Forecast Update
Valid: Mon 11 Aug 2008 22:00 to Tue 12 Aug 2008 06:00 UTC
Issued: Mon 11 Aug 2008 22:32
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook, issued at Sun 10 Aug 2008 18:00 UTC.

DISCUSSION

...N-France...

The attention for the next few hours will be the upper disturbance, now just offshore over the Bay of Biscay. This feature is hard to detect in IR loops, but in the IR/WV combination, there is a rapidly expanding area where clouds disspiate and it looks like this is the intrusion of higher tropospheric/lower stratospheric air, characterizing the position of the feature (which would be well forecast by GFS). The disturbance should reach the NW coast of France during the next 1-2 hours. Regarding BL airmass quality, dewpoints over S/CNTRL France range between 17-19°C, decreasing to 12-15°C over N-France. Surface winds over most parts of France now have a persistent southerly component and this wind field should persist/increase in strength during the night, as the disturbance approaches. Rich BL moisture should be in place after midnight in the broad warm sector with dewpoints well above 15°C. The main concern, nocturnal BL stabilisation, is limited by a thick mid-/upper-level cloud shield with surface temperature readings in the mid-tens. A modified, subtropical airmass will be present in its warm sector, resulting in overall weak lapse rates at all levels so instability release is still the main questionable/limiting factor. However, forcing is strong and if surface dewpoints/temperatures climb only a few degrees higher than currently anticipated, surface based convection will be possible. The wind field is impressive as strong veering and speed shear are already present in wind profilers and LL shear of 15m/s and enhanced SRH-1 values, combined with an intense wind field of 25m/s at 850 hPa will overspread the level-area. Regarding shear, strongest shear will be confined more to the warm front itself, but will stay strong enough over the whole warm sector.

Conditions are favorable for a few thunderstorms to develop over N-France during the rest of the forecast time. Forecast soundings and current data observations hint on a chance that storms can root into the highly sheared BL. Each thunderstorm, which can manage that will have a favorable environment to organize rapidly with an augmented tornado threat and even a strong tornado can't be excluded.Furthermore, severe wind gusts and marginal hail will accompany stronger storms with an enhanced severe wind gust threat if storms line-up. The final risk level issuance is difficult to determine as uncertainty regarding the final thunderstorm coverage is high but latest radar trends from NW France already indicate the first showers / SFLOC reports offshore. We will stay with a high-end level-1, but with strong wording regarding the tornado threat.

The risk for isolated tornadoes continues over E-France as LL shear increases steadily during the next 6 hours. EL temperatures stay quite warm and deep convection is unlikely due to weak lapse rates and warm mid-levels, but again, not much increase in LL moisture/warmer surface temperatures needed to get some low-end instability and hence a non-thundery level area will be retained due to the tornado risk, not necessarily accompanied by electrified convection.