Storm Forecast
Valid: Fri 28 Dec 2007 06:00 to Sat 29 Dec 2007 06:00 UTC
Issued: Thu 27 Dec 2007 17:10
Forecaster: TUSCHY

SYNOPSIS

A potent 140kt upper-level streak is diving south on the west side of a broad cyclonic flow pattern over the nothern Atlantic and is responsible for the amplification and constant neutral tilt of an eastward sliding trough. This feature affects Ireland, Scotland and United Kingdom during the forecast period and is also the main feature of interest regarding severe thunderstorms.
A somewhat more complex situation evolves over the rest of Europe as overall trend is a rise of the geopotential heights downstream of the broad polar vortex . This prevents 2 cut offs to gain strength and those features - one over the W-CNTRL Mediterranean and the other one just north of the Black Sea - don't move a lot during the forecast period.
For the rest of Europe, stable conditions prevail...warmer over western Europe and colder over the NE-part.

Low-end instability release just offshore of NW-Norway is forecast and we issued a thunderstorm area, as thermodynamic parameters indicate a chance for deep convection ( with EL temperatures below -45°C ).... despite the fact that thunderstorms are climatological unlikely during this time of the year.

DISCUSSION

....West-central Mediterranean...

Pockets of enhanced instability ( e.g. at lower levels just offshore of Algeria ) are forecast, but vary a lot during the model runs. Shear is weak and instability marginal at best so no organized thunderstorms are expected.

... Ireland, Scotland and UK...

The tilt of this system is neutral or slightly negative depending on the model / model run, which is not supportive of widespread / robust instability release. Timing of this trough was adjusted slightly backwards during the past model runs , affecting UK mainly during the evening / night hours.
During the day, lapse rates over United Kingdom are weak and despite a moist BL, no initiation can be expected.

Upper-level flow becomes more diffluent during the night hours as a response of a new low pressure development just NW of Ireland. At the surface a cold front crosses UK from the west and rapid cooling of the mid-/upper-levels should be enough for a low-end instability release. Environment is not supportive for more than an isolated thunderstorm risk.

The main focus for organized thunderstorms is a 3-6h time-frame, starting at 00Z, 29th Dez. GFS indicates a fairly compact UVV field, reaching western UK during the morning hours. At lower levels, a convergence zone should reach the western coast of UK between 03Z and 06Z.
Overall, shear relaxes somewhat during this time-frame and despite the enhanced LL shear, non existent helicity values hint on slim chances for rotating storms / shallow mesocyclones, but we will mention the chance of an isolated tornado mainly along the western coast of UK during this time-frame in such a strongly sheared- low - end instability environment.
Marginal hail and gusty winds could also accompany stronger storms.

Uncertainty with this scenario is still high as small departures of the position and strength of the trough could favor another area.

Another story for Ireland between 00Z and 06Z, as pressure gradients re-strengthen and winds at 850hPa reach 30-35m/s. LL shear is impressive with up to 20m/s and so are the helicity values with more than 500 J/kg in the lowest 1000m and 700 J/kg during the lowest 3km.
Aforementioned convergence zone is the main focus for an enhanced severe weather threat. We decided to issue a level-1 area due to a severe wind gust threat even if convection is not electrified and hence only parts of the level- area were included into a TSTM region.
The tornado risk should be maximized along the western / northern coast of Ireland as instability eases rapidly onshore.
This area has to be monitored during the following model runs and modifications of the risk area could become possible.