The pattern change I mentioned in my White Christmas Part 1 blog is set to begin later this weekend.  We have already begun to see temperatures falling closer to normal after the passing of the latest GLC storm with some LES starting to set up over the typical regions.

However the real Arctic air begins to make its way Saturday into Sunday and continue all next week.  The question still remains; does the cold stick around or do we return to a warm zonal pattern?

What will the amplified pattern bring?

Like I discussed in Part 1, the amplified pattern will bring very cold Arctic air into the east.  The result of the strong colder pattern will force most storms to our south and up the east coast.  This means parts of Virginia, Washington DC, New York up to Vermont, New Hampshire, over to Boston and up to New Brunswick, PEI, Nova Scotia and Newfoundland will bear the major impact from storms.  Most of them will likely be snowstorms, potentially major ones,  depending on exactly where the Coastal storm track goes.

This means that Ontario and most of Quebec are left with a mostly cold and dry pattern, with the odd Alberta Clipper storm.  These clippers move in from the Prairies at a quick speed as they are caught in the jet stream and drop less snow as the system is moving fast and is generally moisture starved.  LES also develops in behind these storms as more cold air gets pulled in behind them.  Clippers usually can’t produce more then 5 cm of snow, but sometimes more depending on the amount of cold air increasing the water to snow ratio with the colder temperatures (a temperature of -10c to 15c can easily bump up snow totals to near 10 or more cm as the snow becomes fluffy and less dense and accumulates quicker).

You can see the example of a couple clippers developing around the Great Lakes from Sunday through to late next week from the latest GFS model:gfs_mslp_pcpn_frzn_us_fh60-336

This means that next week until December 20th will be wintery across Ontario.  Although there likely wont be any major storms during this time (again aside from the typical snowbelt regions which could see close to 1 – 2 feet of snow on the ground by Dec.20th) most of Ontario should have at least some snow, albeit maybe only 5 to 10 cms on the ground (especially for E.Ontario which wont really benefit from LES and tend to see weaker 2 to 3 cm clippers).  Again for the purposes of maintaining snow cover for Christmas, this pattern is preferred to a potentially warmer and stormier pattern that could bring more storms but also more rain.

Does the cold amplified pattern stick around?

This is the big question; how long will the pattern last, does it carry through to Christmas and to the end of the month?  As usual this beyond 15 day period is always very hard to predict; to far out for the big three, more reliable, long range models to predict, and too far out for the couple of very long range models to take them seriously.

One such very long range model is the CFS.  The model has the ability to see to the end of the month into January, but at what reliability?  It has got better over the past couple years, but still nothing you would want to bet your house on.

The last couple of runs (although flip flopping from the early morning run and the late day run) have suggested the amplified pattern relaxes towards the end of the month and pacific air returns, along with the threat of GLC cutter storms slicing west, bringing rain to Ontario.  On the model , over the past couple days the pattern relax with cutters has moved from the near Christmas time frame, to towards the end of the month, but do continue to appear.

This is certainly possible, as the lack of support from the teleconnections (the neutral PDO, a neutral NAO and building SE ridge with little up stream blocking) could lead to a return to a warmer, wetter patterns.  However, a slight relax of the cold, allowing storms to stop becoming coastal but impact S. Ontario seems more likely, especially after Christmas.

Just for fun though, here is one of the better CFS runs showing almost all of Ontario with snowcover (in inches) come 8 am on December 25th:snod.conus

The last couple recent Euro weeklies, which are usually more reliable do support the cold remaining for Christmas, however the pattern possibly relaxes a little towards month end:

Bigmt’s interpretation of the Euro model for the week of Dec. 18th:dec4epswk3

and for the week of Dec.25th:dec4epswk4

The Part 2 forecast

The possibility of a pattern relax later this month still leaves me with the feeling that nothing (except for extreme Northern Ontario) is locked in at this point.  The Dec.10th to Dec.20th looks cold with less storms, but has the ability to keep any snow that does fall on the ground.  However the Dec.21 to Dec.25th period is still unclear to me.  If I was betting, I would say the Euro is right, and the real cold air sticks around until at least Dec.26 to 27th, but I am not ready to make that call.

This will become much clearer over the coming days, and by my Part 3 mid week update, I will have a much better picture for you.

For now, my forecast remains almost the same as last week, however I have locked in North Western Ontario as a guaranteed white Christmas.  Nothing suggests that this region will warm up enough to loose their snow cover.  I also expanded the “good” chance more around the LES regions expecting to get the heaviest snow:Week 2 white christmas

Remember, this is a forecast, and does not represent the statistical averages for the regions from Environment Canada.  Here is the link to those averages which bigmt provided last week.

Stay tuned for part 3, where I can hopefully lock in more regions.