Storm Forecast
Valid: Tue 21 Jul 2009 06:00 to Wed 22 Jul 2009 06:00 UTC
Issued: Mon 20 Jul 2009 22:14
Forecaster: GROENEMEIJER/TUSCHY

A level 2 was issued for parts of Belgium, the Netherlands and extreme NW-Germany mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for an area surrounding the level 2 which includes northern Germany, most of the Benelux, northern, central, and southern France, and northern and eastern Spain, mainly for large hail and severe wind gusts.

SYNOPSIS

A cyclonic vortex is situated over Ireland and the NE-Atlantic. Embedded within a strong southerwesterly wind field at all levels, a number of weak upper-level vorticity maxima should affect parts of western Europe during the forecast period. Ridging downstream moves to the east and keeps most parts of central Europe dry. Another upper trough over NE-Europe causes unsettled conditions with scattered thunderstorms.

DISCUSSION

Benelux, Northern Germany...

An elevated mixed layer covering central and southern France will be advected northeastward during the day and should overspread the Benelux and much of Germany during the afternoon hours. Wind shear will be very strong under the southwesterly jet that consists of two separate speed maxima: one should stretch from north-central Spain across France to NE France, whilst the other is located over the Channel region and SE England. A zone of high, 200-300 m2/s2, storm-relative helicity, should move northward along the leading edge of the elevated mixed layer, and values around 200 m2/s2 should also be locally further south across central and southern France. Moreover, 0-3 km shear will be in the 20-25 m/s range, which is strong.

Models are insofar constistent with each other, that they all produce precipitation in the late afternoon and/or evening across the Benelux and NW Germany, and some further east along the warm front as well. But some models produce only little convection. Apparently, the overall forcing for upward motion is not very large.

Still there is confidence that significantly more than just one or two storms will develop. Because of the excellent kinematic and thermodynamic parameters, the storms have a significant potential to become severe, and a level 2 is issued as a consequence. The storms will likely include supercells. Storms across the Benelux and northern Germany will be capable of producing large hail and damaging gusts, and also tornadoes. The tornado risk will be largest with those cells that tap most of the helical low-level flow: those that ingest the most backed low-level winds.

The storms are expected to continue throughout the evening, and new storms could even develop overnight. However, those will probably be elevated and because of low-level cooling, they will be less likely to produce severe winds or tornadoes.

France, Spain...

Despite the weak forcing, convective initiation will probably not only across northern France, but also across central and southern parts of France, and across northern and eastern Spain. Isolated storms are expected across those areas, which again will likely include a number of supercells capable of producing large hail and damaging winds. The storms will likely weaken and decrease in coverage overnight across Spain, but some elevated storms across France may make it well into the night.

South Finland, Estonia, Latvia, Lithuania and Belarus ...

Cold mid-levels beneath an eastward moving trough and seasonably moist low-levels cause moderate instability release in the order of 800 - 1200 J/kg MLCAPE. Shear at all levels remains weak with 10 m/s throughout the lowest 6 km. Diurnal driven, pulsating thunderstorms are expected, which produce marginal hail and strong wind gusts. An isolated large hail event can't be ruled out due to regional MLCAPE in excess of 1000 J/kg. The thunderstorm risk decreases after sunset.

S Switzerland and the western/southern Alps ...

Again, diurnally-driven thunderstorms are forecast. Situated beneath weak ridging, no real forcing mechanism is forecast to affect the area, so afternoon thunderstorms with marginal hail and strong wind gusts are the main risk. An isolated large hail event is possible over south Austria, where DLS temporarily increases to 15m/s, but coverage of expected hail reports should be too sparse for any level.

Ireland and SW-Scotland ...

Isolated thunderstorms are forecast during the passage of the cold mid-level low. Enhanced divergence at upper levels ought to be present as the left exit of a 60m/s speed maximum at 300hPa approaches from the south. Marginal hail and strong wind gusts are the main hazard within the cold and well-mixed maritime airmass.