From ESTOFEX
Storm Forecast
Valid: Mon 19 Jan 2009 06:00 to Tue 20 Jan 2009 06:00 UTC
Issued: Sun 18 Jan 2009 18:34
Forecaster: TUSCHY
SYNOPSIS
A channel of low geopotential heights runs from S-Greenland to the North Sea and dictates the flow over the NE-Atlantic and NW-Europe. Numerous disturbances, embedded in this strong northwesterly flow bring unsettled and cold conditions for NW-Europe. Weak pressure gradients over the rest of Europe, warm mid-levels (Mediterranean) and cold/dry air (E/NE-Europe) result in stable conditions. Surface pressure falls over the western Mediterranean during the night hours, passing in an evolving depression south of the Alps.
DISCUSSION
... Bay of Biscay, parts of France, Belgium, the Netherlands, UK, Ireland, NW-Portugal and N-Spain ...
Frigid air from the frozen Baffin Bay and Davis Street is on its way to W-Europe along the south side of this extensive cyclonic vortex. Airmass modifies over increasingly warm SSTs (10-12°C SW/S of Ireland) while mid-level airmass remains very cold with readings at 500hPa running well below -30°C. So the environment is favorable for widespread CAPE release over the highlighted area and an increase in shower/thunderstorm activity from west to east during the day. EL temperatures drop to -30 to -50°C, parcel layer depth increases in average to well above 1000m and the wind field is shaped cyclonically with various embedded disturbances. The same, messy picture at lower levels as numerous convergence zones cross the highlighted area from the west. Hence, thunderstorms can pop up everywhere over the E-Atlantic and NW-Europe but we want to highlight the areas, where conditions for more concentrated thunderstorms are more likley, including strongest CAPE fields, an uncapped airmass, climatology and synoptic lift. Winds at 850hPa are strong with 20-25m/s, so severe wind gusts in this well mixed airmass are possible with each shower/thunderstorm and a level-1 is needed. In addition, and isolated large hail/tornado event is possible, the latter one especially onshore, where ageostrophic deflection helps to increase LL directional shear.
... Ireland and parts of UK between 12Z and 00Z ...
The focus during that period will be a rapidly eastsoutheastward moving short-wave, crossing Ireland in the afternoon and S-UK in the evening hours. GFS had this feature in its forecast for the past few runs although the strength of this disturbance still fluctuates somewhat. The pressure gradient increases during the passage of this wave and GFS augments winds at 850hPa to 30-35m/s over central/south Ireland, decreasing slightly over SW/S-UK, so severe wind gusts are well possible. The main question remains how robust moisture advection turns out to be just ahead of this wave, which looks marginal at best for now. GFS has a diurnal driven instability signal over UK, decreasing during the evening hours. In addition, temperatures below 3km warm up slightly, also reflected in increasing capping over SW-UK and slightly higher LCLs. The main risk will be severe wind gusts, but the tornado risk has to be monitored in upcoming model runs,too.
