Storm Forecast
Valid: Fri 09 Sep 2011 06:00 to Sat 10 Sep 2011 06:00 UTC
Issued: Thu 08 Sep 2011 17:43
Forecaster: TUSCHY

A level 1 was issued for Ireland mainly for an isolated tornado/severe wind gust report and isolated large hail.

SYNOPSIS

Wave train marches eastwards and places W-Europe beneath strengthening ridge/WAA. Intense trough over E/NE Europe remains in place with ongoing CAA. No real focus is detected for any organized thunderstorm activity.

DISCUSSION

... N-France, W-Germany, Benelux, Ireland, Scotland and United Kingdom...

Robust upper trough/LL cyclone west of Ireland allows gradual moisture recovery as a well mixed maritime air mass from the Atlantic spreads eastwards. However, no signals can be detected for a more robust return flow from the W-Mediterranean with offshore pointing LL streamlines along the coast of S-France well into the night hours. An active warm front spreads northeastwards during the forecast but with latest MIMIC-TPW measurements indicating a TPW minimum over the Iberian Peninsula and further to the SW, concerns arise regarding the moisture content of the warm sector (fostered by yesterday's soundings from that area). This may allow the warm sector to become prone to strong diabatic heating, BL mixing and hence limited BL moisture compared to current model outputs. Finally, EML axis remains well to the west, so forecast CAPE magnitude of more than 800 J/kg MLCAPE (GFS) in some places remains questionable.
Isolated thunderstorm initiation will probably be bound to the warm front, with some elevated thunderstorm activity. Those elevated storms probably also affect parts of the UK and Benelux during the night hours and hence we expanded the 15-% area well to the north. With increasing speed/directional shear, a few updrafts may organize somewhat with an augmented hail risk. However, marginal MUCAPE precludes any level area for now.
There are hints that some surface based instability may evolve over UK around sunrise (the 10th) with strong directional/speed shear in the BL. If this trend indeed already starts in my forecast period, a level 1 may be needed due to an isolated tornado/hail risk. However, current indications are that the main SBCAPE plume sets-up right after 06 Z.

A level 1 was added for Ireland for the 12Z-18Z time frame, as influx of moist air, cool mid-levels, a coupled jet configuration and strong LL shear/modest DLS create a favorable environment for organized thunderstorms. Surface based storms are forecast with an augmented tornado, severe wind gust and isolated large hail risk. The risk diminishes after sunset, as SBCAPE vanishes (becomes more elevated).

Surface based initiation over central France and further to the south seems unlikely with current model data (no real mesoscale focus, pretty dry BL, increasing cap, weak EML). If initiation becomes more likely, a level 1 may be needed due to increasing DLS from the SW.


... NE-Europe ...

Placed beneath a cold-core low, daytime driven showers/thunderstorms are forecast. Weak shear and moderate instability allow pulsating storms with marginal hail and strong wind gusts. Activity eases after sunset.