Convective Outlook 20.07.2016

deutsche Outlook-Kurzfassung (aufklappen)
CategoricalHeavy RainHailWind GustsTornado

Risk Area (sq. km) Population Larger Cities in Risk Area
20 % 675.883 94.888.203
50 % 433.026 120.283.549
Severe Threat 1 169.910 65.748.556
Severe Threat 2 18.400 8.702.991

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Forecast FAQ

 

A Severe Threat 2 has been issued for e Netherlands and extreme NW Germany mainly for large hail, excessive precipitation and to a lesser degree for severe wind gusts

 

A Severe Threat 1 surrounds the Severe Threat 2 mainly for medium-size to locally large hail, excessive precipitation and severe wind gusts

 

A Severe Threat 1 has been issued for N Spain and S France mainly for large hail

 

Synoptic Setup

A deep vortex south of Iceland remains stationary during the forecast period, while a cut off takes place over Eastern Europe. In-between, ridging at first strengthens from N Africa over Spain to Germany. On Wednesday, the ridge gets gradually narrow, and synoptic forcing from the Atlantic-Vortex can easily encroach on wrn and crtl Europe. Within a subtropical airmass at lower levels, an active day concerning convection is expected primarily over Germany and Be-Ne-Lux.

Mesoscale Discussion

…N-France, Be-Ne-Lux and NW Germany…

The most prominent feature will be a secondary trough/PV anomaly, that pivots from GB to the North Sea on Wednesday. As it interacts with a lower positive theta-anomaly over S England, the PV Thinking teaches, that baroclinic development occurs. The resulting sfc cyclone encourages the LL transport of a moist and hot airmass mainly to N France and the Be-Ne-ux countries. Here, sfc temperatures around 35 °C and mixing rations near 15 g/kg are forecast. The highest moisture will be concentrated in the wake of a moisture flux convergence, that ranges from the the centre of LL cyclone over GB to the south-east, as indicated by a tight and linear theta-e maximum. Within a moderate SWly diffluent mid level flow, an EML is advected over the moist boundary layer, yielding potential instability and ML CAPE of 1000-1500 J/kg. As the WLA caps the instability, CI is suppressed until the early evening. We anticipate the first DMC at the northern part of the convergence somewhere between Antwerpen and the Ruhr. This is by two reasons:

1. The most convenient position of that area relative to the surface low with generally maximized cyclonic stream lines
2. Additional forcing via strong upper lvl divergence at the left exit region of a 300 hPa jet-streak, that stretches from the Pyrenees to W Germany

All in all, there is a substantial overlap of all three ingredients over the highlighted area. Besides this fact, shear conditions are not that favourable with 0-6 km DLS between 15 und 20 m/s, disordered hodographs and only weak pre-storm environmental helicity (0-3 km ~ 100 J/kg).
Hence, we expect mostly multicellular storms, but also a supercell is possible, primarily at the tail end of a developing cluster. Large hail up to 5 cm is the dominant hazard in any well organized storm and will be most likely near the German border. Later on, as storms may cluster into a more ore less organized system, the threat switches to severe wind gusts (delta theta-e around 24 K) and local flash floods. Due to high cloud bases and low LL helicity/shear, the tornado risk is negligible.
During the night, additional storms may form further to the south over E France and Luxembourg and move into SW Germany. One or two severe reports are possible here as well, but overall severe potential will weaken due to lowering instability.

…N-Spain, S-France…

Steep lapse rates but less pronounced LL moisture result in 500-1000 ML CAPE. With orographic lift and 15-20 m/s DLS, one or two well organized, high-based storms may develop along the Pyrenees in the evening. The main risk will be large hail up to 5 cm.

Model consistency, additional remarks

Besides the fact that CI is likely over N France, Be-Ne-Lux and W Germany, there are still uncertainties concerning instability-amount and the spatial-temporal evolution of the convergence. As GFS tends to overestimate the BL moisture, we think that the predicted 2k J/kg are unrealistic. Nevertheless, we introduced a small Severe Threat 2 over the region where we estimate the best overlap of moisture, lapse rates, deeper lift and wind shear and where a supercell with large hail is most likely.

Updated: 8. Januar 2017, 18:04 — 18:04
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