Storm Forecast
Valid: Thu 18 Oct 2012 06:00 to Fri 19 Oct 2012 06:00 UTC
Issued: Wed 17 Oct 2012 20:26
Forecaster: GATZEN

A level 1 was issued for north-eastern Spain mainly for excessive precipitation.

SYNOPSIS

A sharp and deep trough continues to amplify to the west of the Iberian Peninsula and eventuelly cuts off after the end of the period. A very strong southerly to south-westerly flow will persist through-out the period across most of western Europe and extends into southern Scandinavia. Warm air masses characterized by an elevated mixed layer originating from northern Africa will spread into the west Mediterranean and southern France, whereas cool and moist air masses advect into the Bay of Biscay and western Iberia behind a cold front. Further east, an extensive high pressure region builds across Central Europe.

DISCUSSION

Western France

A rather moist low-level air mass is present across central France ahead of the frontal boundary. Latest soundings indicate increasing low-level moisture across southern France that will advect north-westwards during the night and morning hours. Additionally, relatively steep low-level lapse rates are indicated and may overlap with the moisture plume. However, current data do not support deep instability. Diurnal heating is questionable in the warm air advection regime across western and central France and GFS may overestimate the instability. Current thinking is that the thunderstorm potential is rather low. However, every storm that manages to form ahead of the cold front across western France will be affected by strong vertical wind shear exceeding 20 m/s in the lowest 3 km and 10 m/s in the lowest km. This may result in fast moving multicells and mesocyclones capable of producing severe wind gusts. A tornado is not ruled out, although the low-level buoyancy is rather weak.

North-eastern Spain

A strong south-easterly low-level flow is forecast across the west Mediterranean between a surface trough at the lee side of the Atlas mountains and the strengthening high over central Europe. Latest soundings indicate rich low-level moisture that will spread into north-eastern Spain. Upsolpe flow will result in increasing instability of this moist air mass and models predict increasing convective activity during the period. Main activity is expected from late Thursday into Friday morning. Given the strong southerly flow aloft, sideward building MCSs are expected with a chance of excessive precipitation. Additionally, strong vertical wind shear can support strong updrafts capable of producing large hail.