Storms are now starting to fire over SW Ontario and should continue to intensify as the air mass out ahead of the storms de-stabilizes.
The graphic below shows where the best chance of potentially damaging (straight-line winds/small hail) storms will be into this evening. Thunderstorms will weaken late this evening and the threat for severe thunderstorms will be done for the overnight period. Tornado threat is low, but you can never totally rule it out with this type of setup. Also, there was a possible tornado yesterday in the Kawartha Lakes area.
Major cool down then a storm
The leading edge of much cooler air will track southeastward over the next day or two then stall out as low pressure forms near Long Island. This storm will drift northeastward into the weekend as a blocking high pressure area out in the Atlantic causes the storm to almost stall for a time. This situation could lead to significant rainfall (25-75 mm) through the weekend over New England and perhaps into parts of the Maritimes as the humid air off the Atlantic overruns the much cooler air coming down from the north.
Heavy rainfall and flooding potential for parts of Alberta Thursday into Friday
The slow-moving upper-low that has been sitting over the Pacific Northwest will drift eastward tomorrow and Friday. The counter-clockwise flow around the upper low will cause a moist, upslope flow into southwestern Alberta, leading to significant rainfall. The cold air aloft will also create more instability.
Upslope winds can lead to significant precipitation as the moist air gets lifted into the higher elevations and cools. As the air cools it condenses out as precipitation.
Below is my latest estimate for rainfall....
Mild air masses likely to outnumber the chilly air masses for a good part of October.
Jet stream pattern across North America will become amplified across North America into next week, which means more extremes in weather.
Warm weather will dominate in the eastern half of the country for the next week. Pattern change possible during the last week of September.
The jet stream will strengthen from the Pacific across southern Canada over the next 1-2 weeks which will keep any sustained chilly/cold air masses up across western Alaska and eastern Siberia.
The latest clues to the long range into the month of October.
Late-season heat in the East and mountain snow in the West. Welcome to the fall season!