25 to locally 50 mm of rain is possible today and tonight through southern Quebec (excluding the Eastern Townships) and eastern Ontario. The heaviest rainfall totals are expected just south and west of Montreal, where Rigaud and Ile-Bizard, communities experiencing some of the greatest struggles from flooding today, are located.
Then on Saturday, skies will clear in E ON and S QC as a mid-level dry slot enters the region, allowing temperatures to reach 20 C in many areas. However, low-level moisture will remain present and the atmosphere will destabilize to the tune of over 1000 j/kg SBCAPE in the Ottawa and Montreal regions in the afternoon hours.
Severe thunderstorms may develop in the corridor between Ottawa and Montreal late in the afternoon and early in the evening hours, producing damaging winds (dry mid-level air and mid-level winds of 50-80 kt plus good low-level lapse rates), large hail (good amounts of CAPE, low PWATs and low WBZs). Discrete supercells seem to be the preferred storm mode, which would introduce the risk of a tornado.
However, torrential rainfall will be the greatest threat, particularly considering the fragile situation in the area expected to receive the storms. At least 30 mm is expected in additional rain with the storms, and some models have over 50 falling due to training as more than one round of storms impacts a given area on them.
This updraft helicity field map points out where the strongest storms will likely form:
Rain will continue on Sunday, albeit light with only a few millimetres expected across the region.