Storm Forecast
Valid: Mon 17 Oct 2011 06:00 to Tue 18 Oct 2011 06:00 UTC
Issued: Sun 16 Oct 2011 22:25
Forecaster: TUSCHY

A level 1 was issued for N-Ireland, Scotland and N-UK mainly for severe wind gusts and marginal hail.

A level 1 was issued for SW/CNTRL UK mainly for severe to isolated damaging wind gusts and an isolated tornado event.

A level 1 was issued for the far W/SW-Black Sea and adjacent areas mainly for excessive rainfall.

A level 1 was issued for far W-Norway mainly for severe wind gusts.

SYNOPSIS and DISCUSSION

A quasi-stationary and broad upper low is centered over SE-Europe and results in unsettled and cold conditions. An intense cyclonic vortex approaches NW-Europe during the forecast period. Ridging in between (CNTRL Europe) constantly weakens with no deep moist convection forecast.

Despite the marginal conditions for organized convection, there will be two areas with somewhat augmented chances.

I) N-Ireland, Scotland, UK, parts of the North Sea and extreme W-Norway

The first focus of anything severe will be the active cold front passage, crossing Ireland during the noon/early afternoon and UK during the early evening hours from west to east. GFS indicates a tongue of modified subtropical air, which becomes advected just ahead of the cold front. This scenario would assist in a concentrated area with agumented LL CAPE ahead of the cold front, probably maximized over CNTRL/SW- UK. Forecast soundings also reveal some modest 0-3km CAPE release and combined with intense forcing, a forced line of convection (LEWP-like) is expected to evolve along the cold front. Kinematics along/just ahead of this front are very supportive with intense LL speed/directional shear and low LCLs. If a forced line indeed evolves, damaging winds and an isolated tornado event are possible. This line rapidly moves eastwards with a gradual weakening trend expected due to the vanishing prefrontal moisture tongue. Intense forcing however may offset that negative effect by a few hours and hence the level 1 was expanded far to the NE/E.
It has to be noted that GFS is the most optimistic model regarding prefrontal CAPE build-up and also has considerable run-to-run inconsistency with that parameter. Hence uncertainties regarding this event remain high.

With deep, postfrontal CAA forecast, delta-T between SST/atmosphere increases rapidly, especially during the evening hours onwards from west to east. Trajectories hint on a modified, polar air mass and enough fetch for constant air mass modification is available (assisted by latest AMSR-E sea ice maps and NE-Atlantic SST data). Also, vigorous convection is already ongoing south of Iceland at 21Z (16th Oktober) and hence strong showers/isolated thunderstorms are well possible. Main concern right now is the fact that this convection occurs in the well mixed post frontal air mass with expected 850 hPa winds of 20-25 m/s. Hence, downward mixing is possible and we decided to issue a level 1, where the overlap of coldest EL temperatures, best CAPE and highest signals for convergence exist (mainly along the westward facing coastal areas). This level 1 covers severe wind gusts and marginal hail. Please be aware that both level areas overlap and merge to that coarse level 1.

Another level 1 was added for far W-Norway mainly for the morning hours (18th) as the cold front moves ashore. Some low-end CAPE, intense shear and strong 850hPa wind field may assist in severe wind gust events during the cold front passage.

II) NW-Turkey, E-Bulgaria and E-Romania

A sharp baroclinic zone is present over the far W-Black Sea due to the cold mid-level upper trough, which stalls over Bulgaria and Romania. This baroclinic zone is enhanced by the warm SST/cold onshore temperature contrast and features the area, where strong cyclogenesis is most likely....e.g. the far SW Black Sea. Models agree in a gradually strengthening cyclonic vortex over the SW/W-Black Sea, which moves NE-wards. Strong wrap-around moisture is expected as a deformation zone sets-up with favorable warm/moist influx from the E/NE. Some CAPE is expected in this area due to steepening mid-level lapse rates and a few embedded thunderstorms are well possible. The main concern will be excessive rainfall with this convectively enhanced band as a 25-30 m/s LLJ assists in rapid moisture advection towards the coastal areas of far NW-Turkey/ far E-Bulgaria and far E-Romania. It is still unclear where exactly this band of convection will evolve and therefore a coarse high-end level 1 was issued for this event.
A reason for sticking with a level 1 is the fact that this looks like to be a classic heavy, wet snowfall event for onshore areas and it is hard to define either the final transition zone between rain/wet snow or the final snow levels (which may limit final QPF magnitude and run-off problems onshore). Therefore we do not want to issue a level 2 despite WRF/GFS constantly indicating QPF amounts of 100 mm or more in less than 24h just along the coasts within the level 1.