This is not a week to spend too much time outside in southern Ontario. High pressure over the region will keep heat and humidity at dangerous levels for most of the week, with daytime highs above 30°C and feeling more like 40 with the humidex. That’s fuelling a fair bit of severe storm risk Monday and Tuesday across almost all of the south, with the humidity in particular setting up any potential storms to feature very heavy local downpours, possibly even triggering localized flooding. All the details on these dog days of summer, below.
MONDAY: HOT AND HUMID, WIDESPREAD SEVERE STORM THREAT
The gradual rise in heat and humidity culminates in this multi-day extreme heat event, with Monday roaring out the gate, temperature-wise.
Widespread daytime highs in the lower 30s will feel much hotter, with the feels-like factor pushing 40 in many areas due to the oppressive humidity engulfing the province.
Heat and humidity are important factors in thunderstorm formation, and with both abundant Monday, storm risk is widespread across much of the region except for a small slice of eastern Ontario.
There’s a widespread chance for storms to cross the severe threshold, in which case large hail, strong winds and heavy local downpours should be expected.
Such local deluges will be substantial. The humidity that’s keeping the humidex at or near 40 for much of this week has packed the air with loads of precipitable water, acting as a reservoir of moisture for thunderstorms to trigger localized flooding. This will especially be a risk in slow-moving storms, which will discharge that prodigious moisture over a given area for a longer period of time.