First snow threat of the season for Vancouver, B.C.

Thursday, December 1, 2016, 10:18 AM - Currently, downtown Vancouver and more specifically Vancouver International Airport (YVR) has been over 1000 days without a significant snowfall; consequently, one of the greatest stretches in recorded history with this snow drought near sea-level.

We have to look back to Feb. 24, 2014 the last time Vancouver reported several hours and several centimeters of snow.

However, things are changing. A more significant blast of arctic air is on the move. Here, I'll show you what I mean:

WINTER IS HERE: With La Niña helping shape global patterns what will Canadians expect from winter? Find out with The Weather Network’s 2016 Winter Forecast | FORECAST & MAPS HERE

For a perfect blast of cold air and below seasonal temperatures to reach coastal B.C., a near-perfect set-up in the upper atmosphere has to take place.

Remember the mega ridge that hampered British Columbia the past several seasons at times, also known as the ridiculous resilient ridge? Well, we need a mega-ridge in Alaska.

When I'm watching for Arctic air and snow potential in Vancouver, we need a massive ridge to develop and the pattern to amplify to allow cold air to move south. Below is typically what B.C. looks like in the winter... a zonal flow. 

But where's the massive ridge?

Well, its humble beginnings are just east of Japan. Watch what happens to the ridge over the next 72 hours. Our forecast models are excellent for the upper air pattern in this time frame: 

I know it's tough to believe, but the blocking pattern over western Alaska is absolutely key to getting cold to migrate towards the Pacific Northwest and allow the cold upper trough and arctic front to dig south and west. Ideally, you want this upper region of high pressure to lock in between 150-160 W over Alaska and hold for a long duration to allow for multiple reloads of cold arctic air to the coast.

First snow threat

As the pattern amplifies and cold arctic air begins to filter down the coast cooling begins late Sunday.

We know a trough will also be hanging out around the coast at this time, but there is significant uncertainty as to how much precipitation will be lingering as cool air spills through the myriad of valleys and inlets in southern B.C. including, the infamous Fraser River gap.

The aforementioned low pressure is a classic 'inside slider' where a low hugs the coast. If it tracks too far inland, it doesn't pick up enough moisture, too far and west and the precipitation stays off the coast. I see most regions transitioning to wet snow starting early Sunday night at higher terrain and then closer to sea level by Monday.

Now, we have high confidence in the atmosphere drying by Tuesday, and then a possible overrunning snow event late next week, where a moist pacific frontal system collides with the shallow layer of arctic air. This type of setup has been known to provide Vancouver's largest snowfalls...but let's get through this first brush with winter first, before I analyze our second potential snow-maker by the weekend.

But, with all of this I have to move the snowmometer up from it's traditional position of 0. Yes, an aggressive 4 right out of the gate, but I've looked over the model data and determined this is a good initial start. There is some potential for a higher number over the next several days and next week.

For those who are new, here's a quick refresher on Vancouver's snowmometer:

• 0 -- sNOw chance. Sorry, not going to happen. (335-340 days/year)
• 1-2 -- conversational snow (5-10 times/year)
• 3-5 -- nuisance snow (4-5 times/year)
• 6-7 -- impactful snow (1-2 times/year)
• 8-9 -- crippling snowfall (2-3 times/decade)
• 10 -- #snowmageddon (2-3 times/century)

Check back for updates as we continue to monitor the forecast.