Storm Forecast
Valid: Wed 01 Aug 2007 06:00 to Thu 02 Aug 2007 06:00 UTC
Issued: Wed 01 Aug 2007 08:28
Forecaster: GATZEN

SYNOPSIS

Central Europe is dominated by unseasonable cold air mass in the range of a weakening trough that is connected to a strong low over northern Scandinavia. During the period, a new strong trough moves into northwest Europe and becomes the main feature at the 500 hPa level. At its eastern flank, the Scandinavian trough propagates northward and weakens while loosing the connection to the central European trough that is expected to be centered as a small upper trough over Belarus/Ukraine by the end of the period. Over west Europe, a cut off low now over southern Bay of Biscay merges to the northwestern trough and accelerates northeastward. It will cross northwest Europe during the period. Ahead of this feature, a tongue of warm air mass will spread northward into France, where highest potential of severe convection is expected.

DISCUSSION

Southeast France, Benelux, southwest Germany

Focus of expected convection is the eastern flank of the cut-off low now located over western Bay of Biscay. A weak surface low has developed east of the cut off over western Bay of Biscay that is expected to move into Benelux until Thursday, 6Z. At the 850 hPa level, a strong southwesterly flow will advect warm air mass originating from the Iberian highlands into France during the day. Latest Santander ascend already indicates an impressive elevated mixed layer reaching up to the 550 hPa level. The EML will likely spread across southeast France during the day. Below the 850 hPa level, cold and relatively dry air mass is situated over most of France. Best boundary layer moisture is located at the Mediterranean Sea that has been advected westward into extremely southern France, where low-level dewpoints have reached 17°C locally in the range of the low pressure center.

Potential of thunderstorms will strongly depend on the evolution of low-level moisture today. Given latest observations, it is hard to believe latest GFS CAPE calculations with CAPE up to 1500 J/kg and low-level dewpoints around 15-20°C. However, GFS seems to be right that instability will be restricted to a small area along the low-level convergence from the Pyrenees into central France, as low-level moisture pooling has started in these regions already. Expect that low-level moisture will increase significantly, and given clear weather and strong diurnal heating, CAPE is also not unlikely to develop aloft of a strong capping inversion. Atop of this inversion, advection of very warm air mass will reach its maximum just ahead of approaching cold front south of the low that moves into central France in the evening hours. Best instability is therefore forecast in the evening hours ahead of the surface convergence.

QG forcing will be limited during the evening hours in the range of the surface convergence, as strong upper jet streak ahead of the low moves north-northeast over western France. The anticyclonic exit will spread across southeast France and may limit QG forcing. Nevertheless, GFS predicts upward vertical velocity due to some DCVA as the vort-max of the low moves eastward.

Current thinking is that thunderstorms will likely form in the evening hours along the surface convergence over southern France. Given strong low-level inversion, surface-based storms seem to be unlikely over most places, with the best potential along the surface convergence, where low-level moisture flux convergence is maximized. Thunderstorms that root to the boundary-layer will likely become supercells given favorable veering profiles. Elevated storms will also have a relatively high potential to develop mesocyclones given strong DLS and some veering in mid-levels. Large hail will likely be the most significant threat, but severe wind gusts are also possible with the stronger storms.

Thunderstorms may likely merge into a MCS given strong southerly inflow ahead of the cold front and persisting QG forcing. However, low-level instability is expected to weak after sunset, and surface-based convection seem to become more and more unlikely. A line of elevated convection is expected to move into Belgium and southwest Germany on Thursday morning. A slight chance for marginally severe hail is not ruled out with the stronger storms.

Northwest France

At the cyclonic flank of mentioned strong mid-level jet streak, QG forcing due to strong DCVA is expected. Additionally, some WAA is possible north of approaching surface low pressure system. Models show that a tongue of warm air mass will spread westward over northern France. At low levels, dew points will likely increase in the evening hours. Uncertainty exists about mid-level lapse rates, but is seems that nearly moist adiabatic profiles are possible at least in low levels, where some instability may develop. Given strong vertical wind shear due to weak surface winds and rather strong southwesterly winds at the 850 hPa level, tornadoes may occur with the stronger storms in the evening hours. However, given great uncertainties of thermodynamic environment, an update may be needed in the afternoon hours today.

Northwest Turkey

Axis of east European trough moves across western Turkey in the evening hours. Latest Istanbul sounding indicates a well-developed elevated mixed layer and rich boundary-layer moisture. Today, increasing northerly winds will lead to some cold air advection, but current thinking is that low-level moisture will not decrease significantly. Upslope flow may be strong enough to break the cap in the range of the trough axis in the evening hours. Thunderstorms are expected to develop. Given strong DLS in the range of propagating upper jet streak at the southern tip of the trough, storms may organize with large hail the most significant threat. In the night hours, convection will likely weaken as low-level moisture decreases to the east.