Storm Forecast
Valid: Mon 17 Sep 2012 06:00 to Tue 18 Sep 2012 06:00 UTC
Issued: Sun 16 Sep 2012 22:01
Forecaster: TUSCHY
A level 1 was issued for parts of W-Finland mainly for severe to damaging wind gusts and an isolated (but potential strong) tornado risk.
A level 1 was issued for E-Sweden mainly for severe wind gusts and an isolated tornado risk.
A level 1 was issued for NW-Turkey, the N-Aegean Sea and parts of Greece mainly for large hail, heavy rain (Turkey) and heavy rain (Greece).
SYNOPSIS
A decaying upper low over the Aegean Sea continues its weakening trend while moving slowly east. Another flat upper trough approaches the Iberian Peninsula during the forecast, inserting lowering geopotential heights. The most dominant feature however will be a potent cyclonic vortex over far N-Europe, which expands to the S/SE. Along its eastern fringe, rapid cyclogenesis occurs over Sweden and Finland.
At the surface, high pressure expands from the W-Mediterranean to Russia, resulting in more stable conditions regarding DMC activity.
DISCUSSION
... Portugal and Spain...
A low, centered in the deep subtropics, approaches Portugal during the forecast. Attendant rich moisture spreads eastwards and starts to affect Portugal during the end of the forecast. Isolated thunderstorms may already affect the W coast of Portugal, but the main activity remains offshore. Given deep tropical moisture, lapse rates remain marginal, so the main risk will be heavy rain.
Another focus for sporadic CI may be the Pyrenees, where a weak mid-level impulse grazes the area during the daytime hours before ridging builds in from the SW during the night. Main question will be degree of moisture advection towards the Pyrenees with GFS the most optimistic model. At least isolated activity is possible and given 20 m/s DLS and locally moderate CAPE, an isolated large hail event can't be ruled out.
... NW-Turkey ...
Placed beneath the weakening upper low, thermodynamics remain supportive for scattered thunderstorms to evolve during the forecast. 15-20 m/s DLS and good directional shear result in a few well organized multicells with large hail the main hazard. Also, the E-coast of Greece may see a few slow moving storms with heavy rain and strong wind gusts.
... Sweden and Finland ...
A potential more widespread low CAPE/high shear wind event unfolds for most parts of the highlighted area with the main risk being severe to damaging wind gusts and isolated tornadoes.
A deepening sub-990 hPa depression moves rapidly to the NE, approaching far N-Sweden during the end of the forecast. Attendant well structured cold front sweeps over parts of N-Sweden, Finland and the N-Baltic Sea with an impressive isallobaric gradient forecast to evolve. The same in the mid/upper levels, where a progressive wave pushes the surface features rapidly to the NE. The main limiting fact for a significant severe event might be the late timing (e.g. front crosses Finland during the evening and overnight hours). However, a well pronounced dry slot overtakes the surface front and probably negates the overall weak CAPE signals due to abundant forcing/moisture overlap.
Despite marginal CAPE (with GFS on the most optimistic side and EZ on the lower end side), shear improves constantly during the forecast with impressive values forecast: 20-25 m/s 0-3 km bulk shear, 30 m/s 0-6 km bulk shear, 15-20 m/s LL shear and SRH exceeding 200 m^2/s^2 significantly.
The first risk of organized DMC may already evolve over parts of E/NE-Sweden along the evolving surface depression, as some wrap-around moisture advects beneath cooler mid-levels. Given improving LL shear, an isolated tornado and strong to severe wind gusts are possible with any longer lived thunderstorm.
The main risk however evolves thereafter, especially during the late afternoon hours onwards, as a gradually consolidating cold front moves off the N-Baltic Sea and enters Finland from the SW. We would not be surprised to see a well structured LEWP-like structure, as a forced line of not necessarily deep convection races E/NE-wards. The main risk again will be severe to damaging wind gusts and features like embedded bows might increase the risk of a local and more serious wind event. Beside that risk, isolated tornadoes are also likely, given amount of LL shear and LCLs at or below 800 m. The line remains active until well into the night, before gradually losing organization over far N-Finland, NE Sweden and far N-Norway.
... Scotland, N-UK and parts of the North Sea ...
Isolated thunderstorms may occur during the passage of numerous disturbances. The main hazard will be marginal hail and strong wind gusts.
