Forecast Update
Valid: Fri 13 Nov 2009 17:00 to Sat 14 Nov 2009 06:00 UTC
Issued: Fri 13 Nov 2009 16:55
Forecaster: TUSCHY

A level 2 was issued for the western English Channel and extreme SW-UK mainly for severe wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for Ireland, Scotland and United Kingdom mainly for severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

Please refer to the main outlook, issued at Thu 12 Nov 2009 21:22.


DISCUSSION

... Parts of the Bay of Biscay, the English Channel and UK ...

Latest remote-sensing data reveals an active warm conveyor belt (WCB), spreading NE-wards. Wave-like appearance in high resolution visible data and recently reported thunderstorms, embedded in this WAA regime, hint on CSI release, responsible for embedded line of enhanced convection (thunderstorms). Despite rapidly increasing wind fields, this activity of thunderstorms over SW/S-UK remains elevated in nature although not much BL modification is needed for near surface based activity. The main risk ought to be a severe wind gust risk although even an isolated tornado event can't be ruled out, given favorable backed wind profile and near zero CIN. Therefore, the low probability thunderstorm area was expanded significantly in all directions, to include active WCB, but also the anticipated more active backside of this belt due to the approaching cold front. Strong dry slot continues to punch northwards, a bit further west than what was expected yesterday. Enhanced convection is just about to reach Ireland, so the level 1 was expanded towards the west. Severe wind gusts and isolated tornadoes are possible over Ireland (most likely over SE/E-Ireland), W/central UK and most parts of Scotland. Degree of LL shear, increasing LL CAPE west of the WCB and low LCLs still indicates a chance for a strong tornado event, especially along the coast, where onshore swashing CAPE and shear overlap.

An upgrade was performed for the time-frame 00-06 UTC, although the severe risk continues later-on. Wrap-around moisture and already developing intrusion of dry high-level air just south of the warm core seclusion point to a developing sting jet event and cloud top loops show a warming trend along this spot. Despite a gradual decrease of deep convection in this area, a broad swath of deep convection is approaching SW-UK from the southwest. Forecast soundings and 06/12 UTC GFS runs still indicate an overlap of some CAPE (maximized over SW-UK, over the English Channel and over extreme NW-France). The overlap of 35-40m/s at 850hPa , approaching tongue of the wrap-around moisture and cool mid-levels indicate that not much is needed for damaging wind gusts producing deep convection and therefore a level 2 was issued to reflect the convective component in this concentrated severe wind event. A tornado event is possible along the SW/S-coast of UK and again, a strong tornado can't be excluded.

The most likely scenario will be an eastward racing forced line (EL temperatures below -30°C and beneath the left exit of a 65m/s mid-level streak) with the sting jet pointing into the backside of this line. After the passage of the line, thunderstorm chances most likely decrease over far SW-UK whereas damaging winds keep going. Thunderstorm chances will still be present further north along the W-coast of UK and Scotland also with severe wind gusts and an isolated tornado risk.

... Extreme NW-Spain and extreme N-Portugal ...

Difficult to determine the environment, where current thunderstorm activity is embedded. At the tip of the WCB, a warm and moist inflow beneath eastward spreading cooler mid-level air supports numerous thunderstorms, and conditions remain supportive at least until "landfall" of this warm conveyor belt. In fact, up to 500 J/kg MLCAPE, roughly 100 J/kg 0-3km CAPE, LCLs below 500m, 20m/s 0-1km speed shear and 200-300 m^2/s^2 SRH1 are supportive for tornadic supercells (a strong event possible), next to damaging wind gusts, given 35m/s at 850hPa. Therefore, a level 2 was included. The severe risk will be maximized probably between 17-21 UTC, decreasing rapidly over the Bay due to the clipping of the inflow by the Iberian Peninsula. The risk also decreases to the south along the west coast of Portugal, as shear gradually relaxes.