Storm Forecast
Valid: Fri 26 Sep 2008 06:00 to Sat 27 Sep 2008 06:00 UTC
Issued: Thu 25 Sep 2008 18:18
Forecaster: TUSCHY

SYNOPSIS

No major changes over Europe for the next 24 hours as opposed to the E/NE Atlantic, where more meridional/blocked flow re-intensifies, which abounds in positive anomalies of latest geopotential height gradient analyses/forecasts for the Atlantic and this period of stronger blocking will persist for the next few days. A wave train of strong upper troughs S of Greenland helped to sharpen a baroclinic zone, which now runs from near 35°N/80°W to 65° N/10°W. A sharp upper trough, analyzed at 40°-60°N/38°-50°W at 12Z, overspread the baroclinic zone yesterday, inducing an area of low pressure, which now is already placed at roughly 53°N/32°W with a nice comma structure in place. Latest CMC upper wind data place this depression beneath a 75m/s streak and models agree in further intensification of this jet maximum to more than 90m/s. The surface depression will race NE-ward along the baroclinic zone during the forecast and the environment is conducive for constant strengthening and it is not beyond question that rapid deepening could occur. Latest available buoy/ship data hint on 20-25kt surface winds, also fostered by the QuikSCAT overpass which already had isolated 30-35kt flags near the center. EZ/GFS and CMC agree quite well with the track of this feature, which will reach Iceland by 12Z. GFS/NOGAPS and GEM are the outlier regarding intensity forecast, bringing the surface pressure to at or below 975hPa E/NE of Iceland at SAT,00Z while the rest of the model pool is more conservative. The intense depression should then move slowly to the east over the Norwegian Sea, but is forecast to stay offshore until SAT,06Z. Positive tilted trough over the NE-Atlantic becomes stationary and a cut-off evolution becomes increasingly likely (see short side-note below). Intense WAA dowstream of this depression re-strengthens ridging with strong positive thickness advection forecast by the models. South of the N/CNTRL-Europe ridge, numerous upper lows spin around (31°N/25°W...40°N/5°W...broad vortex over SE Europe) embedded in a SW-NE aligned channel of low geopotential heights. Weakening of those vortices will be the main difference from today so no mentionable movement expected. A cold and dry airmass affects NE Europe and a cold front will cross NE Europe during the end of the forecast period, but neither the front itself nor the airmass are promising for thunderstorm development.

***A short side-note***

Aforementioned cut-off process at ~35°N/35°W during the next few days has chances for structural changes as this feature resides over a pool of anomalous warm SSTs (positive anomaly of more than 1.5°C). Relaxing shear and a time-frame of a few days for organisation could be enough for gaining subtropical characteristics. No direct impact for the European weather expected so further notification either by the appropriate agency (NHC), or by sporadic side-notes in upcoming outlooks.

DISCUSSION

...Norwegian Sea...

Main concern for convectively enhanced gale-force winds arises after 00Z over the highlighted area, as intense depression rapidly approaches from the west. PVU maps show a well evolved streamer, pointing into this system, superposing a region with intense PVA, so regional release of potential instability is possible in the NE quadrant of the depression. Convection will stay shallow as EL stays low, but nevertheless, downward mixing of intense winds from low-mid levels is likely (H85 25-30m/s and H7 the same). Even stronger winds expected in the warm sector but airmass will be too warm for enhanced/deep convection. A non-thunder level was issued to reflect the chance for severe wind gusts in conjunction with enhanced convection.

...Extreme SW Black Sea...

Broad upper vortex still affects the SW Black Sea where SSTs range between 21-22°C. Atmosphere stays quite warm and lapse rates at mid-levels weak, but models like GFS show an area of low-end MLCAPE/strong SBCAPE release, so scattered thunderstorms will evolve during the forecast period. 0-6km bulk shear in the range of 10-20m/s is intense enough for multicell storms and a marginal hail/strong wind gust threat. A conditional level area was issued, where winds of 20-25m/s at 850hPa and rapidly vanishing instability values overlap as numerous strong/isolated severe wind gust reports can't be ruled out. The level area was expanded well to the south in accord with latest LM output.

... Mediterranean ...

Under the base of the aforementioned broad upper trough channel there will be various places, where thermodynamics become supportive for isolated to scattered thunderstorm development. Only places were highlighted which seem favorable enough for initiation. Over most parts, shear is too weak for storm organisation although locally enhanced LL CAPE release could result in a few waterspout reports, but there is no area to put out a level for spouts as confidence in their coverage/evolution is very low.

DLS increases over the western Mediterranean next to the strong subtropical jet and a level area was issued as a mixture of single cell/multicell storms could produce marginal/isolated large hail and strong to isolated severe wind gusts.