A winter storm watch has been issued for much of southern MB as a low pressure system from the Dakotas spreads an area of precipitation up from the south beginning tonight. The precipitation will start off as band of rain (with possible thunderstorms) near the international border this evening spreading north through the evening. As it moves north, it will encounter colder air aloft and rain will change to snow over southwest MB and the northern Red River valley by midnight continuing overnight into Thursday morning. Precipitation will likely stay mostly as rain over southeast MB tonight before changing to wet snow early Thursday. In between the rain and snow areas, there may be a band of freezing rain and ice pellets cutting though the Red River valley including Winnipeg overnight.
Snow accumulations with this system will be highly dependent on elevation and the position of the rain/snow line.. but at this point, it appears areas north and west of Winnipeg will see around 10 cm of snow overnight with up to 20 cm possible over higher elevations west of the Red River valley. Areas south and east of Winnipeg will see more rain at first before the changeover to snow, so snow amounts in these areas will only be in the coating to 5 cm range. Here in Winnipeg, snow amounts will be a tough call since we'll be right on the rain/snow line much of tonight, but as a first guess, 5 to 10 cm are possible by midday Thursday, although some of that will melt initially due to the warm ground. Note that this system has the potential for heavy precipitation rates due to elevated convection, thus snow or freezing rain could be heavy at times overnight into Thursday morning. In areas that are cold enough for snow, this will mean very poor visibilities in heavy snow, with snowfall rates of 2 to 3 cm/hr possible at times. All in all, a messy commute to work and school Thursday.. be prepared for anything!