Extended Forecast
Valid: Sun 29 Apr 2018 06:00 to Mon 30 Apr 2018 06:00 UTC
Issued: Fri 27 Apr 2018 14:58
Forecaster: GROENEMEIJER

A level 1 was issued across for tornadoes, hail, very heavy rainfall and wind gusts across Northeast France, Luxembourg, East Belgium, West Germany and Southeast Netherlands.

DISCUSSION

Level 1 area...

NWP guidance predicts that, on the eastern flank of a large trough, a shortwave trough will move northward across South and Central France during the afternoon. Ahead of this system, cyclogenesis is expected across Central/North France. The details of this development still differ somewhat from one model to another.

Within a zone of warm air advection east of the developing cyclone, a modest to moderate amount of CAPE is expected to form during the day, generally 500 - 1000 J/kg. Within this region, wind shear is forecast to be strong, with 20-25 m/s 0-6 km shear and 200 - 300 m2/s2 storm-relative helicity (SRH) in many places. Near a slow-moving developing warm front located from near Hamburg to the low pressure centre near Paris, even higher SRH is expected.

Near the warm front, ample convection is expected to develop in the 12-18 UTC time frame in response to frontogenetical forcing. The front's orientation parallel to the shear suggests that clustering will take place rather rapidly, promoting very heavy rainfall accumulation. Further east, the coverage of storms will likely be lower and is not guaranteed.

Given the adequate CAPE and strong shear, a number of storms may develop rotating updrafts and bear a risk of large hail and, given adequate low-level shear, a possibility of tornadoes. These risks will be the highest with storms that remain relatively isolated. Some risk of linear modes with severe wind gusts will exists as well.

The risk of severe weather should gradually diminish after sunset in response to boundary layer stabilization.