Storm Forecast
Valid: Tue 16 Oct 2007 06:00 to Wed 17 Oct 2007 06:00 UTC
Issued: Mon 15 Oct 2007 20:49
Forecaster: TUSCHY

SYNOPSIS

The main features of interest for the upcoming period will be an intense depression west of Norway and a flat trough over SW Europe.
Both bring unsettled weather conditions for parts of northern and southwest Europe.
In-between strong high pressure prevails and brings warm and stable conditions for most parts of the central Mediterranean and central Europe.

DISCUSSION

....Parts of Norway....

Latest IR/WV loops ( 18Z ) show a well developed depression SE of Iceland, moving rapidly towards the ENE. Pressure at lower levels continues to fall due to a strengthening baroclinic environment and nice dynamics ( e.g. embedded in the left-exit region of a 120kt jet , placed over southern Norway). As no compact streak enters the trough on its upstream side, this configuration ( neutral trough axis ) will persist for the period with a slight positive tilt during the end of the forecast period. This has major effects on expected convective activity over Norway, as temperatures at mid-levels stay warm, lapse rates weak and instability release marginal. In addition, this feature never had a real and long lasting connection to higher Theta-E air from the subtropical region due to another trough west of Spain and hence moisture advection at lower levels is marginal at best ( mixing ratio around 5g/kg ).
Current thinking is that the main player for enhanced convective activity will be a decaying vortex, centered just NW of Ireland yesterday at 18Z. This feature has no signs of enhanced convective activity with some stronger cells on its eastern side and SSTs further towards the east ( east of Scotland ) only get colder with no significant change in the thermodynamic mid-tropospheric stratification so no signficant increase in thunderstorm coverage is expected.

Highest potential for thunderstorms will exist along the western coastal areas of Norway mainly during the 06-12Z time - frame, as somewhat more humid LL parcels get advected well onshore. We issued a thunderstorm area mainly along the coastal areas but there exists a chance for potential non electrified convective activity well inland, before meager BL moisture gets mixed out. 20m/s shear at the lowest 3km indicate a risk for strong to severe wind gusts, but we see no reason to issue a level area due to limited convective activity.
Along the coasts, enhanced LL shear will be present, but slim LL instability and aforementioned limited moisture confine the risk for tornadoes, so no level area will be drawn.

... Parts of western Scotland...

As a broad upper-level trough crosses the area, minimal instability release can be anticipated, but otherwise quality of airmass at lower levels does not support any enhanced tornado risk. We did not issue any highlighted area, as no augmented thunderstorm coverage is expected .... this is also in accord with warm convection tops N of Ireland during the latest IR images.

... SW United Kingdom ...

A tricky situation for extreme SW-United Kingdom. The focus for any potential development will be a front, first surging northward as a weak warm front and reaching SW-UK , transforming back into a cold front during the afternoon hours. Rich PBL moisture and warm mid-levels in the prefrontal airmass points to a more heavy stratiform rain - event ( also assisted by maximized moisture inflow in this area ) , but in a short time frame between 14 and 18Z, a marginal enhanced possibility for stronger convection exists. As mid-levels cool down and lapse rates steepen to about 6K/km, strong forcing overspreads the region from the SW and could support stronger convection and maybe an elevated storm atop of a warm 850hPa layer. Low confidence in this scenario and resultant low-end storm coverage don't support any highlighted area.

... Parts of Spain, northern Morocco and NW Algeria...

A broad and ill defined upper-level trough affects those areas and model pool is pretty inconsistent in developing a weak cyclonic feature off the Morocco/ NW Algerian coast. We do not speculate too much as data coverage over those areas is not the best and so are model calculations in such weak-gradient SW-flow regimes.
Environment is fine for scattered thunderstorms to develop in a weak instability and moderately sheared environment. GFS has a belt of better BL moisture just east of Spain coupled with cooler temperatures at 850hPa and somewhat enhanced SRH values. This results in better 0-3km CAPE release which in addition with aforementioned parameters favors a slightly enhanced tornado potential. We decided to issue a marginal level-1 for a small portion east of central Spain, as initial storms could grow into an environment favorable for storm rotation with an isolated large hail and tornado risk.
Betimes, storms should cluster and severe weather threat will decrease, while heavy rain threat increases.
Otherwise, stronger gusts and marginal hail should be the main threat in stronger pulsating storms.

The risk for severe weather would increase south of the Balearic Islands into the level-1 area if indeed a cyclonic swirl emerges from Morocco and drifts northward but discounted this possibility due to aforementioned uncertainties.
This would also significantly enhance the flash flood risk over east-central Spain as forcing in a very moist airmass would be stronger than currently anticipated.