Storm Forecast
Valid: Mon 06 Jan 2014 06:00 to Tue 07 Jan 2014 06:00 UTC
Issued: Mon 06 Jan 2014 00:28
Forecaster: TUSCHY

A level 1 was issued for parts of Ireland, Scotland, UK, France, Benelux, Germany, Sweden and Denmark mainly for severe wind gusts and an isolated tornado risk.

SYNOPSIS and DISCUSSION

What a big surprise ... we have to talk about an intense extratropical cyclone north of Ireland, which shifts to the NE during the forecast. Once again, phase diagrams show a rather impressive warm-seclusion with that one. This cyclone has already seen its peak strength and the ~ 950 hPa pressure around 06Z should increase by 10-15 hPa/24h. Nevertheless, a winter-like intense wind field is forecast along its southern fringe, affecting Ireland, UK, Scotland, parts of France, Germany and Benelux all the way into Denmark and S-Sweden.

This setup resembles past ones with modest (200-500 J/kg) offshore SBCAPE and very strong shear, especially in the lower troposphere. Enhanced convection remains bound to the left exit regions of numerous 40-50 m/s mid-layer jet streaks and to numerous more or less pronounced impulses, which will be embedded in the brisk westerlies. Expect an isolated to scattered thunderstorm potential for all of the highlighted areas through the complete forecast period....especially along the westward facing coastal areas of Ireland and UK. Severe wind gusts will be the main hazard next to an isolated tornado risk. Aforementioned mid-layer waves will be discussed a bit in respect of more organized line structures to the S and E of UK/Ireland:

One wave exits UK during the late morning hours, crosses the S/C North Sea from W to E and approaches Denmark from the west during the afternoon/evening hours. Especially offshore areas (North Sea) and Denmark/S-Sweden should see adequate support (forcing, shear and instability) for organized showers/isolated thunderstorms with a severe wind gust and isolated tornado threat. Any line-up of storms could result in a swath of severe wind gusts.
Further south, over the Netherlands and NW-Germany, forcing will be weaker, but should still help to initiate deeper updrafts. Severe gusts will be the main hazard. In case of short line segments of storms or a bad handling of instability in current model outputs, an isolated tornado risk can't be ruled out (1 km shear aoa 15-20 m/s and 300 m^2/s^2 SRH-1). As past events showed, good forcing could offset meager looking CAPE parameters, especially in global model data. However, placement along the anticyclonic shear side of the mid-layer jet and attendant warmer mid-layers lowers confidence in more widespread/long-lived convection quite a bit.

Another wave affects N-France during the afternoon/evening hours and Benelux and C/N Germany during the overnight hours. This event has more positive signs for the development of an organized line of storms. Increasing mid-layer wave height results in a coupled mid/upper jet configuration with an healthy looking vorticity lobe racing east/northeast. This impulse overspreads a tongue of rather moist low/mid-level air with storm motion vectors aoa 70 kt. There are signals in the BL wind field of numerous convergence zones, which will be aligned near parallel to the background flow. Current thoughts are that an organized line of deeper convection (LEWP-ish) evolves over N-France with a rapid motion to the NE. Bowing line segments (severe to damaging wind gusts) and isolated tornadoes are forecast. Would also not be surprised about marginal/isolated large hail with strongest cores. With mixed model signals however, confidence in such a scenario is not yet high enough to go with a level 2 area. Even without a line of storms, enhanced convection should support downward mixing of 20-25 m/s winds from 850 hPa.

A southbound moving cold-core cut-off low over the far S-central Mediterranean causes isolated to scattered showers and thunderstorms over S-Sicily, Malta and areas to the south ... before leaving the forecast area to the south. Malta could see heavy rain and an isolated tornado risk with slow moving DMC, but limited regional/temporal coverage remains too iffy for a level area. Heavy rain also affects S-Greece and Crete during the forecast, but limited thunderstorm coverage precludes a level area. Isolated waterspouts are still possible.

The rest of Europe remains quiet regarding organized convection. Heavy rain over parts of Portugal should stay without thunderstorm activity during the forecast and this event is therefore not reflected in this outlook.