Extended Forecast
Valid: Fri 03 Jul 2015 06:00 to Sat 04 Jul 2015 06:00 UTC
Issued: Wed 01 Jul 2015 23:10
Forecaster: GROENEMEIJER

A level 2 was issued for NW France and W Belgium

This extended forecast is issued to highlight an elevated potential of severe weather over NW France and W Belgium on Friday evening. There is a risk of strong tornadoes and very large hail.

Other areas are not discussed.

DISCUSSION

An eastward progression on Thursday of the warmest lower tropospheric air and the moist convergence zone on its western fringe across the Benelux and France is followed by a westward advection of low-level moisture on Friday.

This advection in concert with surface pressure falls over the Bay of Biscay in association with the northward advection of extremely warm air over France and an approaching mid-level trough over the eastern North Atlantic ocean. Within the noist air-mass, CAPE may rise over 2000 J/kg per GFS and around 1000 J/kg per ECMWF.

Near a warm frontal zone over the Channel, high amounts of storm-relative helicity (~400 m2/s2) and high low-level shear (10-15 m/s) are forecast. NWP guidance hints that, during a time window in the evening, substantial surface-based CAPE and the strong low-level shear may overlap.

There is some uncertainty about the question if storms will develop in this region and somewhat dry forecast profile. GFS shows the strongest precipitation signals.

Any storms developing in that shear environment have a high probability of turning into supercells with a risk of very large hail and of (strong) tornadoes.