Storm Forecast
Valid: Thu 18 Sep 2008 06:00 to Fri 19 Sep 2008 06:00 UTC
Issued: Wed 17 Sep 2008 18:41
Forecaster: GATZEN

SYNOPSIS

As a ridge axis builds from British Isles to western Russia, upper troughs over France and eastern Europe starts to merge forming a rather large cut-off trough south-eastern Europe on Friday. At lower levels, cool air mass over most of the continent will spread across southern France and west Mediterranean in the range of the southward moving French trough during the forecast period. Another cut-off low will remain west of the Iberian Peninsula leading to warm air advection into south-west Mediterranean. An intense low will be present over the Black Sea region. Lows over the Atlantic will affect the very north-western forecast area.

DISCUSSION

Southern France

Looking at latest ascends over west Mediterranean Sea, a substantial moist boundary layer is indicated, while dry low-level air mass is present over southern France. Ahead of the approaching upper trough over France in combination with south-westerly winds ahead of the trough west of the Iberian Peninsula, warm air advection spreads into southern France during the night. Given rich low-level moisture and quite cool mid-level air mass over France, some instability will likely develop until the morning hours.

During the day, upper trough is expected to move southward over France, while low-level warm air advection is expected to continue, and latest GFS indicates that CAPE may reach some 100s J/kg. In the afternoon and evening hours, a strong upper jet streak is expected to eject from the south-western trough, and models indicate weak cyclogenesis and increasing convergence south of the cold front over central France. Given the good timing, instability will likely be in place and thunderstorms are forecast over southern France and north-western Spain. Although low-level winds will be rather weak, strong deep layer vertical wind shear in excess of 20 m/s may result in some well-organized multicells that may merge into an MCS later on, moving eastward into northern Italy. Initial convection may include some rotating updrafts capable of producing large hail. A tornado is not ruled out in the range of the convergence line, but weak low-level vertical wind shear seems to be an importing limiting factor especially for strong tornadoes. Intense rain will be quite likely given rather high moisture in the range of the slow moving convergence.

During the night hours, main convection will likely spread eastward into Italy, where weak instability and shear will limit severe threat significantly. To the west, models agree that further convection may go on along the cold front over southern France, and local flash flooding may occur.

Southern Ukraine

A moist and unstable air mass present over the Black Sea on Wednesday is pushes to the east and north in the range of intense negatively tilted trough axis. Latest GFS indicates that moist low-level air will spread into southern Ukraine in the morning hours, while strong QG forcing north of the trough axis and associated mid-level jet streak will go on. Although lapse rates will likely weaken due to widespread convective activity, at least moderate CAPE values of some 100s J/kg are expected to be possible, and convection is forecast to go on. Strong vertical wind shear and low cloud bases may be favorable for tornadoes, and a few events are not ruled out at this time. Large hail seems to be not very likely due to limited instability, and convective clusters. Convective activity is forecast to weaken during the period as mid-level lapse rates will weaken and low-level cool air will destroy instability.

Southern Portugal

Ahead of the Atlantic trough, a tongue of moist and slightly unstable maritime air mass spreads northward into southern Portugal late in the period. Along the associated warm front, quite strong warm air advection is forecast. Given a strong mid-level jet streak traveling north-ward during the night, strong QG forcing is likely over southern Portugal region on Friday morning. Expect that thunderstorms will form, strong vertical wind shear in the range of a 15 m/s jet at the 700 hPa level may be sufficient for well-developed multicells and embedded mesocyclones capable of producing some hail. However, it does not seem that severe limits will be reached given bad timing for convection and rather cool low-level air mass near the coast.