Interesting band of rain last night off Lake Manitoba brought a narrow but fairly intense band of lake effect rainfall along the Hwy 6 corridor into Winnipeg overnight. The image attached is from Woodlands radar about 1130 pm last night showing the band coming off the east shore of Lake Manitoba right into Winnipeg. About 10 mm of rain was recorded here in Winnipeg from the band while most areas outside this band saw little or no rain.
Lake effect preciptation occurs when you have cool air sweeping over warmer lake waters creating narrow bands of convective cloud that can give you very localized but intense precipitation. The greater the contrast between the cool air and the warm lake waters, the greater the instability, and the more significant precipitation you can get. Normally, this occurs in the fall when you have cold fall airmasses moving over the still warm lake waters giving you areas of lake effect rain or snow if it's cold enough. It's unusual to get lake effect this time of year as the air aloft is usually too warm. However last night we had a fairly intense storm system drag some very cool air aloft over southern MB, with temperatures down to 3-4C at 850 mb. This air was taken over 18C lake waters to give enough instability to trigger lake effect precipitation. (You need at least 13C difference between 850 mb and lake water temperatures to initiate lake effect. You also need little directional wind shear in the low levels to focus the instability in narrower but more intense bands) The winds in the low levels were aligned from the northwest at about 300-310 degrees which brought the band off Lake Manitoba right into Winnipeg. If the winds had a more northerly component (like 330 degrees) the band would have oriented further west (see lake effect event of Oct 11 2006 for an example of this)
So if you thought last night had a fall-like feel to it.. you were right.. complete with lake effect! Let's hope we don't see another one of these events until the fall .. it's time for some warm dry weather!