Storm Forecast

Storm Forecast
Valid: Mon 26 Dec 2011 06:00 to Tue 27 Dec 2011 06:00 UTC
Issued: Sun 25 Dec 2011 15:33
Forecaster: DAHL

A level 1 threat was issued across Scandinavia and parts of western Russia mainly for severe convective wind gusts.


A progressive upper pattern persists over the N Atlantic and N Europe. A lead short-wave trough and an attendant strong SFC low are rapidly pushing eastward, reaching western Russia on Monday morning but undergoing substantial weakening by the end of the forecast period. The next short-wave trough upstream, which will maintan a rather broad wave cyclone will reach western Scandinavia on Monday afternoon.
High pressure exists over central and southern Europe, and a weak upper trough with an associated surface low is present over the Ionian Sea.


... western Russia ...

It seems that a narrow/shallow line of forced convection may exist along the cold front over western Russia early in the period. As usual with such quasi-linear convective systems (QLCS), there will be a threat for severe wind gusts (augmenting the strong/severe large-scale gradient flow) and for bowing structures and mesovortices, supportive of brief/weak tornadoes. However, given practically absent CAPE and an overall weakening trend of the large-scale system, the overall threat should be somewhat low, but a LVL1 seems to be marginally warranted. Given rather weak/shallow updrafts within the convective system, there may be little or no lightning.

... Scandinavia ... N Baltic Sea ...

There will be two foci for convection with the next system approaching from the Atlantic:
i) cold front: The large-scale system should be substantially weaker but especially in the upslope-flow regime over western Scandinavia, a few thunderstorms may develop along the cold front. Lightning may be quite sparse again given rather weak/shallow updrafts. However, some of the 70 KT flow at 850 hPa could be mixed to the surface, resulting in a threat for severe straight-line winds.
In the lee of the Scandinavian mountains, there will likely be subsidence and a gap in convective activity, but the development of a quasi-linear, strongly forced system is again conceivable. Again, expect an augmentation of the large-scale wind field by the convection due to vertical momentum transport. Also, a brief QLCS tornado or two cannot be ruled out, thought minimal/absent CAPE should be a limiting factor.

ii) Postfrontal cellular convection: This activity should affect primarily the west coast of Scandinavia, where the upslope flow should result in improved chances for more vigorous updrafts capable of producing lightning. Also, a severe-wind threat persists in association with vertical momentum transport.

... southern Mediterranean ....

Scattered weak thunderstorms should persist in association with the Mediterranean upper trough. The shallow nature of the storms and weak shear in the cloud-bearing layer should limit the severe threat with this activity.

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