Storm Forecast
Valid: Fri 19 Oct 2007 06:00 to Sat 20 Oct 2007 06:00 UTC
Issued: Thu 18 Oct 2007 21:27
Forecaster: TUSCHY

SYNOPSIS

Very active weather pattern continues during the next day as an intense upper-level trough over Greenland and adjacent CAA forces modified tropical air towards the north, affecting Iceland with unseasonable warm temperatures ( 850hPa temperatures at or above 10°C which is also reflected in the extreme forecast index of the ECMWF, approaching 1 ). As a result of the downstream development, geopotential heights east of the Geenland trough rise significantly with another trough on its downstream side, affecting most parts of central Europe with the first outbreak of colder temperatures.
The main challenge for this forecast will be a system over the central and later the eastern Mediterranean, affecting Greece, Cyprus and SW Turkey during the latter part of the forecast period.

As a personal side note :

A short review:

Of main interest during the past few days was a well defined upper-level low pressure system east of Tunisia with an intense shear zone at lower levels, running from Libya to Malta. Both, QuickSCAT overpasses and surface observations showed a persistent and strong inflow from the SE towards the center with reported gusts of more than 20m/s over NW Libya. This resulted in the advection of very dry desert air towards the center, slowing down the development of the system in the otherwise favorable environment ( weak shear and SSTs of 24-25°C ). In addition, a well developed polward outflow developed during the day yesterday, as aforementioned European trough continued to dig southward.
The system began a slow eastward motion during the day yesterday and is forecast to accelerate during the forecast period with a constant ENE-movement.
Yesterday's 17Z QuickSCAT overpass missed the area of interest but a new TERRA/MODIS image indicated a sheared depression with the convection displaced towards the E / NE and an exposed but well developed cyclonic circulation.
At 15Z, 18th Oct, very intense convection evolved just next to the center and a few scans later a well developed central dense overcast was present roughly over or just east of the center. We assumed that this would help to establish a low level circulation center during the night hours and this feature could acquire "sub"tropical characteristics during the forecast period. Anyway...satellite appearance continues to indicate an increasingly sheared feature which should continue tomorrow as shear only gets stronger.
It is not unusal for such systems in a not too favorable environment to develop bursts of deeper convection before decaying after a few hours repeating this fore days.

Forecast for this outlook ( 19th Oct ):

Models diverge somewhat regarding the exact track of this system, as UKM and NGP now bring the center over N Libya but still developing a warm-core feature. GFS is more aggressive and still integrates the depression into the westerlies ( the approaching trough ), bringing the feature over S-Greece / Cyprus into SW Turkey although latest runs constantly showed a more southerly track. I decided to take GFS, as system now gets stronger and should feel the stronger steering flow at mid-/upper-levels from the WSW.
Dependant on how far north it will move, influx of very dry desert air plays a more or less important role.
Increasing shear from the west, drier air and an approaching baroclinic zone from the NW should limit the development but we could deal with a possible "sub" tropical feature over Cyprus later the day, if trends of a better defined system of yesterday evening continue.

Again...I certainly won't arrogate to be able / be in the position to declare anything but there is at least a chance for this feature to acquire "sub"tropical characteristics sometime in its lifetime despite increasing shear and constantly cooling SSTs.

Update (s) will be issued tomorrow if this feature indeed continues to develop .

I especially want to thank Dr. Beven ( Hurricane specialist of the National Hurricane Center ) who was so kind to support me in this situation although the Mediterranean is out of his / their responsibility! I again want to emphasize that those lines are only a personal assessment and NOT any official forecast of any institution !
***Update at 00Z,19th Oct. 18Z GFS run just came in and now also dissipates the system over Libya which is also supported by latest IR/WV loops, indicating a system, moving directly to the east.This would bring the center onshore/ very close to the northern coast of Libya during the next few hours. As this is out of our area of responsibility we won't continue to issue any updates on this system.***

DISCUSSION

... NW Poland ...

The streamline maps indicate a more northerly flow pattern over the western Baltic Sea which enhances the fetch over open water during the next 24 hours. In addition, mid-levels stay cold with lapse rates of near 8K/km. Better moisture quality in the boundary layer in combination with those steep lapse rates and weak forcing ( at least during the early part of the forecast ) should support a few thunderstorms. Shear is weak, but marginal hail could accompany those storms.

... Southeastern Germany, the Czech Republic and northern Austria...

Quite tricky to highlight any specific area. Very cold profilers and quite low spreads show the chance for somehow deeper convection with EL temperatures approaching -30°C. As convection will be daytime driven, expect best chance during the early afternoon to early evening hours, as strong forcing in terms of a well developed trough axis spreads southwards. Current thinking is that thunderstorms will stay confined to a region east of the shear axis.
We included northern Austria, as disintegrating band of somewhat higher mixing ratio and a rapidly cooling atmosphere could support an isolated thunderstorm threat.
Kinematic parameters are weak but low WBZ levels and steep lapse rates could support a marginal hail risk in stronger storms.
It makes no sense to focus only on the highlighted area in such marginal conditions as a few SFLOC reports could also occur well outside under the broad cold pool.

...Eastern Belarus...

Downstream of the intense upper-level trough, widespread pressure fall is expected with a developing surface low over northern Romania and Moldova, accelerating northward during the night hours. Moist air of the eastern Black Sea advects northward, reaching SE Belarus during the evening hours.The main focus for not necessarily electrified convection will be the northern quadrant of the surface depression, where wrap-around moisture undercuts cool mid-levels in an environment with strong forcing. Shear at lower levels is strong and SRH 1km values of more than 200J/kg indicate the risk of an isolated tornado. Threat is not excessively high but conditions are quite similar to the possible tornadic event a few days ago over western Russia.


... Greece, Aegean Sea and western Turkey...

A lot depends on the final path and structure of this depression. We decided to concentrate on the general scenario not on the system itself.

One focus for thunderstorms will be a southward moving cold front, slowing down and becoming nearly stationary during the latter forecast period somehwere over Greece. Shear stays quite weak and no real severe threat should evolve out of those thunderstorms.
GFS backed off with the PBL moisture as plume of subtropical air is now forecast to stay further towards the south.
We think that BL over Greece stays quite dry and in combination with warm mid-levels, no more robust instability release can be realized. We therefore excluded Greece from the level area and included the areas offshore, where thermodynamic profilers will be better. Shear, especially at lowest 3km increases considerably with values of 20-30m/s and a few severe wind gusts are expected ( 20Z onwards ).
After 00Z, thunderstorms also increase southwest of Turkey and higher moisture content / stronger shear especially at lower levels hint on an enhanced tornado risk. The exact threat fluctuates with the final strength of the aforementioned system with potential tropical characteristics.

Another focus for an enhanced severe thunderstorm potential arises over the northern Aegean Sea during the late evening and night hours, where influx of higher moisture content and a slowly cooling lower troposphere support an increase in instability release during the night hours. Shear is on the increase and storms should pose an augmented severe wind gust, isolated large hail and tornado threat....the latter one mainly along the coastal areas, where LL shear gets maximized.

Dependant on the final track, an excessive heavy rainfall risk exists over southern Greece, Cyprus and western / southwestern Turkey with flash flooding being a distinct possibility!