Mesoscale Discussion
Valid: Sat 26 Oct 2024 09:00 to Sat 26 Oct 2024 13:00 UTC
Issued: Sat 26 Oct 2024 09:17
Forecaster: TUSCHY

The following Mesoscale Discussion (MD) was issued to highlight an augmented heavy rainfall risk for parts of Sardinia.

Please keep in mind that the validity of this MD does not match the time-frame of the expected severe risk, which covers the overnight hours.

DISCUSSION

An eastbound moving cold front (09Z placement of the cold front is just a rough estimate) approaches Sardinia during the overnight hours from the W and this front is forecast to stall somewhere over/just E of the island (with IFS-ENS tracker losing this boundary in the island's vicinity).

The front is accompanied by a sharp wind shift, which causes a rather strong LL convergent wind field beneath some broad and modest upper divergence signals. As this front interacts with a plume of rather unstable air with MUCAPE in excess of 1000 J/kg, scattered to widespread CI occurs along that front. Evolving or existing convection get fed by a modest SE-erly inflow of moist/unstable air with an uncontaminated fetch all the way into the Strait of Sicily.

The slow frontal motion, a well aligned background flow to the front itself, weak DLS and 0-3km shear values > 0-6 km values all point to an evovling extensive N-S aligned cluster of convection, which impacts Sardinia from the evening onwards (probably with some prefrontal CI well ahead of the synoptic front and mainly along the orography). The main uncertainty remains, where a persistent V-shaped MCS can evolve and if it roots into the offshore airmass or remains stapled along the orography.

Both, deterministic runs and ensemble data show augmented probabilities for heavy to excessive rainfall amounts with local peaks well in excess of 100 l/qm in a short time-frame. Similar to past events (with some of them having a gradient of more than 400 l/qm within 30 km), it is not possible to pinpoint any area with highest likelyhood for extreme rainfall amounts. We highlighted the area of most concern (hatched and blue) but there are some hints that the SW part of the island could see the highest amounts in case of a prefrontal convective line awaiting the cold front passage, while producing high rainfall rates.

Flash flooding is possible with this activity.