Storm Forecast
Valid: Mon 28 Jan 2013 06:00 to Tue 29 Jan 2013 06:00 UTC
Issued: Sun 27 Jan 2013 22:21
Forecaster: TUSCHY

A level 1 was issued for parts of Ireland and UK mainly for severe to damaging wind gusts and an isolated tornado risk.

A level 1 was issued for the Netherlands and extreme NW Germany mainly for severe wind gusts.

SYNOPSIS

Most active part of the split polar vortex resides atop the N-Atlantic Ocean which induces active and intense cyclogenesis over a broad offshore area. This pattern shift starts to affect far W-Europe during the overnight hours with rapidly rising mid-level heights spreading east. Once again, a strong impulse along the eastern fringe of the WAA regime races south over the W-C Mediterranean towards N-Africa as a 50 m/s mid-level jet moves S/SE. Out of this impulse, a closed mid-level low evolves over the Gulf of Gabes with unsettled conditions affecting most of the C-Mediterranean.

A weakening cold-core low exits Romania/Bulgaria to the SE and crosses the W-Black Sea while adjacent precipitation shield also moves offshore during the evening hours onwards.

The system of main interest will be an intense 960 hPa depression, which passes Ireland and Scotland to the north. As seen in past events over the N-Atlantic Ocean (past 24-48 h), this cyclone features a strong warm-seclusion process with phase diagrams reflecting an appropriate signature. Back-bent occlusion will be pronounced and probably the main focus for severe wind gusts.

DISCUSSION

... Ireland, UK and the S-North Sea ...

Regarding thunderstorm probabilities, the current focus shifts to the cold front, which is forecast to cross Ireland around noon. Thereafter, the occlusion point builds south (e.g. probably towards C-UK ) until the front exits UK to the east at roughly 18Z. While affecting Ireland and UK, an authentic signal of a rapidly SE-ward building dry slot exists with a tropopause fold and drying at 700 hPa being present. QPF model signals remain mixed, but there will be a low-end chance for a few deeper updrafts / potentially electrified /. Pooling signals like LL convergence, parcel layer depth and 700 hPa instability, the highest probabilities for an isolated thunderstorm event occur over S-Irland and C/S UK between 12-21 Z. Strong UVV max. /exit region of a 55 m/s 500 hPa jet max enter the scene also in time, so current plan is that a forced line of showers/isolated thunderstorms (LEWP) accompany the cold front passage, capable of mixing winds down to the surface. Kinematics are off the chart, so damaging wind gusts and an isolated tornado event are possible. Some better LLCAPE sneaks in from the W and affects C/SW UK during the afternoon and evening hours, probably also increasing the tornado risk somewhat. A broad level 1 was issued, mainly for the severe to damaging wind gust risk with the expected limited DMC activity and to a lesser degree for the low-end tornado risk. In case a better CAPE environment sets-up and/or the frontal line of showers/thunderstorms becomes more vivid than currently forecast, an upgrade may be considered, given 35-40 m/s just atop the PBL.

A more widespread severe to damaging wind gust risk is forecast with the back-bent occlusion, which affects Ireland during the evening hours onwards. Despite a weakening wind field at 850 hPa, well mixed maritime air should assist in downward transport of winds from higher levels and therefore widespread severe wind gusts affect Ireland and UK during the overnight hours. Missing DMC activity means no reflection in our warning scheme.

We expanded the level 1 far east, including the S-North Sea, the Netherlands and extreme NW-Germany for the frontal passage during the latter part of the forecast. The triple point still resides beneath the exit of a 50 m/s mid-level jet maximum and despite gradually decaying instability, expected profiles still reveal a near neutral stratification and remain supportive for an ongoing forced line of showers with sporadic thunderstorm activity. Severe wind gusts will be the main risk although convectively induced gusts may become blurred by the background wind field betimes as convection weakens.

For the rest of Europe, a few spots see some isolated lightning activity, but nothing severe is anticipated.