Storm Forecast
Valid: Sun 05 Oct 2008 06:00 to Mon 06 Oct 2008 06:00 UTC
Issued: Sat 04 Oct 2008 17:07
Forecaster: TUSCHY

SYNOPSIS

Downstream of an intense N-Atlantic jet, a very diffluent upper tropospheric streamline pattern evolves west of Europe, while numerous intense depressions affect an area SE of Greenland to Finland. Another feature of interest will be a progressive upper trough over SE-Europe, moving to the northeast. Most parts of Europe will be affected by a too hostile environment for organized, deep convection.

DISCUSSION

...Moldova and Ukraine, parts of Romania and Bulgaria and the western Black Sea...

Thunderstorms will already go on during the start of the forecast and will rapidly spread east/northeastwards as the main impulse at upper levels and attendant compact PVA field move northeastwards. Both, the weakly capped warm sector and a strong cold front will serve as foci for thunderstorms during the day as strong moisture advection underneath moderate mid-level lapse rates result in MLCAPE values of 500 - near 1000 J/kg over E-Romania, decreasing gradually to the north/northeast. A 40m/s speed max at 500hPa curves around the base of the upper trough and crosses the western Black Sea region, placing the areas with strongest instability release in the favorable left exit region. Scattered thunderstorm development along and ahead of the cold front is anticipated and an organized line of storms should race to the NE. LLS and DLS are enhanced over the level area and strong LL CAPE release/low LCLs point to an enhanced tornado risk, especially with more discrete thunderstorms. Otherwise the more linear mode of the thunderstorm evolution along the cold front raises the severe wind gust risk. Over the Ukraine, instability release diminishes constantly but strong forcing could offset that effect for some time so the risk area was expanded well to the north.

Beside the impressive kinematic environment, no level-2 was issued as strong LL shear over NE Bulgaria/SE Romania during the early morning hours relaxes betimes and directional shear is forecast to stay quite weak. Aforementioned better LL CAPE release further to the north could still be supportive for tornado development, but expected coverage should not justify a level-2. Offshore over the W-Black Sea, the risk for more severe wind gust reports is higher, but instability release should be less.

Finally parts of N-Romania were also included in the risk level area as wrap-around moisture could lead to some marginal instability release. The past few model runs (GFS) changed the magnitude of instability release all the time, so there is a high degree of uncertainty with this part of the level area. The environment would be very favorable for tornadic storms ( LL shear 15m/s, SRH1 200-400 m^2/s^2 and low LCLs).

... S-UK (06Z-12Z) ...

A complex scenario unfolds for the highlighted area during a short time-frame. At LL a strong cold front crosses UK from the north, reaching S-UK during the early morning hours, the coastal areas during midday and clears UK during the afternoon. A weakening surface depression is forecast to enter the western Channel at roughly 09Z, moving slowly to the east while weakening. Surface wind field ahead of this feature backs with LL shear increasing to 10-15m/s. Airmass ahead of the cold front is very warm/moist (the relict of tropical storm Laura ), so lapse rates at all levels remain marginal, also limiting instability release. However, traces of instability are forecast and forecast soundings indicate a chance for some instability, especially when at least some diabatic heating materializes. If thunderstorms can evolve as cold front pushes to the south, strong DLS in the order of 20m/s should assist in storm organisation with a tornado and severe wind gust risk, given very strong LL winds ( 850hPa at or above 25m/s ).

It should be noted that GFS and GME 12Z runs are the more aggressive ones as surface depression is weaker in the rest of the model pool, but all agree in placement and track of this feature.

Very high moisture content of the airmass also enhances the risk for locally very intense convective rain amounts with a localized flash flood risk. This threat extends well to the east but is not reflected in our risk scheme.