Southern Ontario First Significant Snowfall

The GTA and the golden horseshoe will experience its first significant snowfall of the season tomorrow. Light flurries will begin tonight with light bands of snow moving through the area after the midnight hour. As the night wears on, the snow will begin to pick up in intensity around 9 AM with more consistent bands of snow around the west end of lake ontario in particular which could pick up some precipitation enhancement from the lake. By 11 am the heaviest bands of precipitation will move through SW ontario first with a light burst of snow/freezing rain and sleet before moving northwards and effecting the GTA around noon. This band of snow will slowly move eastwards allowing for light snow to fall from orangeville, through the GTA, niagara, hamilton and the NW shores of lake erie between 10am and the late evening hours. This band of snow will be nothing special for southern ontario but it will promote an ENE to ESE wind off of lake ontario which may work to enhance snow totals. This has been advertised on the RAP, NMM and ARW high resolution models showing enhanced precipitation from hamilton to oakville and even the GTA to a lesser extent. Right now it is hard to tell if and where these bands will set up but my hunch tells me based on wind shear values, light easterly winds and a small temperature inversion that snow will stay light under this system causing 2-5cm amounts across the golden horseshoe. This will be enough snow to coat many cooler surfaces but will not present major problems for road travel. If under the event that lake enhancement does take place these amount can easily double with some areas picking up 5-10+cm of snow. I remember waking up one morning in january of 2011 with a surprise 15-25cm of snow due to lake effect bands off of lake ontario which can pack a quick punch if the correct setup presents itself including heavy snow rates.

NMM Similated Radar:

850mb Temperatures are supportive of snow:

Sounding for mid day friday:

2 Major Storms On The Table For The East

GFS precipitable water at hour 150 shows a system bombing out as it tracks west of japan. This track telleconnects to an apps runner storm track or an GLC track 6-10 days later which lines up with another potential storm between the 10th and 12th of december.

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The EPO is forecast to move towards neutral in that time period which coupled with a -ao and -nao may translate to a sizeable storm. The GFS has really been showing this over the last 8 runs.

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18z GFS:

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Another storm may come into play before hand which will be between the 7th and 9th which has support from the euro but not much on the GFS. A current east asian storm supports this as well. This storm is probably further east and weaker then the first one.

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Precipitable Water:

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Tropical Storm Bopha Recurving

Tropical Storm Bopha is forecast to weaken as it recurves to the east of Japan which will promote a trough in eastern north america in a week to ten days from now.

Forecast Track:

This favours a trough in the east between the 6th and 10th of december which will begin to trigger the pattern change I have been posting about lately. More cold air will begin to usher southwards into the prairies and northern plains before moving eastwards by mid december. Expect the models to trend stronger with this feature as we get closer to the event and warm members turn cooler effecting the mean.
12z GEFS shows the trough:

Even stronger and more frequent trough's will follow suit allowing the pattern change to take place with more western ridging as well as a west based -NAO pattern:

CFSv2 Frigid Long Range, White Christmas For Many

CFSv2 sends the cold charging southwards and is really highlighting the pattern change many have been posting about.
Forecast for the next ten days, ridge is in control but will move out

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Day twenty to twenty-five getting cold:

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Days twenty to twenty five frigid:

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Cold hangs on into January with ridging in the west:

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Predicted snow cover for christmas time will make many people happy:

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For those of us doubting winter, you may end up happy that the cold held off so far because if this turns out correct expect relentless cold for weeks on end and very exciting winter weather. This is still very far out but it is encouraging to see signs of a pattern change from the long range models.

Tropical Forcing Backs Mid December Pattern Change

 The stratosphere has began to warm gradually and will continue to do so over the next few weeks. This is being caused by numerous tropical influences. A major player, the -QBO phase means that warm air from the equator is being transported to the north pole. This means that we are in a pattern that should naturally want to warm the stratosphere, especially as we head into NDJ. In november the stratosphere as a whole has experienced record cold because of above normal solar activity which cools the stratosphere. The sun is quieting down now which will allow the stratosphere to begin warming. 2) Mountain torque values are on the rise in both east asia and north america which act as a trigger for a stratospheric warming event. Above normal torque found near 30N means that we can expect to see the stratosphere begin to warm significantly around the 10th of december. The extent of the warming is still unknown but in east QBO years this is common, usually occurring suddenly.

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Stratospheric temperature change is evident in the 240 hour forecast from the ecmwf:

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This would favour ridging across greenland, a trough over the bering sea, ridge in western canada and trough over the east.

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Remember the fact that modelling is beginning to pick up on this does not necessarily mean that this will occur. It is the observations that i want to look at in order to gain better support for a forecast. Now all of this tropical forcing supports a pattern change to colder weather over central and eastern canada. Now lets see what other indicators we can find too support this. Well we can see the trough recurving east of japan in this time period and AAM beginning to move below normal which supports changes in the pacific, particularly a shift to a -epo and possibly PNA. We need the persistent bering sea ridge to break down in order to bring the cold into the east.

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Pacific Changes On The Way

Recent runs of the OP GFS and to a lesser extent the GFS ensembles are showing the pattern change really take hold by the 10th of december or so. Now remember that the GFS was showing a major pattern change by the 5th and i said that it will be a step down process. This appears to be the case this time but it may take some more time for the pattern to really transition. Right now the jet is shifting further south and maturing as we get closer to winter. right now a persistent -pna and +epo pattern is allowing for warm pacific air to flood north america much like last winter although we are seeing blocking present which should not allow the warmth to last for too long. We will see an impressive warmup for the first week of december but the warmth will be centred west over the american midwest and plains states. As we go on in time the effects of the modiki el nino should allow for more troughiness to form in the aleutian islands due to the enhanced convection around the dateline. This will allow for the persistent aleutian ridge/ alaskan trough pattern to break down which will allow for more cold air to move into eastern and central canada while warmth and dryness returns to the west. This coupled with the negative AO pattern caused by the PV splitting into tow pieces one of which is on our side of the globe. So the pattern we are going in supports front side warmth followed by a pattern change with a step down process to take place in mid to late december. This will allow for a cold january and february pattern to set up. Some support for a trigger to flip the epo phase is the GAM going negative which will allow a ridge to build in the GOA i n the longer range.   Right now we are undergoing some ground work but the rewards will be felt in the heart of the winter! 

December Breakdown

With the AO/NAO forecast to be very negative in the long range it will be very difficult to see prolonged warmups across the east for the start of December. A 3-5 day warm spell will have an impact from the 1st to the 5th of december as the ridge in the plains moves eastwards.
The control is off the charts negative in both the AO and NAO shows by the blue line:

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8-16 day period notice the cold shifting to the west and impressive warmth shifting eastwards centred south of canada:

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A period from the 4th to the 8th of december looks like the time to watch for a trough in the east as we are seeing the east asian trough developing by 48 hours meaning we should look for a trough to develop 7-10 days later. A storm will probably form in this time frame as well in the eastern half of NA which is something to look out for.
500mb chart at hour 240 

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Strong storm developing across east asia matches up with an eastern canada storm in the dec 4-8th period:

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Looking out further another shot of cold air will move southeastwards into the east as a trough plows in from the NW where snowfall is running above normal. The pattern we are going into is one of a negative NAO/AO and -PNA with plenty of cold air available. The ridge in the bering straits will be a prominent feature allowing for more energy to dive in off the west coast. This will pump a ridge in the western u.s causing above normal temps across the southwestern u.s. A battle ground will set up further north will above normal precip across b.c and california. This ridge will allow for a trough to form in the great lakes with a transfer of very cold air from northern Bc the yukon and alaska southeastwards. 
These features will get stronger as we go on in time but i like the positioning. 

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To sum up my main points:
1) A big warmup is in store for the central and eastern parts of canada for the first 5 days of december
2) A very favourable pattern for cold and snow will set up between the 6th and 20th of december with a potential for storms to develop from the midwest to the east coast