Forecast Update
Valid: Fri 21 Mar 2008 15:00 to Sat 22 Mar 2008 06:00 UTC
Issued: Fri 21 Mar 2008 15:32
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Thu 20 Mar 2008 17:49Z.

DISCUSSION

... Belgium and the Netherlands...

High resolution visible images, radar loops and latest VWP products all indicate a small vortex now entering the Netherlands form the NW moving rapidly towards the SE. Sounding reports show the possibility for most unstable parcels to climb well above 600hPa and latest surface data reports show that diabatic warming pushed temperatures to at or above 5°C over the area of interest. Timing is favorable and this additional forcing should support scattered showers and also a few thunderstorms to evolve during the next few hours, spreading southeastwards. GFS has slightly stronger shear signals and this could increase the hail threat although marginal instability release should keep hail size below our criterion.
Convection should start a weakening trend over western Germany as daytime heating vanishes and the vortex decays.

Despite colder cloud tops over N-Germany no lightning activity was seen with this convection, and one reason could be a weak cap at 700hPa which limits updraft strength ( combined with non existing shear ), but diabatic heating could offset that inhibiting factor during the next few hours so we will stick with a low-end thunderstorm area for N-Germany.

... N-central Italy...

Forecast instability after midnight increased over N-central Italy as surface depression became slightly stronger and surface cold front, already blocked by the Alpes, will arrive later so some instability could indeed evolve as upper-level cold air overspreads the area from the north. Main risk will be an isolated large hail threat as shear parameters are fine but we want to see more robust instability signals before reflecting the trend in the probabilities. For now we will stick with a general thunderstorm area.

... W-Turkey...

We decided to upgrade parts of the highlighted area as chances for deep convection between 15 and 21Z increased during the past few runs. Shear is more than adequate for a large hail / severe wind gust risk although weak lapse rates should keep the threat more isolated.