Storm Forecast
Valid: Wed 21 Jan 2009 06:00 to Thu 22 Jan 2009 06:00 UTC
Issued: Tue 20 Jan 2009 23:35
Forecaster: PUCIK

SYNOPSIS

An overall change in the synoptic-scale circulation is expected as a deep trough, stretching from Scandinavia to eastern Spain will quickly decay. In its wake a belt of very strong westerly mid and upper level flow will start spreading over the Eastern Atlantic by the end of forecast period. A ridge over eastern Europe will become more amplified, increasing the strenght of southerly flow over the Central Europe. At the surface a large area of low pressure continues to dominate weather over the most of Europe, except for eastern parts, which are affected by the Siberian high. A further cyclogenesis is expected in the Atlantic and a series of deep lows will start to affect the British Isles and western Europe by the Thursday morning.

DISCUSSION

...Western coast of Norway...

A sporadic lightning is expected especially in the coastal areas, where the strongest convergence signals are simulated by the GFS model as a westerly flow will interact with the mountain range. Despite the fact that only marginal instability will be available ( thanks to the steep mid level lapse rates), EL temperatures will be well below - 20°C and lightning should occur.

....Bay of Biscal and the coast of Algeria/Morocco...

We are anticipating very similar situation to the Norway here although the activity should quickly diminish by the noon hours. Few waterspouts are possible, especially in the coastal areas as most of instability should be confined to the lower levels.

... Belt from southern Ireland to SW England and Wales...


A mid level impulse is forecast to develop in the strenghtening flow , inducing the development of the trough in the low level pressure field. A strong upward motion is predicted to destabilize the airmass, which will profit from the inflow of moist and humid air from the south. Dewpoint temperatures will probably exceed 10°C. Degree of instability is questionable at the moment and different model runs were quite inconsistent in this matter. Nevertheless, strong low level convergence and upward motion are forecast, which will easily compensate for the lack of instability. Judging from the simulated convective precipitation patterns and wind fields a strongly-forced convective line will form. The trough is expected to strenghten the flow, especially in the low levels as a 850 hPa winds should reach 30 m/s. More than 15 m/s of low level shear is anticipated and 300 J/kg of SREH will be available to the developing updrafts.

Convection should enhance the severity of wind gusts and a few tornadoes might occur in the mesovortices of convective line. The overall threat will not be very high, but it still warrants a Level 1. Convective line should affect Ireland around 00Z and Britain around 03Z. An update might be issued during the day due to the uncertainities in the situation.

... Ionian and Adriatic sea...

Two rounds of thunderstorms are anticipated in this region. The first one will be ongoing since 06Z as a mid-level impulse will destabilize the warm and humid airmass, advected on the forward side of the weak surface low. This impulse will become a locus for the thunderstorm development, as suggested by a blob of high lapse rates, induced by the rising motion. MLCAPEs should stay only around 500 J/kg, but deep unstable layers and cold ELs are shown by the model runs, so thunderstorm formation should not be a problem. Wind shear should stay only in order of 10 - 15 m/s in the 0-6 km layer and SREH under 100 J/kg, except for the marginally unstable areas nearby the coasts, where more than 20 m/s of DLS is anticipated and LLS will be over 10 m/s. Also thanks to the ageostrophic component of the wind flow, SREH in these regions will likely reach more than 250 J/kg in 0-3 km layer.

Another impulse will affect the region by Thursday 06Z and a new wave of thunderstorms is expected, persisting well into the next forecast period. Still, wind shear should be a limiting factor and a multicellular storm mode will dominate.

Outside Level 1, a marginally severe hail might be observed in the stronger cells. Strong low level instability release and surface wind field convegence suggest that the potential for waterspouts will exist. Level 1 is issued for the eastern coastal areas of the Adriatic Sea, where the increased values of LLS and SREH might lead to an isolated tornado or a severe wind gust event in the first wave of thunderstorms.