Storm Forecast
Valid: Fri 04 Jan 2008 06:00 to Sat 05 Jan 2008 06:00 UTC
Issued: Thu 03 Jan 2008 22:24
Forecaster: TUSCHY

SYNOPSIS

Impressive 24 hours in store for parts of the eastern Atlantic. As high pressure next to the Azores gets stronger and
geopotential height fields west / northwest of Ireland drop significantly , winds at all levels ramp up to values we haven't yet seen this winter. Winds at 300hPa should increase to 210kt or 105-110m/s and more than 130kt ( ~65m/s ) at 500hPa. Another streak east of the high pressure area covers Spain with 150kt ( 75-80m/s ) at 300hPa supporting an outstanding amplification of an upper-level trough far southward ( to near 20°N during the next few days ).
There's more to it than that .
Such a dynamic high index pattern over the Atlantic is proficient watching for rapid cyclogenesis and GFS has such a feature, which evolves SW of Ireland during the end of the forecat period as lower level temperature gradients sharpen significantly. As it has no affects during the forecast period no further investigations were done and in addition, model pool has a bad handling on this feature with an attendant large uncertainty that far out.

1055hPa high pressure over NE Europe will be present for the next 24 hours and despite a weakening trend, stable and cold conditions prevail for most parts of eastern / northern Europe.

DISCUSSION

.... West - central Mediterranean....

Numerous thunderstorms in the range of an upper-level trough and a developing weak surface depression east of the Balearic Islands should stay sub-severe as thermodynamics and kinematics are too weak.
A quite low WBZ-level SE of Spain may support a somewhat enhanced hail risk, but nothing severe, matching our criterion.

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

As the intense 100m/s upper-level streak approaches from the west , strong cyclogenesis is forecast to occur northwest of Ireland with a central pressure around 970hPa. Embedded in a sharp upper-level trough, locomotion is not too fast as no steering mechanism like an area of strong baroclinity is present and it should slow down gradually during the forecast period. The structure of this upper-level trough is embossed by rapid cooling at mid-upper levels but a weaker cool-down at lower-levels ( due to modifications over sea ). Hence despite the highly negative tilted feature, instability release should be somewhat restricted and this is supported by overall bad connection to an airmass with higher moisture content further towards the south. In fact, a tongue of higher Theta-E values fans out and weakens before reaching the area of interest.
Finally, as typical in such winter-time cases, displacement of streaks at different levels is remarkable with strongest winds at lower levels well displaced to the south. This results in quite weak shear values in the lowest 3 km over UK and Ireland, while 45m/s 0-3km shear can be found well offshore.

Recapitulatory we can say that this is a low-instability and moderately sheared set-up but LL shear of 15m/s and some helicity signals along the western coast of UK / Ireland indicate a chance for an isolated tornado. Expected thunderstorm coverage and tornado threat is too low for issuing a level-1and even a thunderstorm area for parts of W-UK.

To add one more interesting aspect: GFS hints on numerous convergence zones, crossing Ireland and UK from the west during the afternoon and evening hours in combination with still adequate mixing ratios. If convective activity in the postfrontal airmass is more active as currently anticipated, modifications to this outlook may become necessary.

... English Channel and surrounding areas....

A short wave, embedded in the strong cyclonic flow crosses the area between 21Z - 06Z from the west associated with a cold front.
Instability signals are not stunning but low-end values are present as colder mid-levels overspread the region from the NW ( a result of the negative tilt of the sharp upper-level trough ). In addition, the mixing ratio is quite high and LL shear enhanced with 10-12m/s . Winds at 850hPa reach 25m/s during the cold front passage and could support strong wind gusts. We did not issue a level-1 as tornado threat looks quite limited and the same for the severe wind gust risk as downward mixing should get hampered by weaker LL lapse rates.
In addition only the extreme western part of the English Channel was included into a thunderstorm area as moisture abates further towards the east and so do the thunderstorm chances.

.... NW coast of Morocco...

Kinematics are still very strong with 0-6km bulk shear of 30-40m/s along the coast and more than 60m/s well inland but instability release will stay confined to the coastal areas. We went with a low-end level-1 for those areas, because of an isolated tornado / hail threat.