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Major Pacific storm system will spread significant rainfall (by early September standards) into southern BC and Alberta over the next couple of days.
The greatest chance of severe thunderstorms in this region will be Thursday and mainly across extreme southeastern BC during the afternoon then into extreme southwestern Alberta during the evening. Large hail and strong winds will be the main threat in addition to heavy downpours and vivid lightning.
The most widespread, heavier rainfall will occur from Thursday afternoon into Friday night from west to east as the storm drifts slowly northeastward.
Parts of extreme southern BC and into the southern Canadian Rockies could end up with 50-75 mm of rain by early Saturday, which could lead to some river/stream flooding.
Heaviest rainfall should stay just west of Calgary Friday and Friday night and more in the foothills/mountains. This is not the same setup as earlier this summer with the flood. This particular storm system is weaker and will be shorter duration, which will limit the amount of rainfall.
Major temperature contrast across Canada Thursday
More like early October in the east Thursday
An unusually strong trough (pocket of cold air aloft) will allow chilly air from the far north to sweep across a large chunk of eastern Canada through Thursday while a rain spreads up through Atlantic Canada.
Frosty early Friday morning for some
As high pressure builds in Thursday night, the combination of clear skies, light winds and low dewpoints will cause temperatures to drop quickly. By early Friday morning, there will likely be pockets of frost in rural areas away from the Saint Lawrence Valley. For most, this should not be a killing frost, but it is probably not a bad idea to cover or take in some of those more sensitive plants just in case.
Based on the position of the high pressure, it looks like much of eastern Ontario should escape a frost early Friday as slightly warmer air will be moving in on light southerly winds early Friday.
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Early start to summer-like conditions for the West.
Typical spring pattern shaping up across a large portion of the country through the rest of this month.
Rounds of locally heavy thunderstorms for parts of Ontario and Quebec this week