Thursday, September 13, 2007

Lake effect rainbands affecting Winnipeg and area

A band of lake effect rain has developed off Lake Manitoba this morning and is currently affecting parts of west Winnipeg. Cool air (0c at 850 mb) flowing over the warm open waters of the MB lakes (16-18c) has created these bands of lake effect precipitation which should persist this afternoon into tonight. Currently low level winds are from a 300 degree orientation which is advecting the band into west Winnipeg and areas northwest. Rosser has picked up 4 mm of rain this morning underneath the band. Outside the band, the sun is shining in parts of Winnipeg!

For this afternoon and evening, models indicate low level steering winds should become more northerly, with a 330-350 orientation which should push the Lake Manitoba bands westward through Elie and Sanford and areas west. A 330-340 band off Lake Winnipeg would affect the Patricia Beach and Beausejour areas. (see example of 330 degree squalls from last October) I expect the bands to intensify this evening and tonight off both Lake Manitoba and Lake Winnipeg as a cold pool of -5C air at 850 mb moves over southern MB. This will give a 20C temperature differential between the lake water and 850 mb temperatures, which is getting extreme for lake effect precipitation (you need a minimum differential of 13c for lake effect initiation). There's even a chance that the precipitation could mix with or change to wet snow tonight into Friday morning, although I suspect the warm lake waters should maintain the precipitation mainly as rain.

The good news is that we should see a moderating trend this weekend with dry conditions and warmer temperatures as this cold pocket of air moves east.


Lake effect update.. Interesting to see squalls starting to re-orient themsleves this afternoon as low level winds become more northerly. The 300 degree rainbands off Lake Manitoba which had been affecting Winnipeg are now weakening, while a new 330-340 band sets up further west between Elie and Portage. See attached Doppler radar image from 12:20 pm showing this lake effect transition in progress. Note also lake effect bands starting to develop off Lake Winnipeg.


  1. The below freezing temperature at Winnipeg airport overnight should be taken with a large grain of salt... It appears that it was the only environment canada weather station to drop below freezing in the red river valley and interlake.

    The station cleared out only briefly and yet quickly plunged due to cold air drainage effects. Most other stations likely would not have dropped below zero so quickly in that situation .

    I believe the recording station should be moved to somewhere a lot more representative of what is going in the general area... maybe closer to the terminal building or where there is better cold air drainage. Does 'NavCanada' have guidelines/regulations etc that would make that difficult?

    There are obvious agricultural and horticultural impacts on where the recording station is located. The growing season maybe underestimated for all ares surrounding Winnipeg and the mean annual minimum temperatures may be too low. The latter is used to determine what trees can be planted by the city along streets etc.

    It should also be noted that cities like Ottawa, have their 'official' recording station heavily influenced by urban effects. Therefore, how can one compare climate data between Winnipeg and Ottawa objectively?

    Anyways looks like tonite we should escape sub zero temperatures as a return southerly flow gets going ... unless we quickly spike down in the early evening with a lingering northwest flow.


  2. Hi Daniel, I agree that often the Winnipeg airport temp isn't very representative of the actual temperature in the area especially on nights with good radiational cooling as the airport seems to get a good few degrees cooler than most other wx stations around Winnipeg.

    I live just northeast of Winnipeg in St. Andrews and I have a VP1 wx station (not online) and I am always comparing my temp readings to the other local stations. My station usually is very similar however last night I dropped to -0.4C while most others only dropped to around 1.5C. I found that kind of odd because if anything my station is usually on the warm side for overnight lows.

  3. Note also that the station at East St Paul dropped to -0.9C last night.. so the Winnipeg airport is probably fairly representative for areas outside the perimeter especially to the north. Inside the perimeter, it's a different story, and the urban heat island effect makes a big difference to temperatures inside the city core which is well sampled by the EC station at the Forks. Everyone else in Winnipeg is usually in between those two extremes (downtown and airport). So you're right Daniel, just taking values from the Winnipeg airport doesn't tell the true story for Winnipeg area minimum temperatures as clearly demonstrated by our growing mesonet of personal weather stations in the area.

    I've also noted that in light southerly flows, my station in Charleswood will be a degree or two colder than the airport because of open fields just to my south. So open rural fields have a large impact on minimum temperatures depending on the wind direction.