The first significant lake effect event of the season will likely begin tonight as a very cold airmass sweeps over Southern Manitoba, including Lakes Manitoba and Winnipeg. The cold air moving over the 10-11C lake waters will trigger bands of lake effect snow to the south and southeast of Lakes Winnipeg and Manitoba. Guidance is indicating winds of 330-340 degrees in the lowest levels with 850 temperatures of -10C to -12C. This will create very strong instability over the lakes with a lake-850 temperature differential of 20C or more. Generally, lake effect will trigger with a differential of 13C or more. The squalls will likely get going this evening initially as lake effect rainbands and then intensify overnight into snowsqualls as the colder air deepens over the lakes.
Given 330-340 degree low level steering winds, the lake effect bands will likely set up off the south end of Lake Manitoba through Elie and Fannystelle towards Brunkild. Off Lake Winnipeg, the bands will likely set up over Grand and Patricia Beaches south-southeast towards Beausejour and Vivian. (see map) These lake effect bands could be quite intense if they change into snow as expected. Near zero visibility in heavy wet snow are likely through these bands overnight through Wednesday with local accumulations of 5-10 cm per 12 hours possible. Accumulations will be less if the precipitation is mixed with rain which may be the case close to the lakes where the 11c lake water will warm the low levels enough to maintain precipitation as rain.
The orientation of the lake effect bands will depend on the exact low level wind direction. If the low level winds are more northerly (350-360 degrees), the bands will be a little more west than the map indicates. If steering winds are more northwesterly (300-320) the bands will be further east. Note that a wind of 300 degrees can bring lake effect into Winnipeg off Lake Manitoba.
Whatever the case, looks like an interesting couple of days shaping up as Mother Nature sends us an early taste of winter!