Ook forecaster Oscar ziet de CAP als (te) sterk en noemt ook (zie ook de KNMI Guidance v.w.b. het Harmonie-verhaal) 40°C nodig om tot convectie vanaf de grond te komen. Wel is er kans op buien met een hogere basis:
DISCUSSION
...NW France, Belgium, Netherlands, N Germany...
The main dynamics seem associated with warm air advection into a Biscay low coming to the English channel in the evening. Another thermal trough moves from the Netherlands in the early afternoon to N Germany/S Denmark but is not expected to do anyhing convective-wise.
The assessment of instability needs to take into account the differences in low-level mixing ratio between models. Since the airmass is of Saharan/Spanish origin, it is very dry and hot at 1 km altitude and the question is whether mixing ratio can get large enough for a parcel to become positively buoyant. Animation of the 0-1000m mixing ratio reveals an unrealistically strong afternoon increase in present GFS model runs (at least 2 g/kg between 09Z and 15Z). The 09Z mixing ratio looks normal can be extrapolated. So, 12-14 g/kg should be the corrected GFS indication. This agrees with the maximum areas in Hirlam and also with the 00Z Bordeaux, Brest and Santander soundings (13 g/kg in the BL). The problem is that not only this moisture is advected to the north, but also the hot layer above which serves as a cap unless 40°C is reached, or the cap eroded by layer lifting. However, lifting processes are forecast to be weak.
So, as observations as well as models indicate, LFC-LCL differences or CIN remain too high to allow surface-based convective storms, although an isolated elevated convective storm is not ruled out. While the afternoon and early evening may remain rather quiescent, there is now quite a good number of models supporting a late evening and night (21-06Z) thunder episode mainly over the Netherlands. (GFS, ECMWF, diverse WRF, Hirlam 18Z produces at 03-06Z).
The question is whether the 20 m/s DLS will be effective at night, Probably not, and then some 5-10 m/s must be subtracted. It will remain sufficient for organized multicells with severe gusts as the main threat, with large hail as secondary threat. There isn't particularly strong background dynamics going on so the main drive would be strong local cold pools/downdrafts caused by low mid-level theta-e and high LCLs (2000-3000m) - although GFS delta-theta-e (20-28°) is inflated due to the unrealistic low level moisture problem. Still 16-20° is probable. Tornadoes do not seem likely if convection remains elevated, and onion-shaped skew-t profiles (prog soundings) are too dry. Even though Hirlam (more than GFS) produces spots with more than 10 m/s 0-1 km shear.
*edit* inleiding toegevoegd.
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