Storm Forecast
Valid: Mon 08 Nov 2010 06:00 to Tue 09 Nov 2010 06:00 UTC
Issued: Sun 07 Nov 2010 21:59
Forecaster: DAHL

A LVL2 was issued over the Adriatic and Ionian Sea and the western Balkan coastal regions for severe wind gusts, excessive rainfall, large hail, and tornadoes.

A LVL1 was issued across the western and central Mediterranean Sea mainly for severe wind gusts, large hail, and tornadoes.

A LVL1 was issues for the Gulf of Biscay as well as the coastal regions of N Spain and W France mainly for severe wind gusts and tornadoes.

SYNOPSIS

A complex synoptic scenario will be unfolding over Europe on Monday. An upper trough analyzed over central Europe on Sunday evening will continue to move eastward while capturing a downstream vort max that was analyzed over the south-central Mediterranean/S Ionian Sea on Sunday evening. At the same time, a very powerful Atlantic jet streak at the flank of a seasonably intense SFC low, digs southwestwards, reaching the western Mediterranean by Monday afternoon and the western Balkan States by early Monday morning. By that time ... an intense and deep large-scale trough will have evolved, covering the entirety of Europe. The low-level evolution features a Genoa cyclone that is in its genesis as of Sunday evening, and which will be moving off to the northeast on Monday. Another Genoa-type low is expected to develop late on Monday with the approach of the jet streak. The center of the main Atlantic SFC low will migrate into the Channel region towards the end of the period. Altogether, this will result in an extensive SFC low with the two peripheral Genoa-type lows at ist eastern flank.

DISCUSSION

Italy ... Ionian and Adriatic Sea ... western Balkans

There will be manifold convective foci associated with each of the features mentioned in the synopsis. The best thermodynamic profiles will likely persist ahead of the lead trough, where up to 1000 J/kg MLCAPE should persist. In addition, low-level shear of 10 to 15 m/s will be omnipresent on Monday, courtesy of the large-scale SFC low. Deep shear is simulated to range from 15 to 20 m/s. Expect potentially severe storms to persist along and ahead of the low-level thermal boundary associated with the upper trough. Severe straight-line winds should be the main threat, though especially with mesocyclonic storms, large hail and tornadoes are also likely.

Especially the west coast of the Balkans will likely experience excessive amounts of partly convective precip. This latter threat seems to be rather detached from the synoptic-scale forcing and seems to be maintained mainly by upslope flow.

... Gulf of Biscay ... France ... western Mediterranean ...

Upstream ... the intense jet streak could support linear convection along the associated cold front, which will cross the Gulf of Biscay and France in the first hours of the period. However, the thermal structure of the front will likely weaken, so that the chance for organized, strongly-forced convection seems to be limited to early in the period when the system is still off-shore. Still, residual convective line segments could move into France, and given the strong low-level shear, these could spawn a brief tornado or two, along with severe wind gusts.

The subsequent evolution is less clear as the low-level features lose their identity as the system moves over the Mediterranean. There will be an area of strong DCVA that originally has been associated with the nose of the jet streak. This feature will likely be associated with deep convection, possible organizing into line segments with imbedded supercells. This activity will maintain a threat for severe wind gusts, hail, tornadoes and excessive convective rain.

Some instability will persist in the air mass in the wake of this activity over the western Mediterranean. Given that strong shear will be present over parts of this air mass, some severe threat should persist. Again, severe wind gusts, and possibly a few tornadoes could occur. The threat should be somewhat lower than the severe threat farther east as the instability should be shallower.

In general, the low-level shear should be weaker over the sea waters than over land, so that the western coastal regions should be most susceptible to tornadoes during this period.

The French west coast and the Spanish north Coast should see rather shallow cellular convection moving inland from the Gulf of Biscay. Given the favorable low-level shear and low LCL heights, a few short-lived tornadoes could occur in these coastal regions.