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.

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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

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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

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