Winter Weather Forecast Discussion:A significant storm is expected to move into eastern canada from the american midwest by thursday night causing major impacts across southern and eastern ontario as well as quebec. The arrival of this storm will fall just in time for christmas and is expected to dump copious amounts of snow across ontario and quebec increasing white christmas chances significantly. A mixed bad of precipitation will be a threat as well especially across southern and eastern ontario where temperatures will hover around the freezing mark for several hours. The first band of precipitation will move through southern ontario ahead of the cold front late thursday night with an initial burst of wet snow before the precipitation changes over to mainly rain or a mix with ice pellets across the area. This will occur as the low strengthens pulling in warm air out in advance of the cold front. This is typical of very strong low pressure systems which tend to pull warm air into there centers. This is a sign that precipitation is not weakening too much even as occlusion takes place. As the low nears occlusion early friday morning, this changeover to rain will stay very short lived with cold air plunging in from the NW behind the cold front. This will change any backend precipitation back over to a wet snow across southern ontario. Remember that any precipitation west of the cold front will be lighter and more convective in nature meaning short, heavy bursts as opposed to long duration bands. These bands will look very spread out on radar as the deformation band stalls out over michigan and slowly weaken. Heavy bursts of snow will continue throughout the day on friday with a widespread 2-5cm event is expected across southern ontario not including the lake effect snow that will follow friday night into saturday. Across eastern ontario this storm will be a different animal as precipitation arrives by early morning friday with 850mb and 925mb temperatures below freezing which is supportive of frozen precipitation. This is supported by both ecmwf and GFS model guidance. This will promote a mostly snow event for ottawa/gatineau with surface temperatures around 0C. The initial band ahead of the cold front will be very heavy precipitation probably in the form of snow across the ottawa, cornwall and montreal areas. I believe that ottawa is a fairly safe bet in terms of being cold enough for snow but montreal gets a little trickier as an easterly flow will promote warm air advection. There may be some front end mixing but the majority of the event stays as snow across montreal and cornwall. I am expecting an all snow event from trois rivieres northeast to quebec city. By the afternoon hours on friday the upper level trough will go negatively tilted allowing a secondary low to form. The heavy moisture plume will continue across eastern ontario and quebec as 700mb vertical velocity values go off the chart in these bands. This very strong secondary has the chance to create white out conditions across the ottawa, montreal, cornwall, brockville and kingston areas. By this time the cold front will have passed the area allowing for all snow to continue falling with even more supportive boundary layer temperatures. The highest risk with this storm are 850 and 925mb temperatures which will hover around the freezing mark especially in areas along the 401 in eastern ontario and southern quebec. The precipitation will be heavy, so it will all depend on what side of the 0c line you are on to determine if its white out conditions or a wash out with more mixing. Future model guidance may chance exactly where this sets up(50km shifts possible) but models are in good agreement currently. Right now i feel confident that this low is strong enough to pull in plenty of cold air and mixing will be short-lived. As the secondary forms the isobars will tighten across southern ontario promoting additional snowfall on the backside of the secondary area of low pressure. The low will begin to pull away late friday evening allowing precipitation to tone down as the cold air ushers in from the NW.
Here are my preliminary forecast amounts which i will update 24 hours before the event and are subject to change:
GTA north of the 401: 5-15cm of snow including lake effect snow (between thursday night and saturday night). Brief period of rain/ice pellets friday morning
Toronto st clair to the 401: 3-10cm+ of snow including lake effect
Toronto downtown: trace-7cm of snow including lake effect
Barrie: 5-10cm not including LES, brief period of mixing before the change over
Parry Sound: 7-15cm of snow, brief period of mixing before changeover
Cornwall: 10-20cm of snow, front end mixing before radiational cooling allows change over
Ottawa: 20-30cm possible, mostly snow *winter storm warnings likely*
Montreal: 15-25cm of snow, some front end mixing before radiational cooling takes effect *winter storm warnings likely*
Kingston: 10-20cm of snow, right on cusp of larger amounts, shift either way will make big difference
Quebec City: 20-30cm of snow *WSW likely*
Hamilton: trace-5cm of snow including LES
Guelph: 5-15cm of snow including LES
If you have any questions, comments of criticism or would like a forecast for your area feel free to ask away!