Mesoscale Discussion
Valid: Wed 04 Aug 2021 11:00 to Wed 04 Aug 2021 16:00 UTC
Issued: Wed 04 Aug 2021 10:36
Forecaster: TUSCHY
MESOSCALE DISCUSSION
for another round of excessive rain especially in the Lombardy.
The setup is not unknown for an excessive rainfall threat in this region with a progressive low-amplitude trough crossing S France during the day while approaching/crossing N Italy during the night.
Latest WV imagery highlights this wave with a sharpened WV gradient along the western edge of a well established warm conveyor belt (WCB) with cold cloud tops. Embedded in this WCB is a moist air mass with a PWAT anomaly (from the 79-09 M-climate) of 1-2 sigma and TPWs in excess of 40 mm (sampled nicely by the 00Z Novara Cameri sounding).
Increasing upper divergence overspreads N-Italy during the afternoon and evening with rather intense 300 hPa Q-convergence forecast. Mass response features falling MSLP over NW Italy with parts of the NWP guidance indicating a weak LL depression, whereas the rest stays with a sharp convergence zone. A solid 20-30 kn 1-3 km inflow points normal to the Alps with 0-3 km shear around 15 m/s. Enhanced 6-10 km shear points to available ventillation of convection with a prolonged period of separated updrafts/downdrafts as precipitation is blown downwind.
Expect CI around noon onwards with back-building /training convection. The mesoscale dictates, where training will be most dominant (also modified by the LL depression and its ageostrophic response to the wind field). Very effective rainfall rates with a warm cloud depth in excess of 3 km, strong (orographic) forcing and a moist/unstable inflow are forecast.
NWP guidance indicates 12 h rainfall rates (12-00 UTC) of 90-200 l/sm with some LAMs pointing to even higher rainfall amounts on a local scale (max member in the 300-400 l/sm range). The EPS data maxed out with 90-100% for more than 70 l/sm in 12h.
Satellite based relative soil moisture in this area is in excess of .9 but with sharp gradients in the anomaly field. Some river forecasts indicate a significant response in case the most aggressive QPF verifies.
Expect the flash flooding risk to increase with a few significant events possible.
The risk decreases gradually from W to E between 18-00Z.
Further E a similar but less focused setup causes more training convection but extreme rainfall amounts should stay more isolated.
Note: Please keep in mind that the validity of this MD does not represent the duration of this event. It is just an updated information.