Storm Forecast
Valid: Fri 16 Feb 2007 06:00 to Sat 17 Feb 2007 06:00 UTC
Issued: Thu 15 Feb 2007 19:25
Forecaster: TUSCHY

SYNOPSIS

A highly disturbed weather pattern will affect Europe with an amplified trough west of Portugal and strong ridging on its downstream side. Exceptionally warm air will be transported towards the north ( 850hPa values of 11-15°C over the SW Mediterranean and 10°C ! up to the Netherlands ).
This trough will slowly shift towards the east and will affect most parts of Portugal and Spain till the latter part of the forecast period.
Another trough over SE Europe will continue its eastward shift.
Otherwise most parts of Europe will see warm and capped conditions over W-Europe and cold and stable conditions over eastern Europe.

DISCUSSION

...Portugal and most parts of Spain ...

Conditions will become favorable for an augmented severe hail threat with strongest cells mainly during the daytime hours over S-Portugal , SW-/S- and central Spain !

A very warm and dry airmass will be advected northward downstream of an eastward shifting upper-level trough and although the bulk of the strongest WAA will be found east of Spain, surface temperatures over S-Spain will reach 20 to locally 24°C.
The focus for thunderstorm development will be an eastward shifting cold front but movement of this zone will slow down during the forecast period because of the nearly parallel alignment regarding the background flow. Global models also indicate a weakening 0-1 km mean wind streamline pattern along this baroclinic zone mainly over N-Spain, which is also reflected in a weakening UVV field.
As a result of strong diabatic heating, strengthening onshore flow could support the convergence of this zone and therefore a delay of expected weakening of the frontal structure mainly over S-Portugal / Spain .

Timing of exact thunderstorm initiation is still diffuse. Very dry mid-level airmass is present ( reflected at 12Z LEMD sounding ) further inland and a pronounced warm layer at 800hPa will support strong capping.
GFS indicates a cool down at upper-levels as early as 12Z and SW Spain and S-Portugal will likely see initiation as early as 10-12Z.
Dewpoints already higher than global models like GFS indicate ( Td's 10-15°C ) and in combination with constantly steepening mid-level lapse rates, up to 500 J/kg of instability look reasonable.
Enhanced LL shear would support an isolated tornado report mainly over S-Portugal and SW / S-Spain where strong onshore flow and low T-Td spreads support low LCLs.
DLS of 30-35m/s will also foster an isolated large hail / severe wind gust threat.

At 13Z to about 18Z, the focus turns northeastward towards central Spain.
Very steep mid-level lapse rates, a dry boundary layer and DLS of 35-40m/s will be present. 15Z forecast sounding of Madrid indicates an inverted-V-type sounding with an humid layer between 700 and 500hPa and very dry air beneath.
Forcing along the eastward moving cold front and a constant cool-down at mid- /upper -levels will be enough for at least marginal instability release well inland.
This front and a surge of higher PBL-moisture content towards central Spain will be enough for good confidence in thunderstorm development.
Each storm will pose a severe - wind gust and large hail risk with isolated extremely large hail possible.
Right now, the confidence is still too low for a more significant thunderstorm coverage over CNTRL Spain due to very low instability release.
Storms will shift towards the NE and will also affect parts of NE Spain, but rapidly decreasing instability values will suppress the severe thunderstorm threat.

After sunset, main area of severe thunderstorm development will be the coastal areas of S-/SE-/E- Spain, the Strait of Gibraltar and the coastal areas of NW- / N- Morocco.
There exists an isolated tornado threat mainly over NW-/N- Morocco due to enhanced LL shear but DLS will be strong enough,too for an isolated severe wind gust and large hail threat with the stronger cells.

Later model runs and the evolution of the thermodynamic / kinematic environment will dictate if an upgrade over CNTRL Spain and an expansion of the level-1 towards the NE and E will become necessary later on.

...Bay of Biscay and extreme NW France...

An evolving strong LL depression somewhere over the northern Bay of Biscay and cool mid-levels will assist isolated to scattered thunderstorm development. Best shear and instability will stay well separated and hence no significant severe weather threat can be expected.

...SE part of the central Mediterranean and Crete...

Broad upper-level trough and accompanying LL depression will suport a wide area of 400-800 J/kg SBCAPE release. The shear will be weak and LL lapse rates are forecast to be very steep. Won't exclude a few waterspout reports.