Valid: Mon 10 Jun 2019 06:00 to Tue 11 Jun 2019 06:00 UTC
Issued: Mon 10 Jun 2019 04:10
Forecaster: VAN DER VELDE
A level 2 was issued across N Italy, Switzerland, Germany, NW Poland mainly for very large hail, excessive rainfall, severe wind gusts, and tornadoes.
A level 1 surrounds the level 2 mainly for similar hazards with lower probabilities.
A level 1 was issued for Belgium, Netherlands, NW Germany and E UK mainly for isolated excessive rainfall.
A level 1 was issued for N Turkey mainly for large hail and severe wind gusts.
A level 1 was issued for NE Algeria and N Tunesia mainly for severe wind gusts.
A potent mid-level low is centered on the English Channel region with strong southwesterly flow extending across SW to N-CNTRL Europe, and along with it a streak of the Saharan Air Layer with steep lapse rates. At the surface level, the low has three cores, one Genua, an eastern Germany and a Benelux core. Combined with the upper flow, strong vertical shear (15-20 m/s 0-6 km and effective shear) and storm-relative helicity (200-500 m2/s2, in most places 250-350) occur where easterly backed winds occur on the north and west sides of these cores.
Models indicate MLCAPE values over 2000 J/kg in N Italy (Po Valley) and across the area N Austria, Czechia, NE Germany, W Poland. These maxima are capped and need erosion by lifting mechanisms. On the south side of the Alps the upslope flow is already triggering (at night) persistent convective storms with a serious rainfall and hail size threat. This is expected to continue most of the day and the following night.
Around noon, a cold front in the border region of France and Germany will likely be associated with a rainy area or weak convection. GFS model predicts strong upper divergence over it, and all the way into Belgium/Netherlands. From about 18Z, a solid deep convergence and large scale uncapping takes place from N Switzerland to C and NE Germany. Isolated storms may occur over NE Germany and NW Poland in advance.
There is a high potential for supercell development, with (very) large hail and severe wind gusts. The best curved low-level forecast hodographs are found in the NE 1/3rd of Germany, where tornado potential as a result is largest. This region in the center of the level 2 will likely see at least a few significant severe weather events (especially hail >5 cm in diameter).
Storm cluster movement (Corfidi vectors) is N to NNW-ward, with inflow coming perpendicular to the mean wind. This can produce line segments and cells pulling eastward into better feeding conditions while also persistent cell/line training is possible resulting in high local rainfall amounts. The linear mode may not be dominant since forcing is not strong.
The low produces a wave in the front, moving over NE Germany toward Denmark and Sweden at night. The hodographs become smaller and storms may stagnate and the dominant severe weather should be excessive rainfall. Warm frontal storms over S Sweden may also produce large hail at night.
Benelux area has decent shear and weakish CAPE, but hail growth may be impeded by poor mid level lapse rates. Strong low level moisture transport vectors to the north of the low into convergence zones predicted by GFS roughly over SW Netherlands with weak mid level flow could produce rather stationary cells with high rain output, also low level curved hodographs and stronger upper flow create possibilities for tornadoes.
Dry high-based storms in an environment of moderate to strong shear may produce severe wind gusts and perhaps large hail.