Brush With Lowland Snow?

Guess what? It's time to dust off the snomometer 

It has a pulse. 

Our first non-zero chance of lowland snow creeps into the forecast for parts of Southern BC for next week, and a potential modified arctic blast, but significant uncertainty remains. 

But first, some fun. A refresher on the snowmometer scale:

10 point scale from 0 (no snow threat) to 10 (army assistance required). In brackets, I've included how many times a year this often is the case just to help perspective

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0- sNOw chance. Sorry, not going to happen. (335-340 days/year)

The only place you'll find snow is on one of our beautiful local ski hills

1-2 Conversational Snow (5-10 times/yr)

 

  • Typical conversational snow risk,  and often local Vancouver blogs will begin to highlight (i.e. build hype and generate revenue) these levels of marginal snowfall risk 
  • This is at its core conversational snow and nothing more, for now...
    • This isn't to say a significant event can come from a 1-2, and a high impact event with very low confidence will likely get a lower score as well

 

3-5 Nuisance Snow (4-5 times/year)

  • The level of the snowmometer has long moved past the conversation around the water cooler and is dominating all local news sources
  • Tire shops have been inundated by the locals rushing who vow never to join the summer tire club
  • The first stage where you might see your helpless neighbour on bare all-seasons sliding his way along Boundary Rd.
  • Buses to SFU may begin to struggle with the snow with thousands of students praying that classes may be cancelled or delayed
  • NO snowfall warnings have been issued by Environment Canada with amounts generally less than 5 cm (locally higher)

6-7 Impactful Snow (1-2 times/year)

  • Okay folks, we have medium-to-high confidence of some fairly significant snow for this neck of the woods (locally 10 cm+), but I necessarily wouldn't run to the store to buy milk and bread
  • School delays are now likely for portions of Metro Vancouver (higher elevations) along with substantial transit delays
  • Snowfall warnings are likely issued or will be issued by Environment Canada

8-9 Crippling Snowfall (2-3 times/decade)

  • A once in 5-10 year snow event for the South Coast with significant amounts 30 cm+ forecast
    • may become especially treacherous with warming temperatures, localized flooding and intense ponding on roadways as storm drains are clogged with slush
  • Numbers at or above this value will not be used lightly, and may never be used during the life of this blog 

10 - #SNomg #Snowmageddon (2-3 times/Century)

  • Only one or two analog exists, including the Blizzard of 1996 in Victoria, BC
  • Lack of transportation, significant structural damage and accumulations in excess of 50 cm + 

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What is the current risk for a significant snow event for the South Coast: 

Current risk assessment (Probability- Unlikely: Impact - Minor) 

Current risk assessment (Probability- Unlikely: Impact - Minor) 

What are the Models Saying?

Let's use a model that addresses confidence and uncertainty. This particular ensemble model highlights the level of uncertainty with the yellow error bars highlighting the range of the possible values; the larger the range the more uncertainty in the forecast.

It's as simple as that.

Here's an older forecast from earlier today, showing such a wide range of temperatures.

The new forecast is fresh off the press. Let's dive in

The error bars are shrinking, ever so slightly, indicating models are trying to attempt to handle the cool arctic air coming down from northern Canada next week and other models are jumping on-board this solution. Good news for those who like cooler weather. 

The error bars are shrinking, ever so slightly, indicating models are trying to attempt to handle the cool arctic air coming down from northern Canada next week and other models are jumping on-board this solution. Good news for those who like cooler weather. 

Will this forecast environment support snow down to sea-level?

72 hours window for snow? One parameter we could look at is a thickness, or difference between two pressure regions. In this case the 1000mb and 500 mb level. The smaller this thickness, the cooler the air mass; 525 dam is typically when YVR starts to see snow, 50% of the time

72 hours window for snow? One parameter we could look at is a thickness, or difference between two pressure regions. In this case the 1000mb and 500 mb level. The smaller this thickness, the cooler the air mass; 525 dam is typically when YVR starts to see snow, 50% of the time

How is this expected to come together?

High confidence of a ridge setting up in the NE Pacific south of Alaska

Snapshot of the latest run of the GFS highlighting a powerful blocking ridge in the NE Pacific, with the Lower Mainland on the right side, being very susceptible to modified arctic air

Snapshot of the latest run of the GFS highlighting a powerful blocking ridge in the NE Pacific, with the Lower Mainland on the right side, being very susceptible to modified arctic air

Once the blocking ridge is in place, the digging trough down the backside of the ridge can advect much cooler air and the chance of lowland snow. When the block is broken (undercut by a low pressure system), an overrunning snow event is possible if the arctic air is still fully entrenched in the region, before changing over to rain, but this solution is farfetched at this point.

Its very debatable and unlikely that the ridge over Alaska remains in place, and will likely shift east towards the end of end week.

Taking a look a little above the surface, you can truly see the arctic air attempt to move into the lower mainland. 

Here, the 12Z GFS is showing the coolest air heads mainly east of the Rockies, but the gate is open - theres a hole in our defense. BC's mountainous terrain typically holds back the coolest air, while the air also warms as it descends toward sea-level, but confidence is increasing for several days of temperatures significantly below zero

Here, the 12Z GFS is showing the coolest air heads mainly east of the Rockies, but the gate is open - theres a hole in our defense. BC's mountainous terrain typically holds back the coolest air, while the air also warms as it descends toward sea-level, but confidence is increasing for several days of temperatures significantly below zero

Finally, I'll highlight an epic area of high pressure expected to develop under such a setup; looking above a very strong 1046 mb high is forecast to develop over western Canada. Think of this as the pump -- circulation around the high will likely bring some gusty outflow conditions through some of the more prominent inlets along the coast of B.C.

Some atmospheric signals are not suggesting this be a very long-lived prolonged cold, but rather just a brush. 

Are you EPO negative? 

The EPO (The Eastern Pacific Oscillation)

The EPO is forecast to turn SHARPLY negative the next week or so, and arctic air has an opportunity to sneak in down the backside of the building high south of AK.

The EPO is forecast to turn SHARPLY negative the next week or so, and arctic air has an opportunity to sneak in down the backside of the building high south of AK.

Unfortunately, a negative EPO is not effective at building snow, because it lacks the troughing pattern over the NE pacific which is favoured during a positive EPO event.

Current EPO forecast

OFF. THE. CHARTS. 

OFF. THE. CHARTS. 

What this pattern suggests is there's a predisposition to colder air from Siberia infiltrating parts of Central Canada; how far west this cold air travels remains to be seen...

At this point, the high pressure ridge over AK is calling the shots. If it sets up a little further west, Vancouver is looking at a full-blown arctic blast.

But, this is just one teleconnetion. Snowpack, ocean temperature, and many other factors can play a role in driving the large scale patterns across the globe. 

AO (Arctic Oscillation)

  • Can be a good indicator of some cold air outbreaks in eastern Canada too, which would generally leave BC under a ridge of high pressure. See for yourself:
A lot of uncertainty, but no clear signal for a negative AO, at this point

A lot of uncertainty, but no clear signal for a negative AO, at this point