Storm Forecast
Valid: Fri 14 Feb 2014 06:00 to Sat 15 Feb 2014 06:00 UTC
Issued: Thu 13 Feb 2014 18:50
Forecaster: TUSCHY

A level 1 was issued for extreme NW France, parts of UK and Ireland mainly for severe to damaging wind gusts and an isolated but strong tornado event.

A level 1 was issued for the E-Adriatic and NE Ionian Sea mainly for isolated excessive rain, marginal hail, gusty winds and a tornado or two.

A level 1 was issued for Crete mainly for excessive rain and an isolated tornado event.

SYNOPSIS

Another intense extratropical cyclone emerges from the E-Atlantic and affects Ireland, UK and Scotland. In fact, downstream WAA and height rises affect most of C-Europe as well. Further east, another downstream effect will be the southbound digging upper trough over SE Europe with marginal eastward displacement expected through the forecast period.

Numerous vigorous fronts affect NW Europe and result in another round of unsettled conditios for the flood-prone areas over the UK.

DISCUSSION

... Adriatic, Ionian and Aegean Sea ...

Intensifying 40-45 m/s mid-layer jet pushes south along the western fringe of the upper trough and keeps its neutral tilt stable for the forecast period. This ensures separation of most unstable air from strongest deep layer shear.
A sharp W-E mid-layer thermal gradient exists over the Adriatic and Ionian Sea and keeps the unstable air mass along the E part of those regions. Expect diffuse lee cyclogenesis over NE Italy, fostered by the left exit of the aforementioned mid-layer jet streak, to work its way to the SE while weakening. Reinforcement of this depression is first expected over the S-Aegean Sea during the night.

The E-Adriatic and E-Ionian Sea will see a gradual increase of DLS and a decline of MLCAPE betimes. A temporal overlap of forcing and those ingredients should support a few strong to isolated severe storms to move S/SE. Coastal storms could ingest some stronger near-surface helical flow and a few better organized multicells with an isolated tornado, strong wind gust and sleet/marginal hail risk are expected. Heavy rain accompanies those storms, but progressive nature should keep this risk below a level 1. A combination of all hazards warrants a marginal level 1 area however. This risk wanes during the afternoon hours from NW to SE.

For the Aegean Sea, CI awaits the approach of the mid-layer streak and induced surface low development. Those features interact with a moist marine layer, causing MLCAPE up to 800 J/kg. Separation of shear/CAPE however keeps the severe risk low. Intense upper-layer divergence induces a rapid flare-up of DMC and a cluster of showers/thunderstorms likely affects the far S-Aegean Sea and Crete during the overnight hours. Heavy rain will be the main hazard. LLCAPE improves and local models inidcate numerous convergence zones next to Crete. Waterspouts will be possible and therefore a level 1 was issued. By 06Z the convection should have left our forecast area to the SE.

... Ireland and UK ...

Another intense extratropical cyclone affects the forecast area. This depression is already in its mature / occlusion stage as it affects Ireland and UK. Compared to past events however,it has excellent influx of barely modified subtropical air, which features high moisture content and weak mid-layer lapse rates. Two foci exist for thunderstorm development:

a) Strong differential temperature advection regime and numerous NE-ward racing short waves create a slightly unstable warm sector with some MUCAPE build-up forecast. Also, mid-layer temperatures in the warm sector cool down a bit during the evening and overnight hours. This probably results in a broad sector, where isolated clap of thunders are possible. This activity however remains sub-severe. An extensive low-probability lightning area from Portugal to NW Germany was added.

b) From noon to the evening hours, the triple point somewhere between SE-Ireland and C-UK becomes of interest as dry slot draws near from the SW. Interaction of both features could result in a few vigorous updrafts. This scenario is also backed by the favorable placement of a 50 m/s (plus) 500 hPa jet streak. LL flow won't be as intense as further south, but high SRH-1 and low LCLs point to an isolated tornado threat... next to severe wind gusts and marginal hail.
Further south, the dry slot probably lowers thunderstorm probabilities, but an isolated event can't be ruled out over NW France and most of C/S UK as BL moisture remains adequate with mixed-layer mixing ratios in excess of 5 g/kg. Spotty pockets of EL temperatures between -20 and -30 °C also support the idea of ongoing convection potential. Despite lack of confidence in a forced line of storms, any shower/thunderstorm event could support downward momentum transport of very intense winds (e.g. 850 hPa winds aoa 45 m/s with SRH-1 well in excess of 400 m^2/s^2 !). Once again it will be hard to distinguish between the background gradient flow and convectively induced wind gusts, but convection could certainly add a localized damaging wind gust risk at the surface.
Despite the ongoing wrap-up process of the thermal wave, this depression certainly features an higher tornado risk compared to the past events. Extreme LL shear and some modest onshore LLCAPE back that idea and an isolated strong tornado event can't be ruled out especially over SE-Ireland, SW-S and C UK.