The wettest system of the year so far is on the doorstep for southern Ontario, with as much as 100 mm of rain threatening some areas through Thursday. After another humid day, with the feels-like factor hovering near 30, the rain will begin to push into the region later Tuesday, picking up in intensity through the overnight and persisting through the day on Wednesday. Widespread totals of 75+ mm are likely, though with even heavier amounts expected in areas that see embedded thunderstorms. This will elevate the threat for flash and localized flooding in parts of southern Ontario through Thursday. More on the timing and impacts, below.
Visit our Complete Guide to Fall 2021 for an in-depth look at the Fall Forecast, tips to plan for it and much more!
TUESDAY THROUGH THURSDAY: WARNINGS IN EFFECT FOR WIDESPREAD HEAVY RAINFALL, ELEVATED FLOOD THREAT
After days of rain and severe thunderstorm chances on Monday, the rainfall will continue moving east across northern Ontario through the day Tuesday, with the chance of embedded thunderstorms in the afternoon once more.
In the south, early morning rain showers will give way to peeks of sun throughout the day, with most of it remaining rain free. But as a low-pressure system begins to trek further east, showers will creep their way into the Georgian Bay Area late afternoon and then move into the rest of southern Ontario in the evening hours.
Rainfall will pick up in intensity Tuesday overnight and will persist through Wednesday, offering no chance to dry out before the next system moves in early Thursday morning.
Rainfall warnings and special weather statements cover most of the south, warning of widespread heavy totals through early Thursday, with the highest amounts expected northwest and west of the Greater Toronto Area.
Locally, some areas may even end up seeing more than 100 mm of rainfall due to embedded thunderstorms, heightening the risk for flash and localized flooding where the heaviest rain occurs. Motorists should exercise caution as water pooling should be expected.
"Heavy downpours can cause flash floods and water pooling on roads," says Environment and Climate Change Canada in the warning. "If visibility is reduced while driving, turn on your lights and maintain a safe following distance. Localized flooding in low-lying areas is possible."
In some cases, rainfall amounts may near or exceed normal monthly values for September and could even top totals received from the entire year so far in just a couple of days.
The second low-pressure system will trek in from stateside in the pre-dawn hours Thursday morning, bringing more rainfall to what will already be a soaked region. Precipitation will get heavier and move north through the morning hours.
The heavy rain will subside by Thursday evening, with just the chance of lingering showers on Friday morning.
LATE WEEK: COOLER AUTUMNAL WEATHER TAKES HOLD
Temperatures will tumble to more seasonal values in behind the systems, remaining cooler through the weekend with a mix of sun and cloud.
A seasonably cool pattern is expected to continue for the final days of September, with temperatures even a few degrees below seasonal for a couple of days, but warmer weather (relative to normal) is likely to return during the first week of October. Keep in mind however, that "seasonal" values continue to steadily fall.
Be sure to check back for the latest updates on this slow-moving system that kicks off the fall season in Ontario.