A significant winter storm will move into Southern and Eastern Ontario as well as Quebec sunday night into monday. Winter storm watches are up already in parts of quebec and i expect more watches, warnings and advisories to go up later today and tomorrow. A mixed bag of precipitation is to be expected including the potential to see rain, freezing rain, sleet and wet snow. By sunday night we will see an initial push of moderate to heavy precipitation moving directly through the toronto area and extending east towards kingston, brockville, cornwall and through the montreal and ottawa areas later sunday night. This band of precipitation will likely bring an initial burst of snow for ottawa, montreal, and cottage country across southern ontario before a change over to a significant freezing rain event. By the morning hours on monday as the initial batch of precipitation moves out, enough warm air advection will cause the precipitation to change over to all rain or freezing rain for most of the region. Enough cold air should darn as a result of tonights high pressure system which should cool the area to below freezing temperature values. Areas where the most significant freezing rain are possible include the ottawa valley, montreal, cornwall, areas NE of lake ontario along the 401, areas north of the 401 including barrie,orillia, parry sound and northern parts of the GTA where between 0.1" and 0.5" of freezing rain are possible. Confidence is lowest in the city of toronto where the dividing line looks to set up between significant freezing rain, snow and sleet and just plain rain. This will be a tough forecast to make but right now i am thinking an initial burst of sleet and freezing rain but not a long lasting event south of the 401. This is due to past experience with these events where precipitation has trouble changing over in these types of events with marginal surface and 850mb temps. North of the 401 can be a different story with more cold air hanging on allowing for more significant icing. Drivers are urged to take caution and should make travel plans accordingly especially if you do not have snow tires on yet. A combination of low visibility and icy roads will make travelling a huge problem along the 401 corridor sunday after 2pm through monday morning. With that being said these are the potential risks/look for's in tomorrows storm forecast:
1) With surface temps colder then 850mb/ 925mb temperatures, freezing rain will be very likely in areas where surface temperatures can stay at or below 0C. If this is not the case before the event and the surface high does not deliver as much cold air as currently expected less freezing rain will occur across the area. This may occur from kingston and SW along lake ontario if surface temperatures do not cool down enough.
2) With intense upwards motion moving through the area evaporational cooling will be able to cool the parcel of air in areas under heavy precipitation bands. Where these bands set up will be the area effected by significant freezing rain and sleet bands.
With all of these factors in place I am confident in saying that there will be a significant icing event setting up across parts of ontario and quebec as the dynamics are definitely there. It is a question of where and how much at this point! These are my preliminary accumulation forecasts and may have to be tweaked based on short range model output or nowcasting especially due to the variety of precipitation types and difficult forecast period.
Here is my forecast for several cities across ontario and quebec. If I have missed your area just ask and i will add it into the discussion.
Montreal: 5-10cm+ of snow, change over to 5-10mm of freezing rain
Ottawa: 5-15cm of snow, change over to 5mm of freezing rain. If mid levels warm faster then expect more freezing rain and less snow but a general .5-.6" of QPF is expected.
Toronto downtown: Mostly rain with 2-5mm of freezing rain or sleet possible initially
Toronto St Clair-401: 5-10mm of freezing rain or sleet before a change over to rain is likely
GTA north of the 401- 1-4cm of sleet before a changeover to 5-10mm of freezing rain(amounts may need to get taken up or down depending on temps). Rain showers to end off. *Most Volatile Area*.
Parry sound: 5-10cm of snow
Barrie 2-8cm of snow/sleet, 2-5mm of freezing rain
Quebec City- 7-15cm+ of snow, backend freezing drizzle