Storm Forecast
Valid: Fri 29 Dec 2006 06:00 to Sat 30 Dec 2006 06:00 UTC
Issued: Thu 28 Dec 2006 16:50
Forecaster: TUSCHY

SYNOPSIS

Finally, the weather becomes a little bit more active compared to the past few days....at least for parts of NW Europe.
Downstream of a well structured vortex southeast of Greenland, strong warm air advection will support the strong ridging over most part of south and central Europe.
An isolated thunderstorm should manage to develop over the central Mediterranean, but expected coverage will be too marginal for highlighting any specific region.
Another burst of very cold air will affect most parts of extreme W-Russia and will be characterized by a pretty dry boundary layer airmass, so no deep convection can be anticipated.

DISCUSSION

...Ireland, Scotland, United Kingdom...

A possible high shear - low instability event unfolds for the area of interest ! Severe weather can be expected in multiple rounds !

First time-frame of interest will be between 06Z and 15Z, mainly for Ireland, Scotland and northern United Kingdom.
An upper-level wave, positioned in the left exit region of an impressive 170kt upper-level jet over the north-central Atlantic will reach Ireland during the early morning hours, shifting rapidly towards the NE.
A weakening, but still active warm front will reach Ireland during the early morning hours from the west. GFS indicates an approaching dry slot from the SW, intersecting the warm front over Ireland. Resulting steepening lapse rates of 7-8 K/km will be conducive for marginal instability release over Ireland and later on over Scotland, too.
Determining the expected LL lapse rates is difficult due to lack of datas over the Atlantic, but trajectories indicate that airmass at or below 850hPa will be advected from the south-central Atlantic, so a well mixed and pretty warm boundary layer airmass should be present, also characterized by marginal LL lapse rates over Ireland during the morning hours.
Anyway, conditions look fine for at least a few thunderstorms to develop.
Shear will be outstanding and if a line of not necessarily continuous arranged storms can manage to form, damaging wind gusts can be expected.
The tornado threat should be damped by the pretty weak LL lapse rates and warm temperatures at 850hPa, but low LCLs, veering profilers and 300-500 J/kg SRH-1 can't be ignored and hence will mention the risk of a few tornadoes, especially when LL lapse rates will be steeper than expected!
Storms will move rapidly towards the NE, crossing Scotland and N-UK till 15Z. Low-end instability and impressive shear favor a continuous tornado and damaging wind gust threat.

The next round of an enhanced thunderstorm threat evolves during the 18Z - 06Z time frame, when an intense depression will rapidly cross Ireland and Scotland from the SW.
A strong cold front will accompany this system and will be the focus for severe weather.
Impressive forcing will start to spread towards the east and GFS additionally calculates intense divergence values over Ireland,Scotland and UK.
Again, a well developed dry slot is forecast to cross Ireland between 21Z and 00 Z, working its way towards the ENE.
Best instability release can be expected over Ireland, Scotland and N-UK , but also along the coastal areas of W and SW UK.
Any line of not necessarily strongly electrified convection can intensify the already strong background flow and damaging wind gusts can be anticipated, especially if storms line up.
The tornado threat should be enhanced along the coastal areas of SW and W - UK, Ireland and Scotland, where LL lapse rates steepen somewhat during the period and where DLS and LL shear is still very strong.
Main focus for tornadoes will be the eastward racing cold front, especially if it exhibits a LEWP-type structure.

The third and final round of severe weather will finally affect parts of Ireland during the 03Z-06Z time frame and onwards.
A pool of cold mid-level air will support rapidly steepening mid-level lapse rates and hence increasing thunderstorm activity. LL shear will stay strong enough for a tornado and an isolated severe wind gust threat.

Will stick with a broad level-1 due to many uncertainties concerning exact timing, position and strength of various features but an update may become necessary later on !