Storm Forecast
Valid: Tue 03 Mar 2015 06:00 to Wed 04 Mar 2015 06:00 UTC
Issued: Mon 02 Mar 2015 21:02
Forecaster: TUSCHY

A level 1 was issued for parts of Belgium, S-Netherlands, Luxembourg and W-Germany mainly for severe wind gusts and an isolated tornado threat. An isolated large hail event is possible.

A level 1 was issued for Scotland mainly for severe wind gusts and an isolated tornado threat.

SYNOPSIS

Zonal flow persists during the start of the forecast over W/CNTRL Europe before deep-layer ridge arches upward over the far NE Atlantic. This sets the stage for an increasingly amplified flow pattern over W-Europe during the end of the forecast and onwards. Blocking high over Russia dwarfs eastbound motion of an E-European trough. The main change will be falling mid-layer heights over SE-Europe as this trough amplifies to the SE.
The Mediterranean resides on the anticyclonic side of the frontal zone with quiescent conditions. Pressure fall south of the Alps is underway during the end of the forecast as lee cyclone evolves. No influence yet on quiet conditions (thunderstorm-wise).

DISCUSSION

... Belgium, parts of the Netherlands and parts of Germany ...

Embedded in the zonal flow, a mid-layer wave shifts east and affects UK around noon, Benelux during the night and Germany thereafter. Combination of weak upper divergence and the left exit region of a 40 m/s mid-level jet culminate in a rather pronounced low-tropospheric wave well ahead of its mid-level equivalent. This low-tropospheric wave pushes a convergence zone east. This convergence zone is characterized by enhanced BL moisture. Dissolving plume of steeper mid-level lapse rates still overlaps this convergence zone and results in marginal low-tropospheric CAPE ... probably on the order of 100-400 J/kg.

Wind shear is quite pronounced, although no passage of any distinct jet core is forecast. 25 m/s flow at 850/700 hPa results in roughly 20 m/s 0-3 km shear and only marginal weaker LL shear. Not yet sure if updrafts grow deep enough to take profit of 40 m/s DLS as any convection should remain low-topped. In addition, strong directional shear occurs along that convergence zone. This event is an uncertain one as very weak CAPE and weak upper divergence (including wrong placement of the 300 hPa jet) occur, but forcing in the 800-600 hPa layer will be rather strong (which coincides with expected depth of updrafts) so enough confidence in organized convection for a level issuance exists.

Expect growing convection along the eastbound moving convergence zone over Benelux (9-12Z). Best timing regarding peak diabatic heating occurs over E-Belgium/NE France/W-Germany and we would not be surprised to see numerous better organized line elements (some model data even support a broken LEWP event although only for a few hours during peak heating). Small swathes of severe wind gusts are forecast. An isolated tornado event is another possibility due to low LCLs and anticipated strong shear (especially along the rim of any convective line or with advancing (discrete) cells). Hail threat is not yet obvious but any deeper updraft could produce isolated large hail. A broad level 1 should cover that event. After sunset, convection diminishes due to decaying CAPE.

... Scotland ...

An augmented chance for more organized convection is possible over Scotland during a cold frontal passage (between 12-18 Z). Weak CAPE should confine the severe risk to strong/isolated severe wind gusts. Enhanced LL shear by friction also points to an isolated funnel/tornado risk.

Active but non-severe thunderstorm activity occurs over a broad area (offshore) of NW-Europe but also over the Carpathians. Strong wind gusts and graupel remain the main hazard.