Storm Forecast
Valid: Sat 27 Dec 2014 06:00 to Sun 28 Dec 2014 06:00 UTC
Issued: Sat 27 Dec 2014 00:27
Forecaster: BEYER

A level1 was issued for the Ligurian and Thyrrhenian Sea as well as Corsica, Sardinia and the western coast of Italy mainly for severe wind gusts, excessive precipitation and locally large hail.

SYNOPSIS

A broad upper level trough is present over most parts of Europa. It includes several short wave troughs. One is moving eastward with its axis crossing the Aegean Sea and further on reaching the Black Sea at the end of the forecast period. A second through is coming from the British Isles moving southeastward, amplifying on its way and finally becoming an extensive upper level cut-off low situated over the Alpine region and Northern Italy Sunday morning.

On the forward flank of the second short wave trough strong baroclinic forcing leads to a fast intensification of a surface low that is moving from the English Channel into Belgium and Western Germany. During the night hours it will dissolve over southern parts of Germany. At the same time cyclogenesis is induced over Northern Italy by the southward moving cut-off low. With pressure rising over Northern Europe this will change the general flow pattern to an eastern/northeastern one with cold continental airmasses flooding most parts of Central and Western Europe.

The mentioned surface low will be responsible for heavy snowfall in parts of western France, Belgium and the southern half of Germany. At the same time the developing surface low over Italy will induce convective activity for parts of the Western and Central Mediterranean.

DISCUSSION

Ligurian and Thyrrhenian Sea as well as Corsica, Sardinia and the western coast of Italy...

...This region will become most interesting in the afternoon and evening hours as well as during the night when the southward moving cut-off low and the cyclogenesis over northern Italy will take place. The cold front of the developing low is entering the Mediterranean in the afternoon and is only slowly moving southward during the night.

Ahead of the cold-front mixing ratios of 6 to 8 g/kg are predicted. Strong vertical lift connected to the cyclogenesis will lead to strong diabatic cooling and therefore to a steepening of the lapse rates. This in combination with the mentioned mixing ratios will result in CAPE values of about 500 J/kg. At the same time DLS is quite strong with 0-6 km shear around 20 m/s, locally even higher and 0-3 km shear values of up to 15 m/s. This shear values can at least partly overlap with the region where lift and CAPE (humidity instability) are present. This enhances the threat for severe convection especially in the regions where parcel layer depth is highest. In addition a mid to upper level jet enters the region during the evening and night time. Its left exit region should further enhance convective activity due to upper level divergence and resulting lift. A level 1 was issued for this region. Locally large hail as well as severe wind gusts are possible. The latter is due to the high wind velocities in 850 hPa that reach value of up to 25 m/s.

In addition to the named threats also excessive precipitation should play a prominent role in coastal regions. This risk will be most prominent during the night time hours when the westerly flow is strongest and Corfidi vectors indicate that storms can build backward so that storms may influence the same region for a longer time. Finally also orography will help to enhance the amount of precipitation for the western coastal areas of Sardinia, Corsica and Italy. The risk of excessive precipitation is shifting slowly southward during the night.

Norwegian Sea, North Sea and parts of the eastern North-Atlantic...

...With a northerly to northwesterly flow cold upper level airmasses originating from Polar Regions are flooding parts of the above named area. Temperatures in 500 hPa are about -30 to -35 degree. Together with 850 hPa temperatures around -5 degree and SSTs of 5 to 10 degree this will lead to a destabilization of the atmosphere. At the same time only low mixing ratios are present. Thus only CAPE values of 100 to 300 J/kg are predicted. Prognostic sounding show rather dry airmasses in mid and upper levels of the atmosphere. Therefore only low-topped convection is expected that cannot make use of high DLS values near the frontal boundary and should thus stay non-severe. More to the east (North Sea, Southern Norwegian Sea) upper level flow will stay weak until evening and at the same time hardly no vertical wind shear is present. In combination with LCLs around 1000 m there is a chance for waterspouts. Expected coverage, however, seems to be too low for issuing any threat level.

Aegean Sea...

...A surface low connected to a short wave trough is present over the Agean Sea during the beginning of the forecast period. As the trough is the travelling into the direction of the Black Sea also the surface low is moving further to the northeast. Due to that development its cold front is crossing the Aegean Sea from north to south. Ahead of this cold front mixing ratios of about 8 g/kg and steep rather lapse rates result in CAPE values of up to 500 J/kg.

However, DLS is forecasted to stay rather weak and 850 hPa winds also do not show higher values. Therefore only marginal hail and a stronger wind gust prevent from severe weather. Having this weak wind conditions and partly low LCLs (around or lower than 1000 m) there is a possibility for one or two water spouts. But again, overall threat is too weak for leveling this area.

Eventually the overall threat weakens in the evening an night time hours, when the cold front moves further south.