Potential for localized rainfall amounts in excess of 100 mm (4 inches) over western Alberta this week....
A potent, upper-level storm system spinning off the Pacific Northwest coast will track slowly eastward over the next couple of days and then almost stall over extreme southeastern British Columbia on Wednesday.
This will set up an unusually strong, upslope wind flow into western Alberta from Wednesday into early Thursday, resulting in a widespread, heavy rainfall for the region starting late Tuesday and continuing into late Thursday. The higher elevations of extreme southeastern British Columbia will also experience heavy rainfall and possible flooding.
It appears that the heaviest rainfall will occur just west of Calgary and into most of Banff National Park. I personally experienced a heavy rainfall event in that region over 10 years ago and we were almost caught up in a major landslide that took out a good chunk of the Trans-Canada Highway.
Below are 48-hour rainfall forecasts through Thursday night from three different computer models. The first two models are the U.S. GFS and NAM and are in inches (scale to left). The third is the Canadian model with the scale in millimeters to the right.
This type of event could certainly could produce widespread flooding and landslides so caution should be taken if you plan to visit that beautiful park this week. I also suspect there could be an increased avalanche danger in the higher elevations.
Another thing to consider is that the last 30 days have been fairly wet across this region (see below), so the soil in some areas is fairly saturated.
Severe thunderstorm, hail and tornado threat....
In addition to the rain, severe thunderstorms with the potential for flash flooding could impact southern Alberta and southern Saskatchewan Tuesday, Wednesday and perhaps Thursday afternoon/evening. The strongest storms will produce possible damaging winds and large hail. An isolated tornado cannot be ruled out either, especially Wednesday afternoon and near the Medicine Hat region.
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Cooler air to expand from Prairies to eastern Canada late June into early July.
Dry and hotter pattern in store for southern B.C. into next week, while the east turns cooler and more unsettled.