Storm Forecast
Valid: Sun 09 Jul 2023 06:00 to Mon 10 Jul 2023 06:00 UTC
Issued: Sat 08 Jul 2023 22:45
Forecaster: GROENEMEIJER

A level 2 was issued across the eastern Benelux and Northwest Germany for severe wind gusts, large hail, and excessive rainfall.

A level 1 was issued across an area surrounding the level 2 including a large portion of France, Germany, and northern Switzerland for severe wind gusts, large hail, and excessive rainfall.

A level 1 was issued across part of the southern Alps mainly for large hail.

A level 2 was issued across Georgia and northern Turkey for excessive rainfall and isolated tornadoes

A level 1 surrounding the level 2 including parts of Russia, Armenia, Iran, and Ukraine was issued mainly for heavy rainfall, and across a wide coastal zone of the Black Sea for tornadoes.

A level 1 was issued for tornadoes and heavy rainfall.

SYNOPSIS

A cyclonically curved jet is gradually approaching western Europe from the west. Forcing for upward motion is expected to trigger storms in a moderately sheared environment in which substantial surface-based instability should build during the day setting the stage for severe storms.

Near the top of a ridge in the flow around a Saharan elevated mixed layer, isolated hailstorms may form over the Alps.

A deep slow-moving mid-level trough is located over the Black Sea and is associated with plentiful precipitation and thunderstorms in the region, especially near the low-level low and where low-level flow is upslope.

Downstream of it, across parts of Russia, a stagnant warm front is associated with fairly strong low-level shear and slow-moving and training storms potentially leading to flash floods and possibly one or two tornadoes.

DISCUSSION

France, Benelux, Germany, Switzerland...

Model guidance suggests that an elongated zone of convergent winds, related to a cold front, will stretch from the northern Netherlands to southwestern France. Substantial MLCAPE on the order of 1500-2000 J/kg will build in response to the diurnal heating of the most low-level air mass near this boundary. There, approximately 20 m/s of deep-layer shear will be present, suggesting that storms will be well-organized.

In particular, across the level 2 area, quite high storm coverage is simulated. The severe risk will be large (2-5 cm) hail with any more isolated storms, but most convection-allowing models do simulate a quick transition into a linear mode, which would elevate the wind risk. These models suggest the highest risk of wind gusts will develop in case a bow echo propagating north-northeastward along the boundary materializes. However, this does not seem to be the most likely scenario. Regardless of the scenario, scattered severe wind reports are likely, but in case of such a bow echo wind speeds may become extreme and exceed 32 m/s.

Overnight the storms should gradually diminish in intensity and move eastward across Germany and the North Sea into Denmark.

Southern Alps...

Across the southern Alpine flanks, Alpine pumping will create an upslope flow of moist and potentially buoyant air that may be lifted to its level of free convection across the mountains. Given adequate CAPE of ~ 1500 J/kg and moderate 15 m/s deep layer shear, isolated severe storms may form with a primary threat of large hail.

Turkey, Georgia, Russia...

The continued upslope flow of an unstable air mass off the Black Sea onto the northern flank of the Anatolian plateau will ensure the continued development of showers and thunderstorms, leading to locally hazardous rainfall accumulations of 50 - 100 mm in some places, and very locally even higher, in 24 hours. This may lead to flash floods. A similar situation exists across Georgia where upslope flow onto the Caucasus is expected. In the vicinity of an intensifying surface low, high low-level vorticity instability and very low cloud bases, suggest that isolated tornadoes and landfilling waterspouts are possible.

Russia...

Across the northern level 1 area, a stationary warm front is located with continuous convective activity forecast by NWP models. The models' vertical profiles in the region are very humid suggesting a rainfall risk. Moreover, low-level winds are sheared and 100-150 m2/s2 of 0-1 km storm-relative helicity are forecast by the models overlapping with a modest CAPE of ~500 J/kg. Therefore, a few tornadoes could occur.