Severe thunderstorms are possible from Timmins to Edmundston between Friday and Monday.

Multiple shortwave troughs will rotate through Ontario and Quebec during the four-day period, slicing into a very hot and humid air mass. Many areas will make another run at 30 C with dewpoints well into the 20s, resulting in extreme CAPE values that are modelled to be in the 3000-4000+ j/kg range, even on the conservative GGEM (12Z run, Sunday).

With such high values of instability, it would not take much bulk shear to produce widespread severe weather; 20 kt of 0-6 km bulk shear could easily create lots of wind/marginal hail reports when combined with mid-level dryness. In this case, bulk shear should be about 30-40 kt, and potentially 50 kt on Friday in southern Ontario, which will support organized convection, potentially supercellular as winds may back to the SE in many locations during this event.


Here is a simulated reflectivity image from the 18Z 3K NAM for 5 pm Friday.

Sunday has caught my eye as well, with a sub-1000 mb GLC passing through ON/QC.ecmwfUS_850_temp_096


EHI values over 4 combined with 3000 j/kg of CAPE indicates 0-3 km helicity values of at least 200 m2/s2, indicating favourable parameters for supercells.

The only thing that could affect Sunday’s storms is the remnants of Saturday’s convection overturning the atmosphere. With a lack of capping in addition to PWATs approaching 50 mm, lots of storms with torrential rain will quickly form.

Here are projected areas of severe thunderstorm coverage by day:

Friday: southern and northeastern Ontario, as well as far western Quebec. Non-severe elevated storms are possible in southern Quebec and eastern Ontario.

Saturday: eastern Ontario and southern Quebec

Sunday: eastern Ontario and southern Quebec

Monday: southeastern Quebec and New Brunswick