Storm Forecast
Valid: Sat 24 Jan 2009 06:00 to Sun 25 Jan 2009 06:00 UTC
Issued: Fri 23 Jan 2009 20:02
Forecaster: DAHL

SYNOPSIS

A large-scale, negatively tilted quasi-stationary upper trough is present over SE Europe, with a very intense westerly upper flow merging with the SW periphery of this trough. Several perturbations are imbedded in this westerly upper flow, having resulted in a particularly intense SFC low which is expected over western France at the beginning of the forecast period, from where it will move eastwards into the central Adriatic region by Sunday 06Z, undergoing substantial weakening. The subsequent perturbation is simulated to be more extensive but less intense, affecting the westernmost parts of Europe late in the period.

DISCUSSION

... Mediterranean regions ...

Models are in fair agreement about the positioning of the intense SFC low over western France at the beginning of the period. As typical in these situations, convection will likely be in near equilibrium with its environment, and CAPE is simulated to be accordingly low. However, strong mesoscale ascent along the cold front which will be rapidly progressing into the western Mediterranean early in the period, should maintain some convection. Although the dynamics along the front should weaken, instability should increase in the pre-frontal environment as the front moves across the Mediterranean Sea. However, cloud-top temperatures are not expected to drop significantly below -20C, so that lightning activity may turn out to be quite marginal, especially after the mesoscale slab ascent weakens during the day. However, quite strong deep shear and 850 hPa horizontal velocities of about 25 m/s will likely result in linear segments and a threat for damaging wind gusts, and possibly also a tornado or two. Late in the period, pre-frontal convection will likely rejuvenate as more unstable air is encountered over the Aegean region. DLS is expected to be rather low over this region, but strong coastal LLS may be supporting low low-level rotation and possibly a brief tornado or two.

The Low-level circulation center will be displaced from the strongest deep-layer forcing for upward vertical motion already by 06 UTC. Still, mesoscale ascent in the back-bent occlusion may support minimal instability for convection. The low-level kinematic fields will be very strong in this regime, suggesting that the cells may on the one hand struggle to become well-organized/long-lived owing to excessive shear, but during their lifetime pose a threat for damaging wind gusts owing mainly to vertical momentum transport. Also, brief tornadoes may occur. However, the focus for ascent is quite less well-defined that farther downstream along the cold front.

It seems that the convection that originally has started in the region of the back-bent occlusion will move into the Mediterranean in the wake of the cold front. This activity will remain strongly sheared and is also expected to exhibit linear organization. These storms may also spawn isolated tornadoes, though the primary threat will be an augmentation of the already severe background gradient flow.

In the polar air in the wake of the occluded front, which is losing integrity as it moves into the Mediterranean, shallow cellular convection will form. This convection will also be strongly sheared, but likely be quite shallow. Isolated severe wind gusts as well as an isolated/brief tornado or two may occur, but strong post-frontal sinking motion should limit duration and extent of this activity.

... extreme NW Iberia ... W Biscay ... W British Isles ...

GFS comes up with weak instability and cold most-unstable EL heights along the cold front of the next system, which should reach the W british Isles and extreme NW Iberia late in the period. Large-scale forcing for ascent is expected to be rather strong, but it does not seem that instability will be sufficient to sustain deep updrafts and a thunderstorm area does not seem to be warranted at the moment.