Storm Forecast
Valid: Mon 19 Jun 2006 06:00 to Tue 20 Jun 2006 06:00 UTC
Issued: Mon 19 Jun 2006 00:49
Forecaster: VAN DER VELDE

SYNOPSIS

An Atlantic low arrives over the North Sea with a thermal trough over northern Germany. Its cold front will push into the marginally unstable airmass into Germany and Denmark.
An upper cold pool keeps a carrousel of convection going over Eastern Europe, while an old (upper) low still affects Spain with unstable airmass.


DISCUSSION

...SW France, N Spain...

GFS and NMM Sun 06Z indicate CAPE, convergence and a kinematic environment of 17-22 m/s 0-6 km shear vectors, along with SREH 0-3 km in the range of 150-275 m2/s2. This will be sufficient to create a few supercells with a main chance of (very) large hail and severe gusts. Tornado threat should be limited with LCLs around 1500-2000 m and modest 0-1 km shear. Note that 12-18Z model runs downgraded the deep-layer shear and SREH slightly.
According to the model, the difference LFC-LCL will be lowest in western parts, almost zero already at 12Z in northern Spain and at 15Z also north of the Pyrenees. Clustered elevated convection may develop in the evening and night over northern Spain and move into southwesternmost France (GFS precip up to Bordeaux - thunder area probably drawn too tightly there)

...central France into Switzerland and southwestern Germany...

Some deep-layer shear and 0-1 km shear enhanced to over 10 m/s may allow a linear MCS with bow segments with a threat of severe gusts. Somewhat enhanced SREH to over 100 m2/s2 indicated by GFS may induce also a few rotating cells with an enhanced chance of large hail. Organisation into a line is not certain, with no frontal boundaries, upper level lift mechanism or clear lines of convergence indicated by models, but at least isolated severe weather is likely to occur with some storms.


...Eastern Netherlands, northwestern Germany...

The Sun 06Z GFS run indicated a band of significant deep layer shear (15-20 m/s) passing over this area. However, later runs have since decreased this. At the east side of the cold front the airmass is unstable up to several hundred J/kg MLCAPE and strong convective precipitation signals in the GFS model. Because of the lack of enhanced SREH and shear collocated with the instability, main convective mode will be clustered storms, probably grouped in a line along the front.
GFS Sun 06Z indicated already no capping at 12Z, while later runs have instead kept difference between LCL and LFC, which probably means that frontal cloudiness or precipitation prevents sufficient heating. Hence, the level 1 is a very conditional one mainly issued because of the anticipated widespread convection. Main chances of severe weather include marginally large hail and marginally severe downburst winds, on an isolated scale.


...Italian Alps and adjacent plains...

Uncapped instability is restricted to a narrow area near the Alps. GFS typically underestimates the CAPE somewhat. Upslope flow will trigger isolated marginally severe storms under a kinematic environment of moderate deep layer shear and enhanced SREH to over 100 m2/s2. The convection is most likely to locally produce large hail. In the evening convection may cluster or form a small MCS as GFS pushes a line of convergence southward over the region.
[Level 1 has probably been drawn a little too tight]

...Hungary, western Romania, Bulgaria...

With significant low-level mixing ratio values and quite steep midlevel lapse rates, MLCAPE up to 1000 J/kg can be obtained, and with moderate deep-layer shear expect widespread convection in a broad area of some convergence from Hungary into Bulgaria. Storms may produce large hail and/or severe gusts.

...western Sweden, eastern Norway...

With no clear convergence signal or boundary, but within a weak surface trough, GFS indicates a region of enhanced but marginal CAPE. Given the enhanced SREH values to up to 200 near the Norwegian border, as well as moderate to strong (20 m/s 0-6 km) deep-layer shear and low-level shear, can not exclude the possibility of isolated storms developing rotation and a chance of large hail, if instability will verify.

...Ukraine/Belarus, Balkan...
Slow movement of storms in weak flow regime poses a threat of local flash flooding.