Storm Forecast
Valid: Fri 24 Oct 2008 06:00 to Sat 25 Oct 2008 06:00 UTC
Issued: Thu 23 Oct 2008 18:09
Forecaster: TUSCHY

SYNOPSIS

Major large-scale modifications of the streamline pattern are underway with a transition from a more zonal to a strongly blocked pattern from North-America to Europe during the upcoming week. Strong upper trough, currently situated over extreme NE-North-America/SE-Canada shifts eastwards during the following 2-3 days, gaining strength over the N-Atlantic, while increasing its amplitude. Its span of life won't be remarkable high, but still it is worthwhile to watch its evolution as its finaly placement/strength and duration of decay will be crucial for the exact evolution over Europe next week.

Over the NE Atlantic, 12Z-CMC analysis has a strong and very well structured cold-core low over Iceland (04018 has the tropopause well below 5km) and numerous smaller-scale depressions, rotating around the main vortex. Intense CAA upstream of this vortex is underway but as SSTs gradually increase to 9-10°C S/SE of Iceland, rapid modification of the lower column of the atmosphere should occur. Not so at mid-levels, where temperatures reach -35 to-40°C at 500hPa S/SE and E of Iceland, so an extensive area will see steep lapse rates with modest instability release between Iceland - Scotland and NW-Europe. Wind field at lower levels increases over this area in response to an intensifying surface depression (EZMWF 941hPa, CMC 10hPa higher, which is in line with GFS) NE of Iceland, so we have to discuss this area regarding organized thunderstorm potential.

Finally, another depression forms during the night hours west of Ireland, moving rapidly to the east/northeast, producing an intense wind field, which could affect the extreme NW part of the forecast area. As seen in the past few weeks, the position and strength of the strong high pressure area over the Azores assists upper troughs/cut-off lows to dig very far to the south, such as tomorrow, when a weakening upper trough will be placed over far NW-Africa. This feature will be well to the south of our area of responsibility, but embedded in this channel of low geopotential heights, a small cold-core low drops southwards over Spain, resulting in unsettled conditions over SW-Europe. The rest of Europe has high surface pressure with numerous upper disturbances over SE/E-Europe...the overall set-up is alike the Fri 10 Oct 2008 one.

DISCUSSION

... NW/N-Ireland (03Z onwards) ...

Despite the agreement of the occurrence of a developing depression west of Ireland during the night hours, the rate of intensification remains uncertain. Models like the EZWMF, CMC and GFS all show a very tight baroclinic zone which evolves over the NE-Atlantic and also mid-/upper wind field gains strength, so regarding dynamic affects, conditions for rapid cyclogenesis are fine. The main focus for thunderstorms will be the SE-quadrant of the depression, as strong dry slot works its way to the east, overspreading a warm/moist warm sector, while an intense UVV field overspreads the area of interest from the WSW. Some potential instability could be released, resulting in showers/isolated thunderstorms. Wind field in this area is breathtaking with readings of 40m/s at 850hPa and depending on the intensification rate and the timing of the dry slot, the evolution of a sting jet looks possible, which could even augment wind speeds. For now, a level-1 was issued as most of the activity will stay out of the forecast area until 06Z. A thunder area may be needed, if convective activity will be stronger than anticipated.

... NW-Europe...

The environmental condition was already described in the synopsis. In the aftermath of the active cold front, which crossed the area the night before, strong CAA will be on its way to the east. The airmass is moist and global models show various convergence zones / small-scale vortices, which are embedded in this CAA-regime. Both, parcel layer depth and LL convergence signals are exceptionally strong for this area, so combined with modest SBCAPE and very cold mid-/upper levels, deep convection will be a distinct possibility. Current thinking is that main shower/thunderstorm activity will be confined to the passages of those convergence zones and 20-30m/s winds at 850hPa could result in widespread strong to severe wind gusts. Another focus for enhanced convective activity will be the potential evolution of polar lows, as pattern will be quite favorable for their development, although there remain some negative points (e.g. large-scale, weak WAA downstream of an eastward racing trough...see next paragraph). Latest IR animation (16Z) already had an area of very cold cloud tops SE of Iceland with a cyclonic loop.

Finally, an eastward shifting trough approaches SW-Norway after 18Z and winds at 850hPa should increase to well above 30m/s (40m/s at 700hPa) , which should also enhance the severe wind gust risk along the coastal areas of Norway. All those features combined should justify a level-1 area for severe-damaging wind gusts and isolated tornadoes as a result of deep convection.


... SW-Mediterranean...

Beneath the weakening upper trough, conditions for thunderstorms remain favorable. Wind shear, strong during the morning to afternoon hours, diminishes betimes, while instability release increases during the night hours, as mid-levels cool down. Quite warm LL temperatures should limit LL CAPE release, but GFS still has a belt of enhanced 0-3km CAPE release just south of the Balearic Islands during the evening and night hours. The expected reports should be mainly large hail during the morning / afternoon hours, while the risk of an isolated tornado should increase during the early evening hours, mainly over the level area.

Despite a gradual weakening trend of the SW-flow over N-Algeria, subsidence/weak foehn effects just N of the coast should prevent thunderstorm evolution.