Storm Forecast
Valid: Sun 06 Jan 2008 06:00 to Mon 07 Jan 2008 06:00 UTC
Issued: Sat 05 Jan 2008 18:39
Forecaster: DAHL

SYNOPSIS

Intense broad upper zonal flow is stretching from the Atlantic across central Europe into SE Europe, undercutting an upper ridge covering NE Europe and W Russia. The latter feature maintains a sturdy SFC high over the extreme E parts of Europe and Russia, while a large SFC low pressure complex is covering the NE Atlantic and much of Europe. Several wave cyclones are present at the periphery of this system. Quiescent SFC conditions prevail over the Mediterranean.

DISCUSSION

... S UK ... W France ... W Benelux ...

Main challenge this period is the prediction of convection along the frontal wave which is simulated to track across the British Channel on Sunday evening, reaching N France/W Benelux towards the end of the period. The track and strength of this feature is quite coherently simulated by the diversity of available models, so it does not seem very likely that rapid cyclogenesis with an attendant threat for destructive gradient winds, occurs ... though the intense upper-level flow still has the potential for supporting very intense cyclogenesis.

GFS hints at weak CAPE N of the crest of the frontal wave, which is a somewhat unusual location for SFC-based CAPE to develop. Also, weak -- albeit shallow -- instability is forecasted in the warm sector of the low. Spatially, these regions appear to be co-located with regions of maximum DCVA. It could be that the developing instability will be tied to the intrusion of dry upper-level air across the SFC cold front into the warm sector, but this is not certain at this time.
Altogether this seems to imply that some chance for at least shallow convection exists. However, given comparatively weak cyclogenesis, it does not seem likely that the mesoscale forcing along the cold front will be extraordinarily strong ... also, the low-level wind field is somewhat weak (e.g., 850 hPa flow of only 15 to 20 m/s along the cold front). This is also reflected in the 0-3 km SRH which is only on the order of 200 m2/s2 ahead of the cold front ... which is quite marginal given the minimal thermodynamic/dynamic support.
It thus seems that some potential for shallow/forced linear convection exists, but its severe potential especially in terms of severe wind gusts should be limited. Since 0-1 km bulk shear is expected to exceed 15 m/s, there does exist some threat for a brief tornado or two, if isolated cells develop. However, at the moment, the allover severe convective weather threat seems to be too low for a categorical risk. Isolated lightning could occur with this convection, but it should remain too sporadic to warrant a thunderstorm forecast area.
An upgrade may be required on Sunday if it transpires that convection will be deeper than currently anticipated, which would allow it to become more strongly electrified and to benefit from deeper/stronger shear, increasing its potential to produce severe weather.

... S Adriatic Sea ... N Ionian Sea ...

Convection will likely be ongoing over the S Adriatic/W Ionian Sea at the beginning of the period ... and become gradually more shallow while spreading into the E Ionian Sea. It thus seems that -- except at the very beginning of the period -- the storms will not profit from rather favorable 0-6 km shear, and fail to become organized, though scattered lightning may well persist through the period.