Storm Forecast
Valid: Wed 09 Apr 2008 06:00 to Thu 10 Apr 2008 06:00 UTC
Issued: Tue 08 Apr 2008 21:47
Forecaster: DAHL

SYNOPSIS

Deep large scale upper trough is settling off the Iberian west coast, resulting in an intense SWLY upper flow over Europe. Associated quasi-stationary low-level frontal zone is expected to stretch from N Iberia across central France northeastwards into the Belarus on Wednesday. Upper-level vorticity maxima embedded in the SWLY upper flow will promote the development of frontal waves along the low-level front. Strong warm advection is simulated SW of the front over the Mediterranean. An upper short-wave trough is present over the Aegean at the beginning of the period and will move off the forecast domain during the period.

DISCUSSION

... Iberian Peninsula ...

Interesting but challenging scenario is unfolding over the Iberian peninsula as well as over France. The air mass is expected to undergo some destabilization in response to mid/upper cooling ahead of the approaching trough as well as low-level advective moistening. Also, dry Saharan mid-level air should graze SE Iberia, acting to increase the low-level thermal gradient ... which should be focus for convective development, especially in the first half of the forecast period. However, storms will likely tend to be elevated given anticipated stratiform precip which should stabilize the low levels. In fact, GFS does not simulate any appreciable SFC-based CAPE over SE Iberia where maximum convective precip is assumed. However, deep-layer and low-level shear profiles will be more than adequate for severe evolution of SFC-based convection, so any surface-based storm should be monitored for possible mesocyclogenesis with an attendant hail and possible tornado threat. Current thinking is that most of the storms over Iberia will be rather disorganized, elevated, and non-severe, with only a minority of the storms tapping boundary-layer air, which increases their chances of becoming severe. A level one area has been introduced over SW Iberia where frontogenetic forcing should be maximized in the first half of the period, despite the lack of simulated SFC-based CAPE, as possibility of a moderately well-organized and slightly elevated MCS appears to exist.

... S France ...

Scattered storms may also develop over France, especially in the evening hours ... and may become severe given quite strong shear profiles. Uncertainty exists on whether or not storms will be elevated (depending on the degree of cloudiness/precip and insolation preceding the onset of convection). If surface-based storms indeed form, potential for large hail, severe wind gusts as well as a tornado or two will exist. Storms should spread northeastward during the evening/night hours but should gradually diminish as increasingly less unstable air is encountered.

... S Aegean ...

At least weak CAPE and deep shear sufficient for storm organization will be present over the southern Aegean Sea. Expect isolated severe wind gusts as well as marginally severe hail with the strongest cells.