Storm Forecast
Valid: Fri 16 May 2008 06:00 to Sat 17 May 2008 06:00 UTC
Issued: Thu 15 May 2008 19:58
Forecaster: TUSCHY

SYNOPSIS

Upper - level trough over the Gulf of Biscay and the Iberian Peninsula moves eastwards, enhancing the thunderstorm potential over the central Mediterranean. Despite weak ridging over central Europe, environment is still favorable for widespread thunderstorms as weak pressure differences and low geopotential heights prevail. A cold airmass covers most parts of far N - Europe and no deep convection is expected in this airmass.

DISCUSSION

.... Germany / Switzerland eastwards ....

Still under the flat upper-level ridge with weak pressure gradients present so expect scattered to widespread thunderstorm development over those areas as diabatic heating pushes surface temperatures over the essential limit for independent development. Main difference versus past few days is a warm - up in the mid -levels, resulting in weakening mid - level lapse rates also limiting instability release. Forecast parameters still hint on a few areas where mature pulsating storms could produce large hail and strong wind gusts, but confidence is too low to issue a broad level area like yesterday. A marginal level area was issued for SE Poland where forcing by a passing short - wave could create a somewhat better environment for large hail as wind field strengthens ( DLS 15 to 20 m/s ). This short - wave will help to force storms to go on until 06Z over SE Poland / W - Ukraine and N / NW Romania but otherwise thunderstorms will decay rapidly after sunset.

... France ...

Mid - level lapse rates are a bit steeper and DLS stronger with a broad shear axis of 15 - 20 m/s in the lowest 6km. Diurnal driven thunderstorms will be quite widespread as upper-level trough approaches from the west. There is a risk for storms to produce isolated large hail during their mature phase but we do not want to highlight such a broad area with a marginal level - 1. No soundings of yesterday's environment were available so we want to wait for more informations before upgrading parts of the area.


... N - Algeria, N - Tunisia, Sardinia and Corsica ...

Environment becomes favorable for a few supercells with a distinct large hail risk over the area of interest.

As upper - level trough slides eastwards over Spain and Portugal, pressure at the surface falls over N - Algeria / Tunisia and a diffuse surface low evolves somewhere over this area. In addition the left exit region of a strong 50m/s upper -level jet draws near the area of interest during the later afternoon and evening hours. Finally a strong PVA maximum crosses N - Algeria during the evening hours from the WSW moving northeastwards. So abundant forcing will be available for thunderstorm initiation. A more difficult forecast regarding the thermodynamics is in store for the time - frame. As diffuse surface depression strengthens, a strong 20 m/s LLJ evolves over the area, advecting hot and dry desert air towards the north with readings of 15 - 20 °C at 850hPa all the way up to Corsica. This WAA in combination with unchanged mid - levels results in an impressive EML over the area as lapse rates reach 8 - 9K / km. Despite the strong cap, elevated thunderstorms are forecast to evolve at 21Z onwards in an environment with 500 - 800 kJ / m^2 ICAPE and 300 - 500 m^2 / s^2 SRH at lowest 3km. Strong veering is also present above this layer as winds veer from the south at 700 hPa to SW at 500 hPa. Storm coverage will be limited during the evening hours, becoming more widespread during the night hours as main forcing approaches from the SW but each more sustained / discrete thunderstorm will pose a risk for large hail with isolated extreme events possible. We will stick with a broad level - 1 area for the moment as initiation and overall thunderstorm coverage are still uncertain but an upgrade of parts of the level - 1 area may become necessary later - on.

During the night hours, LCLs come down rapidly from averaged 1500 - 2000 m at 21Z to 600 - 1000m at 06Z. LL shear is extreme with LL shear of 15 - 20m/s and SRH 1 of locally more than 250 m^2 / s^2. Uncertainties of exact timing when storms root in the strongly sheared boundary layer and strength of the cap remain and we want to wait for more data before increasing probabilities due to an augmented tornado threat.

... Extreme SE France and NW Italy ...

Main thunderstorm initiation will be between 21 Z and 00 Z as a strong vort. max. crosses the area from the SW. Shear is strong but instability very limited with higher values confined to the coastal areas and offshore. The main arrestive factor will be the uncapped atmosphere during the afternoon hours, which should support scattered showers / thunderstorms all day long creating a mess of decaying convective cells over the area of interest during the evening hours. As atmosphere stays uncapped, widespread re - development expected during the late evening / night hours and hence the chance for more discrete thunderstorms is quite low . Shear at all levels would support a hail / tornado threat but we decided to issue a level - area only for those regions where instability will be the best.

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