En komende nacht en dag krijgen we weer meer van dit alles.
Komende nacht maakt met name het zuiden en westen kans op nog wat onweersbuien. In het noordwesten van Frankrijk kunnen in de loop van de middag en avond zware onweersbuien ontstaan. Ook hier geldt dat er nog onzekerheid is over of de buien ook echt goed kunnen ontwikkelen. Als ze daar gaan ontwikkelen, zijn ze potentieel zeker gevaarlijk. Er is kans op zeer grote hagel, zware windstoten en de omstandigheden zijn zelfs goed voor enkele tornado's.
Die buien trekken in de loop van de nacht richting het noorden en bereiken dan ook het zuiden en westen van Nederland. Hoe heftig het dan nog is, is nog even afwachten.
Uiteindelijk komt er ook weer een zuidelijke stroming met zeer warme onstabiele lucht op gang. Morgen dus weer erg heet. In de loop van de middag en avond net als gisteren weer kans op zware onweersbuien, met name weer in het midden, oosten en noorden van het land.
Storm Forecast
Valid: Fri 03 Jul 2015 06:00 to Sat 04 Jul 2015 06:00 UTC
Issued: Thu 02 Jul 2015 22:17
Forecaster: GROENEMEIJER
A level 2 was issued for NW France and the channel region, for very large hail, severe wind gusts and tornadoes and to a lesser extent for extreme precipitation.
A level 1 was issued for a greater part of Northern and Western France and the Southern United Kingdom for large hail and extreme precipitation.
SYNOPSIS
Friday morning at 06 UTC...a ridge extends from the Western Mediterranean to the Baltic States and a cold core low is slowly drifting southwestward over the western Black Sea. Very high lower-troposperic temperatures are present the ridge. An eastward intrusion of cooler near-surface air has made its way into Germany ahead of a second northward impulse over France of particularly hot air.
DISCUSSION
NW France...
The forecast for this area is particularly difficult. NWP guidance suggests that a warm front will set up over the area during the afternoon and evening, and that steep lapse rates and moisture will lead to ample surface-based CAPE of 1000 (ECMWF) to 2000-3000 J/kg (GFS). In addition, a substantial elevated CAPE is forecast to develop during the evening over the Channel and Southern England.
During the evening, the warm frontal zone features easterly surface winds, a brisk 10-18 m/s southerly flow at 850 hPa and 15-20 m/s southerwesterlies at 500 hPa. This yields a large curved low-level hodograph with 400 m2/s2 of SREH. Hence, strong rotation is expected in any boundary-rooted storm that might develop. A risk of very large hail producing supercells with - given the 10-15 ms/ 0-1 km shear forecast in the 18-21 time frame - a risk of (strong) tornadoes exists.
The difficulty is caused by the fact that NWP models are not uniformly producing surface-based convective storms. Indeed, a significant hindrance to surface-based development is the warm air at 850 hPa of around 18 C. The GFS model initiates what is likely surface-based convection within the level 2 area during the afternoon, but there is very little precipitation in ECMWF.
I have decided to go with a level 2, with the understanding that there is a risk that surface-based convective over NW France may not occur at all, in which case the severe weather threat is limited to large hail and extreme rainfall from any elevated storms.
England, Channel region, Benelux...
Elevated convection should move in from the south during the late evening and after midnight. There is a small risk of large hail and extreme precipitation with these storms.
Bron: Estofex
