Storm Forecast
Valid: Mon 05 Aug 2013 06:00 to Tue 06 Aug 2013 06:00 UTC
Issued: Mon 05 Aug 2013 01:01
Forecaster: GATZEN

A level 2 was issued for parts of France and the western Alps mainly for large hail.

A level 1 was issued for eastern Spain mainly for large hail.

A level 1 was issued for northern France, Benelux,and north-western Germany mainly for severe wind gusts, large hail and tornadoes.

A level 1 was issued for the southern British Isles and southern Scandinavia mainly for severe wind gusts and tornadoes.

SYNOPSIS

A very active severe weather pattern goes on across parts of Europe. A south-westerly flow has developed east of low geopotential across the north-eastern Atlantic. On Monday, a pronounced short-wave trough will move across the British Isles and the North Sea region, the trough axis will extend well into central France and the western Alps. Ahead of this trough, a warm and well-mixed air mass placed across southern France and the Alps will spread northward. Critical issue will be the boundary-layer moisture as cooler and drier air has spread into France and Germany in the wake of Sundays storms, but current thinking is that moisture recovery will be strong enough to allow for storms. In the wake of the first vort-max, another one will follow across the Iberian Peninsula that enters France at the end of the period.

Given the strong south-westerly mid-level flow and backing winds in the boundary-layer, storms will have a rather high potential to organize due to strong vertical wind shear.

Across eastern Europe, lapse rates are weaker in a north-westerly flow, but rich low-level moisture allows for CAPE especially across parts of Russia.

DISCUSSION

Eastern Spain

Upslope flow of moist Mediterranean air below and elevated mixed layer will allow for CAPE exceeding 1000 J/kg during the day. An approaching short-wave trough is expected in the afternoon and evening hours and increasing low-level convergence is expected. Main limiting factor is a strong capping inversion, but current thinking is that the cap will break especially across the mountains with the approaching mid-level trough axis. Strong vertical wind shear (20 m/s 0-6 km and 15 m/s 0-3 km bulk shear) will support multicells and supercells capable of producing large or very large hail. Additionally, severe wind gusts are forecast. Across north-eastern Spain, increasing low-level vertical wind shear will also enhance the risk of a tornado in the evening hours. Main rason for the level 1 forecast is the uncertainty of initiation. With higher storm coverage, a level 2 will likely be required.

Southern/central France into the Alpine region and central Germany

A frontal boundary has formed across southern France in the wake of Sundays storms. The pronounced short-wave trough will advect an elevated mixed layer north-eastward, whereas backing winds in the boundary-layer will support moisture advection into central France. Best moisture is expected to evolve in the afternoon ahead of a cold front crossing northern and western France.

Showers and thunderstorms will move across northern France in the morning hours along the approaching trough axis/cold front. Along the developing convergence line from the Pyrenees to northern France and the Benelux countries, initiation is expected in the noon/afternoon hours. Additional storms will likely develop during the day from the Massif Central into the Alpine region. Best CAPE will be present across central France, where also moderate to strong (20 m/s) deep layer vertical wind shear is expected. Supercells capable of producing very large hail may be possible. Storm clustering in the evening hours will increase the threat of severe wind gusts and isolated excessive precipitation. The potential of severe events will gradually decrease further east across the Alps where vertical wind shear will be weaker. However, excessive precipitation, large hail, and local severe gusts are not ruled out as well given large CAPE and at least 10-15 m/s deep layer vertical wind shear. Storms will go on during the night hours especially north of the Alps in the range of the vort-maximum. Increasing vertical wind shear will result in the potential of severe wind gusts and large hail until the morning hours.

The next vort-maximum will approach across the Iberian Peninsula, and warm advection will again increase across southern France. New storms are forecast to develop and will spread north-east until the morning. With strong deep layer vertical wind shear around 20 m/s, supercells and bow echoes are possible capable o producing large hail and severe wind gusts until the end of the period.

Northern France, Benelux, north-western Germany

From northern France into north-western Germany, weaker moisture and lapse rates will result in weaker CAPE. However, given the strongly-forced synoptic set-up, convection will likely grow upscale into a squall line moving into north-western Germany in the late evening. Severe wind gusts and large hail are forecast with this system. A tornado is also not ruled out given increasing low-level vertical wind shear in the evening hours especially near the North Sea. Given the uncertainty of the low-level moisture recovery, a level 1 was introduced for the northern areas. A level 2 may be required in the case of better moisture recovery on Monday due to the possibility of a severe weather outbreak.

British Isles, North Sea region, southern Scandinavia

A strong low-level jet will move across the area from the west, associated with warm air advection. Strong QG forcing will also allow for increasing lapse rates that will overlap with rather rich low-level moisture ahead of a cold front and a following trough axis. Models indicate some surface-based instability across the North Sea and over the southern British Isles in the afternoon and evening hours. However, current thinking is that most storms will be elevated due to the rather cool and dry boundary-layer. Nevertheless, storms will likely merge into a squall line crossing the southern North Sea and moving into southern Scandinavia. Additional storms are forecast over the southern British Isles along an approaching convergence line. Due to the strong vertical wind shear, bowing segments are forecast capable of producing severe wind gusts. Additionally, tornadoes are not ruled out.