Storm Forecast
Valid: Tue 03 Jan 2012 06:00 to Wed 04 Jan 2012 06:00 UTC
Issued: Mon 02 Jan 2012 23:32
Forecaster: TUSCHY

A level 2 was issued for SE-UK, the Netherlands, N-Belgium, N-Germany, Denmark and adjacent offshore areas mainly for severe to damaging wind gusts. An isolated, strong tornado event can't be ruled out.

A level 1 surrounds the level 2 mainly for strong to severe wind gusts.

A level 1 was issued for the coast of NW-Germany and W-Denmark at 00Z onwards mainly for marginal hail and strong to severe wind gusts.

SYNOPSIS

The major player for this forecast package will be a low pressure area over Scotland, which moves eastwards and eventually approaches S-Norway and Sweden during the end of the forecast period. Models and ensemble data are on a similar track and show a deepening low, which enters the N-North Sea during the early daytime hours. Some discrepancies exist in how far the vortex deepens, with GFS members still indicating substantial spread in the surface pressure fields. Healthy looking satellite presentation, good persistence in past model runs and also satisfying performance in similar, past events let me lean more towards GFS and hence, we will use a blend of GFS/WRF and ECMWF for this outlook.

Modified tropical air from the Bahamas fuels the system, resulting in a rapidly intensifying baroclinic zone west of Scotland. This region of high baroclinity was the place of birth for that depression. Also, a pronounced reservoir of stratospheric air with an high content of IPV can be detected just NW of the developing depression. This air mass becomes incorporated into the rapid cyclogenesis process in form of an healthy, eastward moving dry slot. This set-up assists in an overlap of dry stratospheric air, moist LL air and strong forcing over the North Sea and adjacent areas. Latest phase diagrams indicate a developing (shallow) warm core structure as the feature crosses the North Sea and as a pronounced occlusion starts to wrap around the center. The overall evolution seems to follow a classic, major cold conveyor belt cyclogenesis.

DISCUSSION

... Parts of Ireland, UK, parts of the North Sea, Netherlands, parts of Belgium, N-Germany, Denmark, S-Sweden and the S-Baltic Sea....

The first feature of interest will be the eastward racing cold front, which moves off Ireland during the start of the forecast, then affects UK during the morning hours, Benelux during the afternoon hours and N-Germany thereafter. Of interest is the concurrence between the strong surface front and an IPV streamer, which are both well co-located in past few model runs. Propelled by a stout PVA max, the cold front passage will be pretty active in form of a forced line of convection (LEWP-like) and a solid line of enhanced convection is expected to race eastwards (e.g. over SE-UK, the Netherlands and Belgium, N/NE-Germany, NW Poland and parts of the Baltic Sea). A potent 30-40 m/s 700 hPa jet points into the backside of the cold front with 25-30 m/s at 850 hPa. Forecast soundings hint at a deeply mixed postfrontal air mass and numerous severe to damaging wind gusts are well possible along the cold front. There is not yet an indication that a concentrated swath of damaging wind gusts evolves along the cold front, so the level 2 was cut off onshore pretty fast. Also, with SRH values running off the chart, we can't exclude embedded, rotating cells with a tornado risk. With expected LL CAPE/shear overlap, even an isolated strong tornado event is possible. The cold front gradually outruns strongest wind fields as the depression over SW-Norway slows down, so the overall risk gradually diminishes further east over the Baltic Sea. However, we decided to expand the level areas far to the east as the weakening phase of forced, convective lines are not well handled in global model fields.
The activity gradually diminishes south of the level 2, as the front drops to the south over France and W-Germany. It becomes aligned more parallel to the background flow and will be left behind strongest forcing to the north, so degree of organization becomes less betimes. Nevertheless, sporadic thunderstorms are possible as far south as CNTRL/E-France and S-Germany with cold front featuring either a more solid LEWP or numerous smaller narrow cold front rainbands. Strong/severe wind gusts will be the primary hazard.

Another area of interest will be the E-North Sea, Denmark, Skaggerak and Kattegat, as the back-bent occlusion approaches from the west. Timing will be 18Z onwards although some discrepancies exist with ongoing model uncertainties. However, the set up for widespread low-end/marginal SBCAPE build-up is present as a pronounced pool of dry stratospheric air overruns the backwards banded occlusion at lower levels. 35-40 m/s winds at 850 hPa rapidly evolve over the E-North Sea and spread eastwards. Favorable downward transport of those extreme winds and scattered shallow/deep convection assist in a swath of damaging and potential life threatening wind gusts over Denmark and surrounding offshore areas. Also, an isolated strong tornado event can't be discounted, given the aformentioned overlap of shear and CAPE.

...S-CNTRL Mediterranean...

A southward moving upper low sparks isolated to scattered storms over the S-CNTRL Mediterranean. Nothing severe is anticipated although storms may produce locally heavy rainfall amounts over Sicily due to slow storm motion.