Lake effect season is here

This morning is providing a good example of a common phenomenon in southern MB in the fall - lake effect precipitation. Lake effect precipitation develops when you have cool air aloft moving over the relatively warm lake waters, creating narrow bands of instability precipitation to the lee of the lakes. You need at least a 13C difference between the lake water and the temperature at 850 mb, and winds need to be well aligned in the lowest 10000 feet (i.e. little directional shear) to form these narrow bands. This morning we have 850 temperatures of +2C over 18C lake waters, along with low level winds aligned in a 330-340 degree orientation giving lake precipitation to the southeast of Lakes Manitoba and Winnipeg. Typically these lake effect bands form in the cool air behind a cyclonic storm system and will dissipate as subsidence moves in with an approaching ridge of high pressure.

Early in the season, these precipitation bands are in the form of rainshowers. As we get further into the fall with colder temperatures aloft (i.e. 850 temps of -5C or colder), these lake effect bands will produce narrow swaths of snow to the lee of the lakes.. sometimes quite heavy. See an example from October 11, 2006 for an early season lake effect snow event over southern MB.. notice that the lake effect bands from that case are oriented virtually identical to those this morning (except it's rain today, not snow!) Lake effect bands off Lake Manitoba commonly set up between Portage and Elie, but can come into Winnipeg if the low level flow is in the right direction, from around 300 degrees, as seen in this example from last September.