Storm Forecast
Valid: Tue 09 May 2006 06:00 to Wed 10 May 2006 06:00 UTC
Issued: Tue 09 May 2006 08:40
Forecaster: VAN DER VELDE

SYNOPSIS

A 500 hPa high is situated over southern Scandinavia, with 500 hPa lows over the Ukraine and Italy. A jet at the south side of the Italian upper low and a jet at the west side of the Ukrainean upper low create favorable deep layer shear conditions combined with instability.
The surface flow is such that drier air advects into central Europe from the northeast, while Atlantic cool airmass is advected northeastward. This causes a line of convergence and confluence of the moister air over central Europe into the Benelux and UK.

DISCUSSION

...southern Benelux region...

Compared to the previous day, higher concentrations of boundary layer moisture, of about 6-7 g/kg mixed over the lowest 1000m, will be present according to the GFS model. Part of this is due to advection from northern France and from Poland, ahead of the drier airmass. Consistent precip signals in GFS and NMM over the past few runs and a few hundred J/kg CAPE will allow deep convection. The necessary trigger will mainly be the zone of strong 10m wind convergence in the level 1 area. Outside of this area very localized spots may trigger convection.
The reason for the level 1 is a concern of downburst strength due to evaporational cooling in the dry environment with high LCL heights (around 2000m).

(next lines amended Tue 9 May 8:40 UTC)

In addition to this, models predict an enhanced level of storm-relative helicity with northwestward storm motion, and low-level shear may overlap the CAPE region as well. This may enable some updraft rotation. Such cells may be efficient producers of (marginally) large hail. High LCL heights suggest the threat of tornadoes will be small.

...UK...

Narrow line of convergence may trigger some deep convection, probably isolated, which may profit from slightly enhanced storm-relative helicity, with some chance of marginally large hail.

...Italy...

Deep-layer shear maximum of more than 30 m/s over 0-6 km over central Italy may support well-organized cells whose updraft may rotate and generate (marginally) large hail.

...Ukraine into Moldova...

Band of 20-25 m/s 0-6 km shear will wrap around the upper low, when GFS and NMM models create a few hundred J/kg MLCAPE with mixing ratios of only 4-5 g/kg, apparently by steep lapse rates of 7-8 deg/km between 2000-4000m. The upper low may create PVA to trigger the storms, the NMM model also progs a line of convergence at the northeast side of the Carpat mountain chain. If storms indeed form, they may produce (marginally) large hail and (marginally) severe gusts.

...Balkan...

Thunderstorms in stagnant flow will be mainly induced by orographic features, and produce local flash flooding. Landspouts are a real possibility given the steep lower lapse rates and abundant low-level CAPE such as that indicated by 12Z Budapest sounding of the previous day.

...central Europe...

Dry, marginally unstable air may allow very isolated convection with high cloud bases producing strong downburst winds due to evaporational cooling.