Storm Forecast
Valid: Wed 04 Jan 2012 06:00 to Thu 05 Jan 2012 06:00 UTC
Issued: Tue 03 Jan 2012 23:24
Forecaster: PUCIK

A level 1 was issued for a broad area from the British Isles to NW Germany and S Sweden for severe wind gusts and tornadoes.

SYNOPSIS

A very active baroclinic pattern will persist over most of Western and Northern Europe. Very deep low pressure system with central pressure below 970 hPa will progress from Southern Sweden towards Southern Finland during the forecast period. Weakening dynamics, e.g. strenght of the mid and upper level jet will have an adverse effect on the low, which is already well occluded and is expected to slowly fill. Another strong disturbance has started forming over the Atlantic, fueled by 80 m/s westerly to northwesterly 300 hPa jet-streak. Short wave developing in the exit region of the jet-streak will amplify during the day, as ridge begins to build over the Atlantic with prevailing flow becoming more northwesterly in nature. Again, deep low pressure system will be the result with very strong low level wind fields, especially in its warm sector. Center of the low will likely pass just to the north of the British Isles towards Southern Norway and Denmark.

DISCUSSION

... Morning hours Denmark, N Germany to S Sweden ...

Dry mid-level airmass characterised by high lapse rates will advect over the region with mild values of dew points from the proximity of the sea. Persisting very strong flow will continue to induce high values of wind shear, especially at lower levels. Marginal values of instability are predicted by most of the models for this region, especially for coastal areas. Post-frontal cellular convection will likely form, only very weakly electrified with chances of tornado due to the high values of LLS and low LCLs. Downdrafts can also help in slightly strenghtening the force of wind gusts. Despite high values of wind shear, marginal instability and weak dynamics at this stage preclude more significant threat and Level 1 is sufficient.

... Late evening and night hours British Isles to BENELUX, NW Germany and W Denmark...

With the arrival of another major baroclinic system, model fields suggest significant dry intrusion behind the jet-streak, possibly spreading partly over the low level warm sector of the surface low. As an interesting note, dry air is expected to penetrate below 700 hPa as suggested by GFS cross sections! A strongly forced convective line seems like a possibility, especially when taking into the account very strong dynamics of the system, even though instability will likely be only marginal at best. 850 hPa flow over 40 m/s ensures very strong wind shear at lower levels and points to the possibility of severe wind gusts and tornadoes within such line. Where exactly will such a line pass is a matter of question, on which models do not agree. Therefore a broad Level 1 is introduced to cover range of possibilities. An update with even an upgrade to Level 2 might follow during the day if situation becomes more certain.