Storm Forecast
Valid: Fri 17 Oct 2014 06:00 to Sat 18 Oct 2014 06:00 UTC
Issued: Thu 16 Oct 2014 21:31
Forecaster: BEYER

A level 1 was issued for Ireland and Western Scotland mainly for severe wind gusts and tornados.

SYNOPSIS

A zonal flow is present over most parts of Europa at the beginning of the forecast period. Two main troughs can be found. One of them is influencing eastern parts of Europe and Western Russia. A second trough is present over the eastern North Atlantic influencing Portugal and Great Britain. In between an initially weak ridge can be found that is overrun by several short wave features.

In the course of the forecast period the western trough is considerably increasing its amplitude. As a consequence the ridge over central Europe is strengthening and a strong southwesterly flow develops advecting unusually warm airmasses into most parts of Western Europe with 850 hPa temperatures clearly in excess of 10 degree C.

Rather humid airmasses are still present over the Mediterranean. With a more or less westerly flow they are advected onshore on the western coastlines.

DISCUSSION

Ireland and Western Scotland...

...the frontal system coming from the main low which has its center west of Ireland with a central pressure below 970 hPa is influencing this region. In the second half of the forecast period the occlusion/cold front is arriving in Ireland and most parts of Great Britain. A few hundred J/kg are forecasted for the passage of the front as well as for the following cold sector. Thus, several thunderstorms should develop. The main threat with this convection are severe wind gusts since mid-level winds are rather high (850 hPa: up to 50 kn). In addition one or two tornados are possible since LLS is quite strong (0  1 km: about 15 m/s) and LCLs are only between 400 and 800 m.

Apart from that other areas over Europe can be found where convection is possible. However no threat level was drawn since severe weather is rather unlikely.

Over the Mediterranean humid airmasses are available but a capping inversion prevents the occurrence of convection over most parts of the sea. Low level streamlines are advecting those humid airmasses (between 11 and 13 g/kg) to the western and northwestern parts of Italy as well as the western coast of Corsica and Croatia. Diurnal heating leads to a steepening lapse rates over land and thus to enhanced instability. As a result a few hundred J/kg of CAPE are possible throughout the day for Italy and Corsica. Since no real trigger can be found coast line convergence, orography as well as other mesoscale lift mechanisms are needed for initiation. LAM show the strongest signals for the region of Piemont. If a storm may forms it can make use of wind shear values between 15 and 20 m/s (Croatia: 25 m/s). Thus, a few better organized storms are possible having the chance for excessive precipitation (ppws up to 30 mm) and a large hail event. However, there are many uncertainties and the overall coverage seems to be too weak for a LVL1 area.

Concerning parts of the Balkan (bordering region: Serbia, Hungary and Croatia) and the Black Sea (later on), a short wave trough could provide the needed lift for convection. Widespread cloud coverage, however, inhibits insolation and thus a stronger steepening of the lapse rates. Thus, CAPE values are mostly rather weak with a maximum of about 500 J/kg predicted by the models. If thunderstorms manage to develop they can benefit from rather strong DLS. Storms can then be accompanied by excessive precipitation given ppw's between 30 and 35 mm. In addition severe wind gusts are possible with the cold front when a convective line manages to form. But again questionable coverage and initiation prevent from a LVL1.

Thunderstorms are also possible with cold upper level airmasses over the North Sea as well as the southern Baltic Ocean later on. Rather warm SSTs in combination with those cold airmasses lead to a steepening of the lapse rates and thus the development of a few hundred J/kg of CAPE. Most of the expected convection is cold air convection without any severity. However, having rather slow mid-level wind velocities at the beginning of the forecast period and saturate low levels a waterspout event over the North Sea cannot be ruled out.