Markham, ON - An incoming system from the United States is going to be one major weather maker for many Ontarians towards early next week and possibly into mid-week. A ridge of High Pressure currently over Ohio River Valley is encouraging strong southernly flow (High pressure circulates clockwise) to return into much of Southern Ontario. As a result, temperatures in Southern Ontario is going to be influenced by the Gulf of Mexico. Towards next Tuesday, the southerly air is so strong that temperature could bump up to 4 C.

In the meantime, with such warm air incoming, a strong northerly cold airmass situated near Hay River, Northwest Territories, wanted to invade south. The centre of this air mass has a daytime high temperature of -35 C (with wind chill close to -50 C). The collision of these two airmass is going to happen near mideast US (around Michigan, Illinois, Wisconsin, etc.) , and results in a strong low pressure system (as the GFS suggests, a centre with 986 mB. The lowest air pressure we had experienced in the Greater Toronto Area this year was 990 mB.). This strong low pressure system could stir up some nasty mix precipitation. As to how much we are getting, more details would be followed.

On the east side of the low pressure will be warm air, and thus, as this low pressure enters our area late Monday night at around 21:00 to 00:00, the precipitation will start as wet snow (nighttime temperature still remaining below 0 C). The wet snow is going to last throughout the night, and eventually change into rain Tuesday morning, as the temperature climbs to 4 C, with a wind chill around -2 C, and high southerly winds. The light rain is going to last throughout the day as well as the night. At around 05:00 to 07:00 EST, we are projecting the cold front of the system pushing through the Greater Toronto Area. This is going to create some intermittent heavy precipitation, as well as very high wind gusts, switching from southerly to a northerly flow. It looks like the heaviest part of the precipitation is going to stay as rain, with temperature around 5 C.  Around noon hours, the cold front successfully passed through, and temperature is going to drop rapidly throughout the day. At noon, temperature is going to drop down to -2 C, with precipitation changing from rain to freezing rain to light snow. Rapid freezing of meltwater and water on the roads could also occur, creating hazardous driving conditions. Strong northwesterly winds are also accompanying this,  wind gusts could go as high as 80 km/h. With the wind chill, it could feel more like -10 C. The bulk of the precipitation is going to exit around 00:00 on Thursday night, with around 5 cm of snow on ground.

Lake effect snow could follow after this bulk of the precipitation due to the high northwesterly winds in the traditional snowbelt areas. More than 15 cm of snow is expected in the core of this band. This band could even produce a flurry or two to the GTA, while a couple cm of snow around the outskirt areas. More details to follow.