Storm Forecast
Valid: Thu 07 Nov 2013 06:00 to Fri 08 Nov 2013 06:00 UTC
Issued: Wed 06 Nov 2013 22:20
Forecaster: GATZEN

SYNOPSIS / DISCUSSION

A very sharp but intense short-wave trough will move across eastern Europe on Thursday. Strong DCVA will overspread this area and a relative well-mixed air mass is expected near the rough axis. Latest models indicate that rich low-level moisture ahead of the trough will be replaced b drier air masses from the west. Remaining moisture across the Ukraine may be sufficient to allow for weak CAPE in the noon and afternoon hours, so that isolated showers or thunderstorms are not ruled out near the troughs base. However, the likelihood and coverage of storms is too low for a thunder line.

Further south, the southern portions of the trough form a cut-off low that slowly moves east across the Mediterranean. Especially in the morning hours, rich low-level moisture across the Aegean Sea and to the south of Turkey will be present due to easterly winds, and combined with the relatively steep lapse rates weak CAPE is expected. Showers and thunderstorms are forecast in the morning hours. Weak vertical wind shear will limit severe potential expect of some waterspouts forming near the coasts along land-sea-breeze convergences.

In the wake of the sharp trough that moves across eastern Europe, a weakly amplified ridge affects central and eastern Europe. Along the northern periphery of the ridge, a strong westerly jet has spread into Europe. This jet weakens and turns to south-west ahead of an amplifying Atlantic long-wave trough.

At lower levels, another well-developed frontal wave including a broad warm sector moves across central Europe. Moist maritime air will spread into the region and is well separated from steep lapse rates further north in the range of the low geopotential. In the wake of the frontal wve, a frontal boundary will remain from central Germany to the Bay of Biscay and further into Portugal. Rich low-level moisture and slightly steepening lapse rates will increase the thunder potential a little, so that storms may form across Portugal and north-western Spain in the afternoon, spreading into the Bay of Biscay during the night hours. Given the strong low-level vertical wind shear along the frontal boundary, a tornado is not excluded with this convection. Further north-east, stable lapse rates will keep the thunder probabilities too low for a thunder line.

The potential of CAPE is also expected over regions where the polar air masses overspread the warm sea surface. Most storms will form in weak vertical wind shear. However, increased low-level vertical wind shear near the coasts may support some mesocyclones that may produce a tornado. Best overlap of weak CAPE and strong low-level vertical wind shear is forecast over the central British Isles and the south-western coasts of Norway, but the threat is too marginal for a level 1 at this time.