Storm Forecast
Valid: Sun 12 Apr 2009 06:00 to Mon 13 Apr 2009 06:00 UTC
Issued: Sat 11 Apr 2009 20:37
Forecaster: TUSCHY
SYNOPSIS
Low geopotential heights prevail over Europe. An active upper low affects N-Algeria, Tunisia and Sicily and unsettled conditions are forecast over most parts of the south-central Mediterranean. In addition, a low-level vortex evolves just west of Sicily and this feature drifts slowly eastwards during the forecast. Otherwise, numerous small upper lows continue to be the foci for locally enhanced thunderstorm probabilities. High pressure dictates the weather over central Europe.
DISCUSSION
...Slovakia, extreme S-Poland and extreme W-Ukraine ...
The cold-core low, which had an impact on the area of interest for the past few days, is on the point of sliding southwestwards over Slovakia. A combination of moderately steepened lapse rates between 2-4km, dewpoints at the surface just shy below 10°C and some diabatic heating are enough for another round of scattered thunderstorms over a concentrated swath. Instability builds up to well above 500J/kg and some buoyancy will be found in the favored hail growth zone in addition to augmented directional shear just on the fringes of that low. Large hail is the main hazard until storms tend to cluster betimes. The sub-cloud layer is well mixed/dry, so strong downdrafts are possible and because of the hail/strong wind gust risk, a level-1 was issued. Although decreasing in coverage and intensity after sunset, isolated thunderstorms continue well into the night over Slovakia as thermodynamics remain supportive.
...E-France, Belgium, the Netherlands and Luxembourg ...
We will stick with a persistent forecast. Not many changes at the synoptic and sub-synoptic scale to talk about. Geopotential heights are on the increase during the day, which ought to keep thunderstorms isolated to scattered at best, especially over E-France. A short wave crosses the North Sea leisurely from the SW during the forecast and some forcing approaches the highlighted area from the west during the daytime hours. Past GFS runs kept this vorticity lobe more to the west at each run and latest thinking is that it may not even play a role anymore for initiation. Therefore, initiation will be tied to orography, a tongue of better boundary layer moisture, which runs from E-France N/NE-wards and weak convergence zones. The wind field at all levels is very weak, so short-lived single cells are likely with the strongest activity over E-France.
DLS increases over Benelux and combination of modest instability and up to 15m/s DLS would enhance the risk of hail, but initiation is very uncertain as ridging strengthens somewhat during the day. GFS backed off convective precipitation whereas NOGAPS and WRF still show signals for initiation. For now, we stick with the more reluctant solution of GFS and concentrate on the zone with strongest convergence/best BL moisture. No level area was yet issued due to all those uncertainties.
... Sicily and S-Italy ...
A gradually evolving surface depression over Sicily results in unsettled conditions. Marginal instability, weak lapse rates and overall weak shear preclude any severe risk. Scattered thunderstorms occur in the highlighted area during the upcoming 24h.
... Extreme NW Algeria ...
A compact cold core low crosses the extreme western Mediterranean and enters NW Algeria after midnight from the north. Steep lapse rates and moist low levels yield moderate instability in a sheared environment (DLS 15-30m/s). There will be a large hail risk and hence a small level-1 was issued.
