Storm Forecast
Valid: Fri 13 Nov 2009 06:00 to Sat 14 Nov 2009 06:00 UTC
Issued: Thu 12 Nov 2009 21:22
Forecaster: TUSCHY

A level 1 was issued for parts of the Bay of Biscay, UK, Scotland and Ireland mainly for severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

An extensive baroclinic zone has established between 10-40°W and 40-50°N. This region will be the setting for a rapidly developing depression and phase diagrams of numerous model data indicate a warm core seclusion with this feature. Slow forward motion during the rapid intensification stage (until 18-21 UTC) gives way to a more rapid NNE-ward motion during the night hours, as a strong jet ejects out of the base towards the north. At the surface, a cold front pushes eastward, trailing over the Atlantic with a more aggressive propagation over Irlenad, UK and Scotland.

Ridging downstream affects central Euorpe with cold/stable conditions over far east/northeast Europe.


... Bay of Biscay, NW-France, extreme NW Spain and N-Portugal ...

Aforementioned warm seclusion is in full progress west of Irleand and assists in a rapidly strengthening wind field at all levels over the area of interest. In fact, 20-30m/s 0-3km shear does not reflect at all the strength of the overall wind field with 45m/s at 700hPa, increasing to outstanding 65m/s at 500hPa, approaching the Bay during the day from the NW. The increasingly negative tilt of the upper trough axis and the gradually increasing inertial advective wind component along the base of the trough causes overall jet strength to decrease during the evening and night hours, with one stronger branch exiting the area to the north (UK) and a re-developing one along the zonally aligned baroclinic zone over the Atlantic.

Overall synoptic pattern and intense LLJ (40m/s at 850hPa), pumping rapidly mixing/thinning tongue of subtropical air towards the north, probably establishes an extensive warm conveyor belt (WCB), running from extreme NW-Spain/N-Portugal all the way to NW-France and UK. WAA keeps atmosphere neutral/stable with moist adiabatic ascent and there is no risk for deep, moist convection. However, a region for isolated, deep convection could evolve along the back (west side) of the WCB, as the cold front and somewhat drier/cooler
upper-level air approaches from the west. Another feature, which fosters this scenario is the split front appearance in forecast model data/cross sections of the cold front itself. Given the magnitude of the LL wind field, at least a level 1 was issued due to the limited coverage of showers/very isolated thunderstorms. The main risk of this activity will be confined to the NW-Bay, NW-France and extreme SW-UK. Overall environment with low ELs and weak updrafts is hostile enough for not even issuing a low probability thunderstorm area.

... United Kingdom, Scotland and Ireland...

The outlook for UK turns out to be even more complex in respect of the degree of thunderstorm coverage. The WCB with more stratiform rain ought to keep the atmosphere very moist and stable for vertical ascent, so the main focus for convection arises during the evening hours, when the cold front draws near from the west. Overall set-up looks promising for a flare-up of convection during the cold front passage over S/central UK as surface cold front gets topped by WAA above 5-6km AGL with aforementioned 65m/s streak at 5km covering SW-UK during the sunset-midnight hours. Despite the unfavorable placement of the front beneath the indirect thermal jet circulation, overall diffluent upper streamline pattern and left exit of a streak at roughly 3km, crossing SE-UK during that time, point to adequate conditions for low-topped convection. Forecast soundings indicate some LL CAPE release and the possibility for storms to evolve in roughly 30-40m/s 0-5km shear (even stronger, if deeper convection verifies). Speed and directional shear, low LCLs and NE-ward racing vorticity lobe create a favaorbale environment for a strongly forced convective line (LEWP-type) with damaging wind gusts and tornadoes possible. Model discrepancies are still high, so new model data has to be evaluated to determine the final severe weather risk. A coarse level 1 was issued, but if the forced line with embedded thunderstorms verifies, a level 2 may be needed.

Another area of concern arises over SE-Ireland and east/northeastwards at 15 UTC onwards, as the triple point of the warm/cold front and occlusion approaches from the SW. Enough instability for stronger and more persistent updrafts and forcing/shear in the extreme range indicate a severe wind gust and tornado risk and an isolated strong tornado event can't be ruled out. This risk spreads northeastwards, affecting S-Scotland thereafter. Decreasing instability onshore may be delayed by the left exit of the 50-60m/s mid-level streak and hence the level was expaned well towards the north.

... South of Ireland and extreme W/SW-UK 21 UTC onwards ...

The intensification of the depression just to the WSW of Ireland will be already in the end pahse and occlusion starts. GFS and ECMWF are in line with the overall scenario, whereas the rest of the model pool is still oscillating around their solution. For now, we stick with the American/European model solution, which sounds quite reasonable, as both models had the most persistent scenario during the past few runs. A back-bent occlusion starts to wrap around the center beneath a pool of drier high-tropospheric/low stratospheric airmass, covering the center of the depression. A rapidly intensifying sting-jet event is forecast to unfold at roughly 21 UTC onwards, affecting the offshore areas south of Ireland. Model output forecast wind speeds indicate gusts in excess of 35-40m/s, which sounds reasonable, given 40-45m/s at 850hPa and a well mixed postfrontal airmass. This evaporatively cooled air is not favorable for deep convection and therefore the damaging wind gusts risk is not reflected in our risk level forecast, but damaging and potential life-threatening wind gusts in excess of 35m/s affect SW-UK after 03 UTC and spread eastwards thereafter.

Just for the sake of completeness, the last similar event (although not comparable in respect of forcing and wind speed) was severe extratropical cyclone Klaus over the far S-Bay of Biscay. A few thunderstorms occured in a similar environment offshore and onshore, so the risk for isolated showers/thunderstorms can't be ruled out completely. An upgrade may be needed, if this concern increases.