STORM FORECAST
VALID Mon 27 Feb 06:00 - Tue 28 Feb 06:00 2006 (UTC)
ISSUED: 27 Feb 01:23 (UTC)
FORECASTER: VAN DER VELDE

A threat level 1 is forecast across the Ionean Sea and its coasts

SYNOPSIS

Due to a blocking high pressure zone from Iceland southward to the British isles and building to Spain, a northern flow of unstable maritime arctic airmass will reach into western Europe by the end of this forecast period, behind a cold front associated with a deepening low over the North Sea.

An upper cut-off low with superposed surface low drifts off the Spanish coast into the southwestern Mediterranean late Sunday evening. A new upper vorticity max comes out of Tunesia by Monday 18Z and with low-level warm air advection a low deepens. Elevated instability should be available at this system.

DISCUSSION

...Baltic Sea...
just a few weakly electrified showers possible at the convergence zone-induced convection, tops reaching into levels colder than -20C.

...North Sea and coastal areas...
The Netherlands may first see some shallow convection in the early hours of the forecast period, with strong buoyancies and lapse rates possible below the subsidence inversion at 800 hPa... with weak low level winds this might induce a waterspout or two.

Later in the period, instability behind the cold front may be weakly electrified. Uncertain is the level of instability at the front itself - seems that generation of a line of organized convection and thunder is quite possible due to localized forcing and a tongue of moisture, present in the GFS model. Do not expect much chance for severe weather given the low deep layer shear, and disposition of storm-relative helicity and low-level shear ahead of the front.

...central Mediterranean Sea...
At the vorticity max and low-level convergence zone near Sardinia convection may spin up a few waterspouts. This weak barotropic synoptic situation appears quite typical for this, though the grade of instability may be on the marginal side, approaching a level 1 (which starts from 3 tornadic events in a 500 by 500 km area)

...Ionian Sea and coastal areas...
Cyclogenesis takes place at an upper vorticity max and advection of warm air, while upper winds are strong. GFS and NMM forecast increasingly unstable airmass to develop of a few hundred J/kg MLCAPE... but mostly elevated. At the same time DL shear will range from 20-30 m/s and LL shear over 10 m/s, with SREH in the instability zone of >250 m2/s2... supporting mesocyclonic updrafts... however only part of this would be effective in elevated convection. Especially if surface-based convection manages to develop, a few tornado- and large hail events will be possible.

#