Storm Forecast
Valid: Thu 05 Feb 2009 06:00 to Fri 06 Feb 2009 06:00 UTC
Issued: Wed 04 Feb 2009 23:28
Forecaster: GATZEN
SYNOPSIS
Large amplified long-wave trough will also affect western Europe on Thursday. Its axis extending from British Isles to north-western Iberian Peninsula and further to the Canary Islands on Thursday morning will move eastwards during the period. Ahead of this trough, a strong westerly flow stretches from northern Africa and southern Iberian Peninsula to south Mediterranean Sea. Strong jet streaks will eject into west Mediterranean ahead of the trough axis.
Southern Spain
Maritime air mass will affect southern Iberian Peninsula in the range of the trough axis. Latest soundings indicate low-level steep lapse rates and quite rich boundary-layer moisture. On Thursday, quite strong QG lift will likely result in deeper instability, and deep convection becomes quite likely as an upper trough axis will move north-eastward across the region. Given a strong 20 m/s westerly low-level jet curving around the troughs base, quite strong low-level vertical wind shear will result, and convection may evolve into mesocyclones given around 200 mē/sē helicity in the lowest kilometre. Severe wind gusts and isolated tornadoes are forecast. Marginally severe hail is also not ruled out. Convective activity will likely weaken in the afternoon as cols air advection sets in to the west of the trough axis.
South-eastern France
A strong mid-level jet streak moves north-eastward across southern France on Thursday. A strong southerly low-level jet is forecast especially in the morning hours that will advect moist low-level air into south-eastern France. As mid-level air mass is rather cold, instability is likely to develop during the day due to QG lift to the north-west of the jet streak. Thunderstorms are forecast that will likely spread eastward across southern France. Northern extension of deep convection is mostly dependent on the moisture transport, and current thinking is that low-level air mass will remain rather cold, leading to elevated convection. Over southern France, surface-based convection in a strongly-sheared environment will be capable of producing severe wind gusts and tornadoes. Large hail is not ruled out. Late in the period, convection may form again in the range of an approaching trough axis. Near the coasts, convection may be severe again given strong vertical wind shear.
West Mediterranean
Unstable maritime air mass in the range of the west-European trough and several vort-maxima travelling across the region will be sufficient for convection through-out the period, spreading eastward into Italy late in the period. Given strong vertical wind shear, isolated mesocyclones are forecast capable of producing tornadoes and severe wind gusts.
Bay of Biscay
Unstable maritime air mass is also expected across the Bay of Biscay. On Thursday, the trough axis of the west-European trough will cross this region, leading to QG forcing. Showers and thunderstorms that form will have a low potential to rotate given favourable veering hodographs. An isolated tornado is not ruled out especially near the coasts until the evening hours, when cold air advection sets in.
