Storm Forecast
Valid: Fri 11 Jan 2008 06:00 to Sat 12 Jan 2008 06:00 UTC
Issued: Thu 10 Jan 2008 20:51
Forecaster: TUSCHY

SYNOPSIS

Very active weather pattern continues for the forecast period. A developing depression in the southwestern qudrant of a huge cyclonic circulation moves southeastward, reaching the Bay of Biscay during the early morning hours of Friday. As this feature moves further to the east a new depression develops over extreme NE Spain, reaching the western Mediterranean during the evening hours.

Stable conditions prevail for the rest of Europe.

DISCUSSION

... NW Spain, N-Portugal and the coastal areas of N-Spain and SW France ...

An upper-level trough approaches the area from the west during the morning hours, driven by an intense mid-/upper-level streak in its southwestern quadrant. This supports a negative tilt of this feature with attendant growth of the large-scale wave's amplitude.
At the surface, a depression is forecast to strengthen as being placed in the dale of this strongly cyclonic curved trough ( also being placed in the left-exit region of a potent 55m/s mid-level streak ).
As this depression moves eastward, reaching the northern Bay of Biscay during the morning hours, pressure gradients at lower levels tighten and support winds at 850hPa at or above 25m/s.
Instability release is faint due to a shallow moisture layer at lower levels, which rapidly gets mixed out onshore. 100-200J/kg along the coastal areas ( a bit more offshore ) should be enough for an isolated severe wind gust risk as wind at lowest levels should get mixed downwards due to a well mixed BL with steepening LL lapse rates and no obstructive inversion being present.
We decided not to issue a large level-1 as set-up looks too marginal.

Best chances for a short spin-up exist over extreme NW Spain / NW Portugal starting between 12-15Z and lasting until the early evening hours. As a response of an approaching trough axis in the surface pressure field, helicity values increase in the postfrontal airmass and an isolated thunderstorm with a LL mesocyclone in a well mixed airmass can't be exlcuded. Environment is not too conducive and hence we only issued a small level-1 along the coastal areas.
Threat should diminish after sunset.

... NW France...

Another area with a low-end tornado threat exists over NW France between 18Z and 00Z, stretching far inland. As surface depression comes onshore, a surge of at least marginal better LL moisture overspreads the area from the W/SW and as mid-level lapse rates continue to steepen, at least low-end instability release can be expected in an environment with 150-250 J/kg helicity values at the lowest 1000m.
Strong wind gusts can be expected, too with a few gust approaching our criterion mainly along the coastal areas.
We decided to issue a low-end level-1 for the coastal area expanding the thunderstorm area well inland.


...SE France and extreme northwestern Italy...

As pressure falls over the western Mediterranean a nice moisture return pattern develops over SE France. 6g/kg mixing ratio gets advected well to the north, even reaching extreme western Germany during the afternoon hours.
Temperatures at all levels stay warm in this WAA regime with weak mid-level lapse rates over SE / E-France, increasing over SE France to 6K/km. During the evening hours ( 18-21Z ), mid-levels cool down and lapse rates steepen somewhat. The main negative point for enhanced thunderstorm probabilities looks like to be a lull in the UVV fields as this area is located between 2 surface low pressure areas.
Best chance for a few thunderstorms will be over SE France and an isolated tornado can't be excluded although overall set-up looks quite marginal.

We decided not to issue a thunderstorm area over E-France and western Switzerland, but an isolated embedded thunderstorm in a more stratiform rain shield can't be excluded.

An isolated tornado is also possible along the coastal areas of NW / western Italy and Corsica / Sardinia during the early morning hours. Threat is too low for issuing any level area.