Storm Forecast
Valid: Wed 09 Jun 2010 06:00 to Thu 10 Jun 2010 06:00 UTC
Issued: Wed 09 Jun 2010 00:09
Forecaster: TUSCHY
A level 2 was issued for extreme W-Switzerland, E-France and SW-Germany, mainly for large hail (a significant event is possible) and strong to severe wind gusts.
A level 1 surrounds the level 2 mainly for large hail, strong to severe wind gusts and to a lesser extent for an isolated tornado.
A level 1 was issued for SW-France mainly for large hail.
A level 1 was issued for W-Turkey mainly for excessive rainfall.
SYNOPSIS
A strong trough is situated over the Bay of Biscay with no eastward progression expected. Downstream, ridging is forecast although LL air mass and gradually lowering geopotential heights assist in enhanced thunderstorm activity well into the ridge axis. A shallow upper low crosses W-Turkey with widespread thunderstorm initiation forecast. Cool and more stable conditions persist over N-Europe.
DISCUSSION
... Switzerland, Germany, W- Czech Republic and parts of Poland ...
Seems to be a transition day for this area, as the potent upper trough to the west still intensifies on site with no appreciable eastward progression. Hence, weak ridging at mid-levels persists with numerous waves creeping in from the SW during the forecast. Therefore, initiation occurs over a prolonged period with no significant upper support and hence more sporadic in nature. However, mid-/upper streamline pattern achieves a more diffluent pattern during the night hours, which may assist in enhanced convection. Also, local convergence zones, rough topography like the Black Forest, foehn effects along the northern Alps and regionally moisture pooling along decaying or diffuse surface fronts induce even more uncertanties in respect of where highest thunderstorm coverage may occur. For now, we went with a blend of better forcing, best BL air mass quality and best model agreement.
Yesterday at 22Z, GFS was on track with dewpoint forecasts, but quite unrealistic jumps are forecast tomorrow noon/afternoon. Some daytime mixing and modest moisture advection in mind, we keep surface dewpoints a bit lower than GFS, but agree with EZMWF and foresee roughly 1000 J/kg MLCAPE (with even higher SBCAPE/ICAPE magnitudes) from S-Germany east/northeastwards. In addition, mid-level lapse rates undergo modest steepening during the forecast wich foster widespread CAPE build-up. CIN decreases from south to north, but expected more isolated initiation may not inhibit insolation during the day with full surface mixing expected. The cap will be conquerable and it could keep thunderstorms more discrete for a longer period over S/central-Germay.
DLS increases from east to west (10-20 m/s) with strong anvil layer shear. Main concern right now is quite widespread and strong directional shear over S-Germany and Switzerland. Given favorable shear/CAPE overlap in the level 2 area, good CAPE build-up in the hail growth zone and a well mixed subcloud layer (foehn effects), large hail will be the main risk with significant events possible. Strong to severe downdrafts are also forecast given supportive environment. Initiation will be more isolated at 12Z onwards with more widespread development during the later afternoon hours along the Black Forest. This activity may grow upscale into a cluster of storms, and despite weak to modest shear, cold pool driven thunderstorm line(s) may pose a strong to severe wind gust and hail risk, while affecting central/north Germany thereafter.
An isolated tornado event can't be ruled out but right now there are no real foci for a more concentrated/augmented tornado risk.
Over E-Germany, Czech Republic and Poland, shear remains weak, but SBCAPE could become substantial, if GFS is on the right track (at 22Z, dewpoints already reached 18°C over W-Czech Republic). Hence, pulsating storms will evolve and a few of those could become severe with large hail and strong wind gusts. After sunset, activity gradually diminishes.
... Parts of Portugal, Spain and W-France ...
Constantly intensifying surface depression moves in from the Bay of Biscay. A cold front affects Portugal between 08-12Z and Spain thereafter. Postfrontal showers/thunderstorms occur, but shear remains weak at all levels. Marginal hail and strong wind gusts will be the main hazard.
W-France looks better for at least an isolated large hail report, as SBCAPE is more robust and DLS increases to 20m/s. Thunderstorm activity temporarily decreases after sunset, before a second round occurs after midnight, although the second surge is a bit uncertain as a lot depends on the exact track of the surface depression.
...W-Turkey ...
A shallow upper low crosses W-Turkey atop a moist low-level air mass with surface dewpoints in the mid to upper tens. Shear is nearly non existent with very slow storm motions forecast, next to enhanced lift due to orographic effects and backward building MCS propagation vectors. Widespread thunderstorm initiation is expected with rapid clustering. Heavy to locally excessive rainfall is forecast. We decided to stick with a high-end level 1 as we miss some feeder effects and better anvil layer shear. Nevertheless, flooding could become a problem beneath strongest/slow moving updrafts.
