Storm Forecast
Valid: Tue 20 Jun 2006 06:00 to Wed 21 Jun 2006 06:00 UTC
Issued: Tue 20 Jun 2006 02:04
Forecaster: VAN DER VELDE
SYNOPSIS
A similar distribution of moderate instability over Europe as yesterday, but some areas will now experience more vertical wind shear and storm-relative helicity.
A mid/upper level trough approaches Europe from the west and deepens over the UK. At the same time, the upper ridge over Italy slightly stretches out northward. The midlevel height gradients between these systems tighten slightly, causing winds at 850-500 hPa to reach values between 15-25 m/s (35-45 kts) in the level 2 area, through the unstable airmass.
A thermal low has developed last night and generated a MCS over southwestern/central France. 18Z GFS model shows a wave in low-level winds moving from France into northeastern Germany/Poland by the end of the period. This feature is also present as a shortwave trough in midlevels.
The other synoptics correspond closely to yesterdays description.
DISCUSSION
...Northeastern France into northern Switzerland and southwestern Germany...
A level 2 has been issued, meaning a threat of widespread severe weather, and the possibility of isolated extreme weather. As a shortwave through moves through the area it will cause widespread convection.
The strong midlevel winds, described above, would generate 15-20 m/s deep-layer shear (0-6 km), while SREH will surge to values over
100 m2/s2 over a large area, possibly >250 m2/s2 over a smaller area. These numbers are equal or better than we have seen yesterday, which proved to be sufficient for supercells. More supercells, with a capability of producing isolated large or perhaps extremely large hail and severe gusts will be likely today. There is an increased threat of tornadoes, because the low-level (0-1 km) shear reaches over 10 m/s, in combination with LCLs not too high, between 1000-1500 m.
As convection is expected to coagulate into a rapidly moving MCS (squall line with bow echoes) towards or during the evening, the threat of especially widespread severe gusts will increase.
...northeastern Spain, southern France...
Basically the same discussion as the previous day - nothing much changed. SREH and moderate deep layer shear create an environment in which, again, some supercells with a chance of large hail in particular are able to form.
...rest of Germany, Czech Republic, Austria...
Instability expected to be comparable to yesterday, but the vertical shear and SREH are a notch higher. Expect isolated to scattered severe storms, some supercell, with large hail, and some severe gusts the main issue, especially where midlevel lapse rates are expected to be steepest, towards the south. GFS has some convergence areas that would help more widespread convection, but produces not that much convective precipitation.
...Balkan, Ukraine...
Essentially the same kind of situation as the day before. Note that Sofia Mon 12Z sounding showed very strong CAPE and 50 kts at 500 hPa, which would possibly have caused extreme hail... expect midlevel winds to ease down a bit, but GFS may still be underestimating MLCAPE for this area.
...former Yugoslavia...
In this area, moderate 0-6 km shear and some SREH come together in unstable airmass. Previously this area has seen CAPE but strong capping, but GFS signals with strong convergence, a bit convective precipitation and no cap (small LFC-LCL difference). With relatively dry conditions (LCL at 2000 m, dry midlevels) expect chance of severe gusts and large hail, from isolated storms, possibly supercells.
...Sweden, Finland...
some marginal instability will develop under moderate (15 m/s) deep-layer and moderate (7 m/s) low-level shear conditions, also enhanced SREH (>100 m2/s2)... allowing marginally severe gusts, rotating updrafts may develop and enhance the chance of marginally large hail, also a tiny chance of an isolated tornado given not too high LCLs.
