It appears that the greatest severe weather potential today will be in southern Ontario and southwestern Quebec where a squall line may produce damaging winds and isolated large hail late this afternoon and this evening. Scattered thunderstorms may also form between Cornwall and Montreal early this afternoon, which may produce over 25 mm of rain due to slow storm motion.

The likelihood of a major severe thunderstorm event on Sunday is increasing in eastern Ontario and southern Quebec. Surface winds are still expected to back to the south and southeast in southern Quebec, resulting in sufficient helicity for supercell development in addition to over 35 kt of effective bulk shear. CAPE over 3500 j/kg combined with DCAPE over 1000 j/kg, 45 kt low-level winds, and good low-level lapse rates will result in conditions conducive to downbursts. Low LCL heights and STP values over 2 locally may result in isolated tornadic development. Stronger cores could also produce large hail.


06Z NAM 3K simulated radar for 5 pm Sunday.

After sunset, CAPE will inevitably decrease but PWATs will increase to over 2″ in a saturated sub-freezing layer. Low-level winds will weaken resulting in slow storm motion and the potential for storms with very frequent lightning to produce torrential downpours with over 50 mm possible.

Linear convection moving from the Eastern Townships of Quebec to western New Brunswick will occur on Monday. Faster, unidirectional low-level winds will prevent training and flooding from occurring in those regions. Damaging winds and large hail is possible.