A disturbance from the Pacific Northwest will borne a low pressure system on the leeside of the Rocky Mountains early this weekend. Low pressure is then expected to intensify as it approaches the Great Lakes on Monday. By Monday, the system is likely to reach its mature stages and develop into a “classic” mid-latitude cyclone structure. This structure consists of a series of “conveyor belts” which make up the air flow patterns within these cyclones. In the simulated satellite image below (that is valid for Monday evening), a rough depiction of these conveyor belts is shown.
The Warm Conveyor Belt in the case of this system will feed moisture-rich air from the Gulf of Mexico into the cyclone. This warm, moist air ascends up and over the warm front providing ample lift for the development of precipitation. The Cold Conveyor Belt originates ahead of the warm front, and ascends more gradually toward the centre of low pressure (underneath the Warm Conveyor Belt). This air stream becomes saturated and turns clockwise away from the system in the upper levels of the atmosphere. A TRough of Warm Air Aloft (otherwise known as a TROWAL) establishes itself in a counter-clockwise orientation on the backside of the low, and can be thought of as an extension of the Warm Conveyor Belt. The TROWAL is an area of enhanced lift, and often coincides with high precipitation amounts on the backside of a low pressure system (in this case, falling in the form of snow). Lastly, the Dry Conveyor Belt develops as a result of the intrusion of dry air from aloft into the mid-levels of the cyclone. As in the image below, this air stream can be characterized by a lack of precipitation in behind the cold front. As this dry air advances into the low pressure system paralleling the Warm Conveyor Belt, it sometimes can give way to a region of lower precipitation amounts.
At the current time, the centre of low pressure is anticipated to track over the Upper Peninsula of Michigan late Monday then toward areas northeast of Georgian Bay by Tuesday afternoon. Precipitation will develop across Southwest Ontario late in the day on Sunday, for the GTA by early Monday morning, and for Eastern Ontario by Monday afternoon. Precipitation should primarily fall in the form of rain for Southwest Ontario, although freezing rain appears likely for a large swath of Southern and Eastern Ontario beginning in the morning on Monday. An area of high pressure over Quebec will support cold air near to the surface as warmer air ascends aloft ahead of the warm front. This is a perfect recipe for freezing rain. The question is – how long will this freezing rain last? Ultimately, it all depends on the track of the low. With the track mentioned above, a prolonged period of freezing rain may last several hours in some areas before tapering from southwest to northeast early on Tuesday. For areas from the French River and north, this track supports an event that is predominantly snow. Snow begins across Northeast Ontario by Monday afternoon, then tapers from west to east through the day on Tuesday. Snow may also impact portions of Algonquin Park and the Ottawa Valley beginning on Monday afternoon if this track does indeed verify. In regards to winds, a gusty easterly flow develops across Northeast Ontario on Monday afternoon and persists into early Tuesday morning. The strongest winds, however, are expected across Southern Ontario on Tuesday as the cold front passes.
Some uncertainty still surrounds the exact track of this low pressure system. A track that is farther to the north – roughly from Sault Ste. Marie northeastward across southern portions of Northeast Ontario – could mean lesser impacts from freezing rain across Southern and Eastern Ontario. Warm air would advance further northeastward in this scenario resulting in a quicker transition from freezing rain to rain. Impacts from freezing rain are still to be anticipated if this solution holds true, especially for Central and Eastern Ontario and for areas of higher terrain (i.e. the Dundalk and Haliburton Highlands, the Oak Ridges Moraine). This track would also support snow becoming mixed with ice pellets and freezing rain for locations near to the North Channel, thus lowering snowfall totals in those areas. This track would push the axis of heaviest snowfall further to the north and west across Northeast Ontario.
So, what can we expect? Snowfall amounts of 15-25 cm are possible within the axis of heaviest snow with local accumulations to 30 cm. It is still too early to address where exactly these snowfall totals may fall. Impacts from freezing rain are most uncertain at this time. If a prolonged period of freezing precipitation does develop for sections of Southern and Eastern Ontario, disruptive to significant ice accretion cannot be ruled out.
Once low pressure develops on the leeside of the Rockies early this weekend, the models will better be able to initialize in order to arrive at a more stable solution. Monitor ShieldsWeather on Facebook and on Twitter (@ShieldsWeather_) for the latest updates as details on the storm become clearer.