19-07-2007 22:52 | Estofex | m.b.t. 19-07-2007
Storm Forecast
Valid: Fri 20 Jul 2007 06:00 to Sat 21 Jul 2007 06:00 UTC
Issued: Thu 19 Jul 2007 22:52
Forecaster: GROENEMEIJER
SYNOPSIS
A baroclinic zone initially stretches from SW France to the SErn Benelux over central Germany and southern Poland to Belarus. In the mid- and upper troposphere this zone is associated with a jet, that is zonally-oriented in the east. To the west... a SWly jet streak over France -ahead of a trough approaching the European continent- is expected to move northeastward as cyclogenesis occurs in its left exit quadrant over northern France.
DISCUSSION
NE France, E Benelux, NWrn half of Germany...
Models tend to agree that rather widespread convection will form in response to the vorticity maximum in the left exit of the aforementioned jet. At daybreak, some storm systems will however already be ongoing, most likely over central and N France. Over NE France, W-central Germany and the SE Benelux solar heating and upward vertical motion should be capable of creating about 1000 to possibly up to 1500 J/kg (50hPa-)MLCAPE. Rather widespread storms should form as a result. The amount of insolation and instability being dependent of course on the prior evolution of convective storms. The most likely convective mode is linear convective systems and locally supercells capable of producing some hail and strong winds. It is possible that one or more rather well-organized lines produce isolated severe winds while moving northeastward during the morning.
Especially with supercells, tornadoes will be possible as well. The reasoning is that a narrow band of strong helicity and low level shear > 10 m/s should develop just ahead and to the east of the surface low expected to track northward near the Benelux/German borders according to a consensus of GFS/AFWA-MM5 model runs. However, because of the strong forcing, convective lines will likely dominate over isolated supercells which lowering the overall tornado threat. At the moment, the overall threat appears just to fall short of the level 2 criterion that requires either widespread severe or isolated extremely severe events to occur. An update and/or upgrade may however well be given for certain parts of the forecast area.
SErn half of Germany, E-central France, Switzerland...
Although similar large values of instability are forecast over this area, weaker forcing will probably cause storms to be more isolated. They will bring a risk of severe wind gusts and large hail. The main convective modes should be eastward-moving multicell clusters and possibly a few supercells. Although storm initiation during the afternoon is likely, storm coverage is forecast to (further) increase from the west during the evening hours.
Austria, Czech Republic, Poland ...
Low storm coverage is expected across much of this area at first, as a strong elevated mixed layer should succeed to effectively cap a moist warm boundary layer in most places. During the latter half of the afternoon, however, the cap is forecast to break mostly over west-Central and central Poland, close to the axis of the baroclinic zone. It is however not ruled out at all that this will also occur further south especially near mountain ranges. CAPE values of around 2000 J/kg and deep-layer shear around 15 m/s suggest a substantial severe potential with any storm that forms, mainly consisting of severe downdrafts and large or even very large hail. The low allover coverage and unceratinty about its location however prevents the issuance of a level 2 at the moment.
S England, S Wales...
A relatively weak but strongly cyclonic low-level flow is expected south of a large-scale precipitation area. This in combination with moist air and instability with low LFC heights suggest that thunderstorms, which could be rather widespread, may be accompanied by a few waterspouts and weak tornadoes. Some small hail will be possible as well.
Verwachtingen/waarschuwingen
