Storm Forecast
Valid: Thu 18 Sep 2014 06:00 to Fri 19 Sep 2014 06:00 UTC
Issued: Wed 17 Sep 2014 22:30
Forecaster: GATZEN

A level 2 was issued for the north-west Mediterranean mainly for large hail, excessive precipitation and severe wind gusts.

A level 1 was issued for the western Mediterranean mainly for large hail and excessive precipitation.

SYNOPSIS

A ridge is present across central and northern Europe. Its northern flank will weaken due to a cut-off low moving east across the southern Baltic Sea. The ridge is flanked by a deep quasi-stationary trough centred west of the Bay of Biscay, yielding a strong south-westerly flow over the west Mediterranean. At lower levels, a plume of warm air has spread into western Europe, with an axis from the central west Mediterranean to western France. In the same region, rich low-level moisture is present, whereas dry boundary layer air masses dominate across eastern Europe.

DISCUSSION

Mediterranean west of Italy

A strong jet stream affects the west Mediterranean region through-out the period. Rather small and weak mid-level vort-maxima are embedded in this flow, leading the some DCVA. A well-developed short-wave trough will pass by late in the period, leading to QG forcing east of Iberia.

With the south-westerly flow, steep lapse rates are continuously advected from the Atlas mountains over the west Mediterranean Sea. At lower levels, moisture pooling has lead to 0-500m mixing ratios up to more than 16 g/kg. High CAPE in excess of 100 J/kg is sampled by latest soundings, and GFS indicates 1500-3000, locally more than 5000 J/kg over the west Mediterranean on Thursday. The capping inversion is strongest in the southern portions, whereas weak capping is present from the Ligurian Sea to the Gulf of Lion.

Current thinking is that land-sea-breeze convergences and outflow-boundaries of overnights storms will support scattered initiation through-out the period, with a focus from north-eastern Spain to the Ligurian Sea. More isolated initiation is forecast further south and east. With rather high instability and 0-3 km vertical wind shear around 10-15 m/s, intense and rather well-organized storms are expected, capable of producing large or very large hail. Severe wind gusts and excessive rain are not ruled out as well.

Later in the period, storms may become more widespread in response to the QG forcing, and may merge to one or two MCSs. If this scenario comes true, severe wind gusts will become the dominant threat together with excessive precipitation, especially near the Gulf of Lion.

France into Belgium and western Germany

A plume of warm air with steep lapse rates has spread across western Europe. Due to adequate low-level moisture and diurnal heating, CAPE will again develop on Thursday. Initiation is forecast in the afternoon and evening, when weak QG forcing spreads north-east over France. Storms that form will be mostly non-severe, however, vertical wind shear is slightly stronger over France, and some large hail events are not ruled out. Additionally, excessive precipitation may occur with slow-moving storms across western France, Belgium, and western Germany. Overall threat seems to be too low for a level 1 at this time.