Mesoscale Discussion
Valid: Sat 31 Jul 2021 21:00 to Sun 01 Aug 2021 00:00 UTC
Issued: Sat 31 Jul 2021 19:30
Forecaster: TUSCHY

MESOSCALE DISCUSSION

for updating the evolving nocturnal severe weather event along/just S of the Alps.

Latest WV loop shows a potent vorticity maximum/enhanced moisture gradient at mid/upper levels, which now emerges from E Spain and races NE towards NW Italy. In the meantime, a weaker lead impulse evolves ahead of the main forcing on a slightly polward displaced track over the Alps. The cap, which confined CI mainly along the mountains, should weaken beyond 00Z, as stronger height falls and decreasing H7/H85 temperatures spread E.

Until 00Z, isolated CI over N-Italy into SE Austria (apart from the mountains) is still forecast in the capped airmass - which occurred already during the past few hours on a local scale. Further discrete CI on a isolated to scattered scale is forecast between 00-06Z with multicells/supercells possible - especially within the level 3 area. It has to be seen how many updrafts can mature in the obviously more hostile environment than anticipated yesterday. The main risk remains (very) large hail, severe wind gusts and heavy rain. A few tornadoes are possible e.g. over SE Austria with favorable LL shear/low LCLs.

The next focus is a potential serious flash flood threat for parts of the Lombardy and surrounding areas. With the approaching wave from the SW, strong WAA atop a moist BL inflow from the S/SE assist in a favorable setup for training convection. Strongly curved hodographs with 15-25 m/s 0-3 km shear, a 20kt southerly jet increasing to 35kt at 700hPa with the shear vector pointing normal to the orography and the prolonged period of lift all point to a risk for very effective/excessive rainfall rates/amounts. In addition, CAPE/shear space still supports an ongoing/embedded severe risk with hail and an isolated tornado event.
Some EPS members still show 6h rainfall rates in excess of 150 l/qm, which is certainly possible. Flash flooding is likely in such an environment.