Storm Forecast
Valid: Wed 08 Oct 2008 06:00 to Thu 09 Oct 2008 06:00 UTC
Issued: Tue 07 Oct 2008 20:25
Forecaster: PUCIK

SYNOPSIS

A blocking pattern has developed over Europe with deep trough at 500 hPa over the E Atlantic / Iberian peninsula, slowly moving eastwards. On the contrary, large ridge will lie over the Central Europe and Scandinavia. Another trough has settled over W Russia and Ukraine. Steering flow will be split into two parts, with the first branch over N Scandinavia / N Russia. The second one will be found rotating around the trough centered over Iberian penninsula. At the surface, most of Europe will be dominated by high pressure system, stretching from Atlantic into Sweden. This feature will guarantee widespread stable conditions for Western / Central Europe. Surface lows will be affecting NE Atlantic and W Russia. Shallow low with several centers as sugggested by both GFS and ECMWF is anticipated over the W Mediterranean, especially the Balearic sea.

DISCUSSION

...W Mediterranean...

Warm SST with quite cool airmass being advected at midlevels should yield slight to moderate destabilization across the region. GFS simulates that more than 1000 J/kg of MLCAPE might develop, with very little CIN. It is quite probable however, that these values are a bit exaggerated, so 500 J/kg seems like a more realistic value. Approaching trough will provide enough forcing to sustain widespread initiation of thundestorms across the region. Furthermore, strong flow at higher levels will increase DLS values, which should reach 15 - 20 m/s on the fringe of the trough. Shear confined to lowest 3 kms should be in order of 10 m/s. So environment will be conductive to development of organised storms in the belt of stronger mid-level flow which will slowly travel eastwards during the forecasting period. It should start to affect the region by late evening, around 21Z, so the highest threat is expected overnight. Also, strongest forcing will be avaible during this period. With few well organised multicells , small to marignally large hail can be expected.

A waterspout threat is anticipated across the W Mediterranean, as robust low level instability development is expected with nice convergence signals shown by GFS.

Furthermore, flash flooding will be a serious hazard in Morocco and Algeria. Thunderstorms should persist for a long time over the area, as favourable forcing will support their develoment. Strong convergence signals and moisture advection from the sea also point to the chance of very high rainfall accumulations during the thunderstorms....

A marginal Level 1 was issued for the region, where the best instability / shear and forcing overlap will exist. Here, best chances for waterspouts and some hail exists. Weak low level wind fields and strong storm clustering should prevent from higher threat.