Wednesday, July 18, 2007

Western heat wave spreading east this weekend

The heat wave that has gripped the western Prairies for the past week with temperatures in the mid to upper thirties will be spreading east over southern Manitoba beginning this weekend. A ridge of high pressure over southern Manitoba on Thursday will move east into the Great Lakes by Friday. A return southerly flow west of the ridge will draw increasingly warm air from the western U.S. and Canada into southern MB over the weekend, with temperatures expected to climb into the low to mid thirties by Sunday. The hot weather is expected to continue through much of next week with temperatures likely persisting in the thirties most of the week. Little precipitation is expected over the period, except for a possibility of some showers or thunderstorms Tuesday and Wednesday as a weak cold front tries to move through southern MB.

Officially, a heat wave is defined as a period of at least 3 consecutive days with maximum temperatures of 32C or greater. This is a throwback from the Fahrenheit days when we used 90F (32C) to define a "hot" day. Unofficially, we have started to define a heat wave in Canada as a period of 3 days or more with maximum temperatures of 30C or more. It's a nice round number, and it's a good threshold for most Canadians to define as a "hot" day. Either way you slice it, it looks like an extended period of hot weather is coming to southern MB!


  1. I see the predicted highs are gradually deflating. No more 37 C and the like. I'm not surprised, I suppose it was another of those automated forecasts with no local input.

  2. That's right.. those temperatures in the extended forecast have been a little over done. It's certainly going to be hot, but 37C was a bit of a stretch for Sunday for Winnipeg. For Winnipeg to hit 37C, we need a southwest or west flow tapping hot air from southern Saskatchewan. It looks like the peak of the heat will be over southern SK and southwest MB over the next day or so, pushing into Winnipeg by Monday or Tuesday. The whole upcoming week looks hot, with afternoon temperatures of 35C or so through Friday and humidex values of 40-45C. Nightime temperatures will be warm as well so people without air conditioning are going to have a tough week sleeping. This will be a prolonged heat wave that can be very dangerous for the elderly and people in poor health.

  3. When I fist saw the forecast for 37 I figured they'd overdone it a bit. I forecast weather as a hobby (down here in Lancaster, MN) and I look at model data every day. I didn't see evidence that it would get that hot today, and I don't think it will reach 38 on Tuesday. I'm going for 36 (97F) in Lancaster for Tuesday, and that's about what it looks like for Winnipeg as well at this point. Models actually show 38 or 39 for Tuesday, but the dew points with this air mass are very high, and that high moisture content retards daytime heating a bit, so I've been trimming the highs from what's shown in the data. Of course, the high dew points also mean very warm nights. It was 22 this morning here as well. A slightly drier air mass will let temps dip a little more tonight, but that won't last. Right now, there is some evidence that there will be a short break late in the week, especially with nighttime temperatures, but the heat builds by next weekend (especially Sunday) and lasts through the middle of the following week. However, IF I can believe the model data, round 2 of the heat wave will be a bit less miserable, with dew points in the 15-17 degree range as opposed to the low to mid 20s we have now. Models show the heat wave breaking down around the 2nd or 3rd of August, but since the models have been flip-flopping a lot, it's too early to say for sure when we'll be done with the heat wave.

  4. Hi Bruce, and welcome to Rob's Obs! Nice to hear from our southern neighbours.. I think we passed through Lancaster once on our way to Thief River Falls.

    As mentioned, our long range temperature forecasts have gone a little overboard on the forecast highs during this heat wave. They've consistently been trending downwards as we get closer to the day. Certainly it will be hot on Tuesday, and it may FEEL like 38C (or more), but the actual air temperature will be more like 33 to 35C for highs in Winnipeg. As you mentioned, it's difficult to get those upper 30 temperatures with high dewpoints.. you need low dewpoints, and a dry surface (like in a drought year) It also helps if Winnipeg can get a west or southwest surface flow, which is a bit of downslope wind for us and can add a couple of extra degrees.

    Regardless, it's going to be a hot stretch until that cold front comes through on Wednesday! (note that earlier models weren't bringing that front though until Friday.. subsequent model runs have pushed that front through faster now)

  5. Thanks, Rob. It's good to have someone to talk to regarding the weather. I noticed that the models have sped up the front. We could have a few storms late Wednesday into early Thursday, as the front comes through (at least down here, but I would not be surprised if scattered storms were to occur up there). Yes, it has been too wet to allow temps in the high 30s to about 40 like the models were showing. When Larry Updike talked to me Friday morning on CJOB, I hinted that the 37 degree high for Sunday was overdone, but that lower 20s for lows were quite likely at times with this type of air mass. Lancaster is on Hwy 59, and that was the road you took to Thief River Falls. Of course, with the 55 mph (88 km/h) speed limit, it is a slow drive. By the way, we had a cousin who lived in Charleswood (on Charleswood road to be exact).