How a plume of heat out of the U.S. will impact temperatures in Canada

CTV meteorologist Kalin Mitchell tracks a rainy weather system across the Maritime provinces.

A large ridge in the mid-to-upper levels of the atmosphere will allow for some potentially record-breaking May heat to build into parts of the Canadian Prairies late this week.

Part of the ridge is the jet stream, which is a band of faster moving winds aloft in the atmosphere. The jet stream often marks the boundary between warmer air to the south and cooler air to the north.

When the jet stream is tied up in this type of ridge, it often forms a blocking pattern for weather. That means the heat could last several days before moderating. At the same time, dual troughs located on either side of the ridge will bookend Canada with cooler temperatures on both the west and east coast.

Bringing the heat

Temperatures will climb rapidly in the Prairies this week. Southern areas will hit daytime high temperatures in the low-30’s as early as Tuesday. By Thursday, some communities in southeast Alberta, southern Saskatchewan and southern Manitoba could reach the mid-30’s. Daytime high temperature records for late May will be challenged. Here are some of the standing records for May 29, which will coincide closely with the peak of the heat in the forecast.

• Lethbridge, AB: 33.1 C set in 2025 - forecast high 30 C
• Medicine Hat, AB: 36.1 C set in 1936 - forecast high 33 C
• Saskatoon, SK: 35.1 C set in 1986 - forecast high 35 C
• Regina, SK 37.2 C set in 1934 - forecast high 35 C
• Brandon, MB: 35.2 C set in 1988 - forecast high 35 C
• Winnipeg, MB: 34.4 C set in 1934 - forecast high 34 C

Parts of the Northwest Territories will catch the northern edge of the ridge. Yellowknife will likely reach a high temperature close to the mid-20’s on Friday, which is several degrees above the late-May average of 14 C.

The standing record high for Yellowknife for May 29 is 27.9 C.

Fair weather finish

Neither of the remaining northern territories are expected to end up directly under the heat ridge.

In the Yukon, May is expected to finish with fair weather and seasonable temperatures. For Whitehorse that means high temperatures in the low-to-mid teens and overnight lows near zero.

Western parts of Nunavut could feel some of the warmth for the final weekend of May. At Baker Lake, daytime highs on the weekend could reach the low-to-mid teens with a mix of sun and cloud. The average late-May high temperature there only two degrees.

Iqaluit will have more moderate temperatures for the last few days of May. Highs are forecast to reach the mid-to-high single digits.

Warmer early this week

The westernmost communities of Northern Ontario are expected to feel some of the heat late in the week. The high temperature for Kenora could reach 28 C on Friday. It will be warm at Thunder Bay with high temperatures in the low-to-mid 20’s for much of the week, which is slightly above the seasonable high of 19 C.

Southern Ontario will be warmest through the first part of this week. Toronto will have high temperatures near 27 C for both Tuesday and Wednesday. After that, the highs are expected to fall into the low-20’s for Thursday through the weekend.

Likewise, Montreal and the eastern townships in southern Quebec will be warmest Tuesday and Wednesday. Highs will be in the high-20’s. After that, temperatures will cool through late week with a chance of rain and showers Friday into the weekend.

Quebec City and northern areas of Quebec will end up in the cool part of this weather pattern by the end of the week. Highs on Thursday and Friday will fall into the low-teens, below the late-May average of 20 C.

Cooler coasts

What goes up, must come down. And the ridge of heat will come down into two troughs with cooler air pooled in them for both the west and east coast this week.

The south coast of BC is expected to mostly stay out of the coolest conditions. Temperatures could briefly fall towards the mid-teens for both Victoria and Vancouver on Friday before recovering on the weekend.

Parts of the central and northern coastline of BC will feel the cooling effect a little more. Communities such as Prince Rupert can expect to end May with temperatures a bit below seasonable and with a higher chance of cloudy and showery weather.

Cooler weather will be felt more earnestly in Atlantic Canada to close out May.

For the Maritime provinces, seasonable high temperatures for the end of May range 16-to-20 C. Daytime highs Thursday through the weekend are likely to struggle in the high single digits and low-teens for most communities in those provinces. The cooler weather will be accompanied by cloudy periods and a chance of showers. It may take at least the first few days of June for temperatures to moderate for the region.

Labrador will close out May with near normal high temperatures. Newfoundland will feel the effects of the cooler air. St. John’s and the Avalon Peninsula will be chilly with high temperatures to finish the week and start the weekend in the single digits. The below average temperatures will be accompanied by a cloudier sky and periods of showers or drizzle.