Storm Forecast
Valid: Tue 31 Oct 2006 06:00 to Wed 01 Nov 2006 06:00 UTC
Issued: Tue 31 Oct 2006 05:10
Forecaster: VAN DER VELDE

SYNOPSIS

Two important depressions determine convective severe weather occurrence. A vigorous low over the North Sea and southern Scandinavia looks currently not much convective, but this is expected to improve during the period. Its cold front is scheduled to arrive at the coast of The Netherlands, Germany and Denmark at 18Z, according to GFS and MM5. It is followed by a trough during the night hours. Strong pressure gradients may cause gale-force winds over sea and coastal areas.

This is also the case over the southern Aegean Sea where also very tight pressure gradients will be present. Very unstable airmass streaming up against southern Turkey is going to yield large amounts of precipitation and flash flooding.


DISCUSSION

...Netherlands, northern Germany, southern Scandinavia...

With 850 hPa winds progged to reach 30 m/s during the passage of the postfrontal shortwave trough at night, any convection could potentially produce severe gusts, on top of the already gale force winds. However, GFS/NMM models do not really predict very deep convection at this disturbance with LI remaining over 4-6 K.

The cold front passing in late afternoon/early evening could be associated with deeper convection (LI<2), organised into a line and within good DLS zone of the upper jet, with >17 m/s overlapping unstable air. LLS at the front is progged to reach over 10 m/s in NMM, although no significant low level veering with height is indicated. It appears to become rather a linear event than discrete cells, with the main risk being severe gusts (speed at 850 hPa >22 m/s) but an isolated tornado is not ruled out, as is an isolated marginally large hail event.

Southern Scandinavia, especially Sweden, sees a better overlay of (similar) DLS and LLS and deeper instability. Convection there may group more into cells and clusters rather than lines, and hence more of a tornado threat, albeit not very significant due to almost absent veering with height in the majority of places. Hail and gusts could reach severe thresholds in isolated spots.

...Aegean, southern Turkey...

Near-saturated, neutral to moderately buoyant profiles are present. The western part of the area can expect severe gusts with storms in a tight pressure gradient zone, whereas southern Turkey sees conditions of pretty strong DLS (15-20 m/s) and dito LLS (>20 m/s possible over land, NMM) though SREH stays behind. Storms in this area may become supercells and produce tornadoes/waterspouts and large hail, and also flash flooding due to the very large amounts of rain.