Convective Outlook 23.06.2016

deutsche Outlook-Kurzfassung (aufklappen)
CategoricalHeavy RainHailWind GustsTornado

Risk Area (sq. km) Population Larger Cities in Risk Area
20 % 661.260 93.117.157
50 % 343.873 90.318.295
Severe Threat 1 337.988 84.496.076
Severe Threat 2 111.647 35.799.168

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A Severe Threat 2 has been issued for N France, parts of Be-Ne-Lux and NW Germany mainly for large to very large hail, severe wind gusts (isolated damaging wind gusts possible), excessive precipitation and a tornado event


A Severe Threat 1 surrounds the Severe Threat 2 mainly for large hail, severe wind gusts and excessive precipitation


A Severe Threat 1 has been issued for the western Alpine region mainly for large hail


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


Synoptic Setup

An amplifying upper trough over the E Atlantic will be the prominent feature during the forecast period. On its eastern fringe, southerly winds support the influx of warm subtropical air masses towards West and Mid Europe. In the lower troposphere, homogeneous pressure fields with weak gradients prevail across the forecast region. The anabatic sfc cold front, that corresponds to the trough, is presently situated west to the the European coastline and will slowly be displaced to east. Prefrontal pressure falls may induce one or several convergence lines within the subtropical air mass. Regarding severe weather, a potentially dangerous set-up will be unfolding over Be-Ne-Lux and W Germany.

Mesoscale Discussion

…N France, Be-Ne-Lux, NW Germany…

Current remote sensing and sfc data represent plenty of DMC popping up from Asturias to the Channel. A supercell could also be reported over N France a few hours ago. This convection shifts further to the North and Northeast influencing primarily W France and S England in the night hours. Although most storms will become elevated, one or two well organized cells with severe reports cannot be ruled out.
Tomorrow morning the coastal regions may still see remnants from the overnight convection and THR-cloudiness, that initially could suppress insolation. To the east model-guidance only expects a few clouds so that diurnal heating can easily push 2m-temperatures to the thirties.
Within the WAA regime, a spanish-plume, that emanates from the Spanish highlands, is advected to the area of interest. As a result of abundant rainfalls during the last weeks, soils are nearly saturated as indicated by meteoam. With additional moisture advection, mixing ratios may frequently exceed 15 g/kg. With a pronounced EML around 700 hPa (500 hPa lapse rates around 7 K), a loaded gun situation is forecast to evolve from Be-Ne-Lux to NW Germany providing ML CAPE between 1500 and 2000 J/kg with peaks up to 2500 J/kg.
In the wake of a Mid-Level Jet ,1-6 km DLS of 20-25 m/s is superimposed with the highest instability, yielding conductive conditions for well organized convection. With quite linear hodographs SRH reaches 100-200 J/kg and locally higher values near the coastline. In contrast to energetics and kinematics, there are still greater uncertainties concerning the forcing mechanism. As the streamlines at upr levels are very linear/uncurved and the main Jetstreak remains over GB, there is no substantial QG Forcing, that could overcome the WAA-induced capping inversion. Nevertheless, model consensus anticipates CI in the afternoon over Belgium/Netherlands in the vicinity of the prefrontal sfc low. We are still somewhat sceptic, as the today’s CI and severeness over N France was overestimated by WRF and EURO4.
Moreover, overnight convection should be monitored as it may care for a different overall set-up development (outflow boundaries and aforementioned cloudiness). If BL-rooted CI takes place as simulated by the models, convection will rapidly become multi and supercellular bearing a risk of large to very large hail, excessive precipitation and severe wind gusts. EURO 4 and WRF 4 km both indicate clustering and an evolution of a cold-pool driven MCS with bowing segments. Such a system would move into NW Germany somewhere between the Lower Rhine and Eastern Frisia in the evening and live on in the night hours (MU CAPE > 1500 J/kg). The development of strong cold pools seems to be realistic due to high Delta-Theta-E > 20 K. In such a system, the main threat switches to severe and damaging wind gusts > 32 m/s and locally significant severe precipitation. Within a small temporal window between LCL lowering and BL stabilization, a tornado event is not unlikely especially along the coastline where LL vorticity may locally be concentrated. We decided to go with a broad and solid severe threat 2, keeping in mind the high severe potential but also the uncertainties of the set-up. We will update the Outlook or publish a Mesoscale Discussion if necessary.

…Wrn Alpine Region, S France…

CAPE between 1000 and 1500 J/kg (higher values near the Alps) and orographic lifting will result in isolated CI in the late afternoon. The main threat will be isolated large hail, especially in S France where a dry Foehn layer is present in mid levels. The latter may on the other hand inhibit convection, but we issued a severe threat 1 to point out the enhanced severe potential due to hail. In general it can be stated, that CI near the Alps/Black forest is more likely than in S France.

Model consistency, additional remarks

—see text—

Updated: 25. Juni 2016, 13:44 — 13:44
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