Storm Forecast
Valid: Sat 10 Jul 2010 06:00 to Sun 11 Jul 2010 06:00 UTC
Issued: Sat 10 Jul 2010 06:27
Forecaster: VAN DER VELDE
Level 1 and 2 were issued for N France, Belgium, Luxembourg, Netherlands and W Germany for large hail, excessive precipitation and severe wind gusts.
A level 2 was issued for the mountains of Romania for excessive convective precipitation, level 1 for surrounding areas.
A level 2 was issued for S Ukraine mainly for large hail and severe wind gusts.
A level 1 was issued for central and SE France/NW Italy for locally excessive precipitation.
A level 1 was issued for N Turkey for large hail and severe gusts.
A level 1 was issued for N Spain for isolated large hail and severe gusts.
SYNOPSIS
An unstable airmass has been advected in the past days from the northern Iberian Peninsula to France, and today will cover a still larger area, while afternoon MLCAPE values soar to values of 1200-2200 J/kg. Under the influence of approaching low geopotential and vorticity advection at mid levels, and continued warm air advection in low levels, development of storms will likely become more widespread over France, Belgium, Luxembourg and the Netherlands.
The other major body of unstable air is associated with a confluence zone between the Polish high and an intense mid level low centered near Moldova, advecting very humid air to eastern Romania (again).
DISCUSSION
...France, Belgium, Luxembourg and the Netherlands...
The 00Z Trappes sounding confirms the presence of a deep 14-15 g/kg moist layer in the boundary layer, which is even more than predicted by GFS 18Z (12 g/kg). The model, however, increases it to such amount during the day (18-21Z northern France, Belgium). Weakest cap is found according to GFS model LFC-LCL differences initially over C France, E Belgium and W Germany, with strongest convergence near Luxembourg, then develops a cyclonic circulation towards the evening apparently in response to the QG lifting. This low pressure center drifts off to the north or north-northwest over Belgium and Netherlands during the evening/night and will most likely be accompanied by one or more MCSs (possibly a mesoscale convective complex, MCC, given the interaction with dynamics and large instability). This scenario seems also supported by 00Z WRF models and ECMWF (Fri 12Z). The cyclonic circulation is already present before GFS develops a gridscale 'convective bomb', and is confirmed by current 04Z surface winds.
The severe weather potential is initially limited to flash floods over central France where storm motions are relatively slow and the airmass close to saturated. To the north (Belgium) 0-6 km shear is a bit higher (10-15 m/s) as well as the MLCAPE. This may already organize supercells with large hail, and after 15Z the storm-relative helicity over the lowest 3 km grows from 100 to 300 mē/sē which enhances the chance of rotating, strong updrafts. On the other hand, clustering will force the storms to coagulate into an MCS, turning the threat into mainly severe outflow winds and excessive rain. A factor which increases the wind potential from wet microburst is the large Delta-Theta-E (locally 20°). However, the storm motion will drag mostly along the boundary and shear vector direction, which may limit the spread of gusts. For this reason only a small level 2 was issued.
...Romania...
The strong transport of a very humid airmass with large precipitable water content onto the eastern flank of the Carpathian mountains combined with conditional instability (storms) should lead to widespread cases of excessive convective precipitation and flash floods, in the already saturated area.
...S Ukraine...
1000-2000 J/kg MLCAPE on the GFS maps is found in this area. Lifting and moderate wind shear (in particular directional) are both present, and a tongue of enhanced SREH (>100 mē/sē) stretches northward. The conditions can lead to organized multicells with large hail and locally severe wind gusts (delta-theta-e of 20°)
Destabilization and low level convergence are enhanced by rising motions in the left jet exit region.
...N Turkey...
Same story as for Ukraine except that wind shear and SREH are stronger in magnitude (250 mē/sē), but CAPE lower.
...N Spain...
Enhanced SREH and mid level shear vectors (1-4 km) offer some potential for isolated storms to organize rotating updrafts with isolated large hail formation, as well as an isolated severe convective gusts event due to strong evaporational cooling (high LCL and delta-theta-e >20°). Initially there is subsidence predicted at the rear of the French shortwave trough, which could inhibit growth.
