Scattered thunderstorms are expected today in S ON and extreme S QC, with some strong microbursts and heavy rainfall rates possible. But, that will be nothing compared to tomorrow’s threat.

A deepening GLC will move through the central GLs tomorrow. In its warm sector, substantial surface heating will occur, resulting in widespread CAPEs over 2000 j/kg and over 3000 j/kg locally. 0-6 km bulk shear will be higher than during the rest of the week, anywhere between 25 and 40 kt. Values of around 15 kt have yielded severe wind and hail several times this week so we’ll see what happens this time around.

Although convection earlier in the day will primarily affect SW and SC ON, the best combination of CAPE and shear will exist in extreme E ON and S QC where scattered multicell clusters are expected to develop in that area in the late afternoon. Veering winds will result in the potential for brief supercellular structures.

Whilst those storms affect the eastern sections of the severe threat zone, a squall line will develop over S ON throughout the day, reaching favourable parameters in E ON early in the evening hours; the dynamics of the line will result in H85 winds over 50 kt near it, as well as 0-3 km helicity values over 200 m2/s2 region-wide (up to 400 m2/s2 in the vicinity of the line), which may cause some bookend vortices to break out along the MCS, which could produce an isolated tornado.

Bow echoes are also possible along the line, and widespread damaging winds gusts are likely considering that the 50-60 kt low-level winds will be the only alteration to an environment that has produced up to 110 km/h wind gusts this week, with much weaker wind fields.

The SSW to NNE movement of the line will result in training and rainfall amounts over 50 mm in some areas.


Updraft helicity fields from the 06Z 3K NAM. The green/yellow along the eastern Ottawa and St. Lawrence rivers is from afternoon convection whilst the rest in ON is from the squall line.

The squall line should hold its own somewhat throughout the overnight hours due to residual elevated CAPE and strong low-level winds, and continue to produce damaging wind gusts. The remnants could fire up again on Saturday morning in S QC and produce additional severe weather if destabilization occurs. However, there is considerably uncertainty regarding that threat.

However, even the timing of the squall line is uncertain, as there are many solutions that have the initial MCS in SW ON at 8 am hold together throughout the day, then hit S QC late in the afternoon or early in the evening hours. The premise that the squall line hits S QC in the morning on Saturday is based on that MCS weakening and moving north into the nickel belt and SW QC, and a new MCS forming near Toronto in the afternoon hours.