Storm Forecast
Valid: Wed 20 May 2009 06:00 to Thu 21 May 2009 06:00 UTC
Issued: Tue 19 May 2009 18:30
Forecaster: TUSCHY

A level 2 was issued for west central France mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for parts of France, Belgium, Luxembourg and extreme west Germany mainly for large hail and severe wind gusts.

A level 1 was issued for west Turkey mainly for large hail and severe wind gusts.

SYNOPSIS

A positive tilted upper trough just west of Europe remains stationary for the forecast and strong ridging persists downstream over central Europe. The cut-off over the far eastern Mediterranean continues to move eastwards whereas cold and stable air advection is still underway over far NE-Europe.

A SW-NE aligned surface boundary over France is the focus for convective initiation and this front is forecast to shift slowly to the north during the evening/night hours. Otherwise there are no clear frontal signals over the rest of Europe which could serve as foci for initiation.

DISCUSSION

... France...

Another set-up with missing mid-/upper forcing ahead. A vigorous WSW flow covers France during the forecast period with weak wind maxima embedded. However the position and strength of all those jet maxima differs significantly in the model outputs. Otherwise, various impulses cross France from the SW during the day but the overall strength is not that impressive. However, the liability to more widespread initiation increases from the west during the late night hours, as an upper trough over UK draws near.

At the surface the main focus continues to be a convergence zone, which starts to move northwards as a warm front during the day and past model runs brought this front all the way up to N-CNTRL France until 06Z. A cool and more stable stratified airmass is present north of that boundary, while increasing moisture and steepening lapse rates over S-France create a quite unstable environment just south of the front. A check of the latest synop data illustrates that GFS has a good handling on the current boundary layer moisture with ECMWF running a bit too low. However, we challenge the rapid increase of surface dewpoints to roughly 20°C of GFS along the boundary and we therefore went with a compromise of both models. Indeed, a concentrated swath of enhanced boundary layer moisture could evolve just along the convergence zone with maximized moisture advection from the south. Areas too far to the south, e.g. coastal areas of S-France, stay capped with temperatures at 850hPa increasing to 16-17°C during the day. An EML fans out over S-France but keeps mid-level lapse rates steep enough during the whole forecast period. Combined with the aforementioned boundary layer quality, widespread 500-1000 J/kg MLCAPE are reasonable to forecast with locally significant higher values just along the front.

The environment remains strongly sheared with 20-30m/s 0-6km bulk shear, but also quite impressive 15-20m/s 0-3km shear. Shear at lower levels gets maximized along the warm front with intense directional shear (SRH-1km 200-300m^2/s^2 and SRH-3km around 500m^2/s^2).

Timing of initiation will be late and we may see a delay until sunset. GFS/WRF are quite optimistic that initiation occurs over central France and then northeastwards whereas ECMWF indicates development over W-CNTRL France and then has the initition all the way to E-France until 06Z. Right now we lean more to the ECMWF solution as better forcing and boundary layer moisture indeed create a more supportive environment over W-France. Nevertheless, initiation is possible all the way up to Belgium with WRF moving a cluster all the way to NW-Germany. Thunderstorms over W and CNTRL France are able to produce all facets of severe weather, including very large hail, severe wind gusts and an isolated tornado. Thunderstorms over NE-France tend to be more elevated with the main hazard being large hail and severe wind gusts. If a cluster of storms manages to evolve over NE-France, front-parallel wind field with 20m/s at lowest 3km could promote an organized squalline/bow echo with a more widespread severe wind gust risk.

A level-2 is issued for those areas, where confidence is high regarding initiation. The level-1 was expanded well towards the E/NE and may be extended even more towards the east, if thunderstorms over central/NE-France develop earlier than currently anticipated. In addition, parts of the level-1 may see an upgrade to a broader level-2 region.

...W-Turkey ...

Departing upper low still affects most parts of Turkey. Winds at all levels constantly decrease during the forecast period but remain strong enough for organized thunderstorms (e.g. DLS around 15m/s and augmented directional shear). Scattered thunderstorms are forecast with a large hail/severe wind gust risk as subcloud layer is well mixed with LCLs between 1-2km. Thunderstorms continue well into the night hours.

... Rest of the general thunderstorm areas....

Diurnal driven thunderstorms develop but either shear or instability or both are too marginal for organized thunderstorms. Those storms pose a marginal hail/strong wind gust risk although an isolated large hail report is possible over the Balkans. Storms decrease in intensity and coverage after sunset.

Thunderstorms over central UK have a short time-frame during the afternoon hours, where isolated deep convection is possible. 20m/s DLS and an augmented wind field at 850hPa could result in strong to severe wind gust and marginal hail but expected thunderstorm coverage is too low for any level area.

Thunderstorms over extreme SE-Sweden take profit of moderate CAPE and 20m/s DLS and isolated large hail/strong wind gusts are possible during the daytime hours. Again, coverage of storms ought to be too low for a small level-1.