Storm Forecast
Valid: Tue 11 Mar 2008 06:00 to Wed 12 Mar 2008 06:00 UTC
Issued: Tue 11 Mar 2008 01:58
Forecaster: VAN DER VELDE

SYNOPSIS

A large Atlantic low pressure area is situated with its cores near Scotland. A sequence of active weather systems affects northwestern Europe. A longwave trough shifts east over Italy and southern Balkan.

At 06Z, the period starts with a warm front invading Ireland and France. At 12Z, this front (at surface level) has reached central France, and the occlusion is produced near Wales (GFS model). The crest of this wave is predicted to move rapidly eastward near the jet axis (left exit region), at 18Z it should be over The Netherlands. The occlusion will be over Poland at the end of the period, while its cold front extends almost zonally into northern France.
An extra wave is clearly present in BL mixing ratio fields at 21Z near Land's End, over Belgium at Wed 06Z with again possibly convective activity.

Key issue is whether instability will be strong and deep enough to pose a convective severe weather threat (gusts and tornadoes).


DISCUSSION


...northwestern Europe...

The GFS model gives indications of marginal instability over France, Belgium, Netherlands and the Wrn/Srn UK and later also over Germany.
CAPE fields (especially 0-3 km integrated) show slight instability over the ocean, Srn UK and English Channel and into Belgium during the early evening, at the cold side of the occlusion. However, 2000-4000m lapse rates reveal that this CAPE is present under a subsidence inversion, allowing not very deep convection.

The strip of poor mid level lapse rates in the wake of the occlusion can be tracked nicely and probably corresponds with descending air, so any chances of strong convection would be at the occlusion and cold front itself, where lift, very moist and likely neutral profiles should be present or perhaps a narrow fringe zone where deep instability can be released. Convective precipitation is produced in GFS.
A PV anomaly is missing, and so the faith in a convective touch to this front is low. However, if any sustained convection develops it may impose a threat of tornadoes besides strong/marginally severe gusts (bowing lines), because of the 15-20 m/s low level shear, 60 m/s deep layer shear and >300 m2/s2 0-3 km SREH capable of inducing brief mesocyclones in storms. It was chosen to indicate a large, rather conditional level 1 area for this reason.

The second wave gives a small convective signal over Belgium at the end of the period and, if realized, may induce similar conditions as described before.


The cold front is likely to become more active during the early Wednesday hours, when CAPE is predicted along it between Bretagne and central Germany - especially after 06Z during the next period. Isolated thunderstorms may occur then and shear conditions (as described above) are still favorable for tornadogenesis.

Also in the early Wednesday morning, CVA from the trough sinking SE-ward into central/southern UK can set off convection in high shear area - but this is under weak 2000-4000m lapse rates. More northward near the shortwave trough (possible comma?) deep convection will be able to develop, and steep pressure gradients (but curved isobars) cause severe wind speeds at the mesoscale. Convectively enhanced gusts or an isolated tornado are a possibility, the latter because of >400 m2/s2 SREH.... but actually, CAPE and SREH appear to occur separatedly, reducing this risk.

General picture of marginal instability and simultaneous forcing conditions in this area looks insufficient for drawing a large thunder area at this time, but may need to be updated later.


...southern Aegean Sea....

Convergent flow in unstable airmass and decent shear conditions (>15 m/s DLS, 10 m/s LLS, 200 m2/s2 SREH) after 00Z suggests an organized convective line with possibly supercellular elements at its southern end... primary threat seems isolated large hail, though CAPE is modest.

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