado are not ruled out. However, this scenario seems too unlikely to issue a level 1.

A second round of possibly thundery showers may affect the W coasts of Ireland and Scotland well behind the cold front in the evening and the first half of the night. Within a deeply mixed air mass and under a further relaxing wind field, they are not expected to pose a severe weather risk any more.

... sea waters around Malta, Tunisia and Sicily ...

Patches of augmented low-level moisture still allow a little CAPE near the old trough axis (refer to Thu 12 UTC's Tunis and Trapani soundings). Isolated to scattered showers may form, but their chances to grow deep enough to produce lightning further decrease during the forecast period. Nonetheless, one or two waterspouts are not ruled out in coastal areas in the morning hours.

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