Significant Lower Mainland Snow

Well, another round of significant winter weather is shaping up to move into the South Coast.

I want to break down timing and impacts, to better help you make decisions about potentially leaving work early or planning your weekend around the rounds of active weather.

Sorry, I won’t a note to your employer or school, though.

What we know.

A weak coastal low or trough will slide in from the northwest, but will pick up more than enough Pacific moisture.

But, weak doesn’t mean low impact—weaker lows tend to keep the modified arctic air in place; consequently, this low pressure system has significant snow potential. Northwest flow always increases the risk of lowland snow, and there’s a couple rounds on the way.

TIMING

Friday AM

·        No issues with morning commute for Vancouver

·        Shortwave still NW of the Lower Mainland

·        Precipitation and periods of snow developing late morning on Vancouver Island

·        Late morning 9-11am you’ll notice the cloud deck lower as snow begins aloft across the Lower Mainland and over higher elevations. Likely still dry in the Fraser Valley

timing1.png

Friday PM

  • Periods of snow spreads across the entire South Coas
  • Risk of ice pellets and rain/snow mix late afternoon at sea level, with Victoria likely to change over to rain into the late afternoon and evening.
 Canadian Model, highlighting the weak trough offshore, and showing all regions likely starting as snow, before a wintry mix and rain develops for southern Vancouver Island and areas immediately beside Georgia Strait

Canadian Model, highlighting the weak trough offshore, and showing all regions likely starting as snow, before a wintry mix and rain develops for southern Vancouver Island and areas immediately beside Georgia Strait

Now, for the fun stuff…

Friday Evening-Saturday AM

  • Heaviest snowfall rates expected across the Lower Mainland
  • Friday evening commute may be particularly impacted. If possible, leaving a couple hours earlier may help significantly
  • This burst of snow is well advertised on a wide array of computer models, giving moderate to high confidence
  • 10-15 cm across parts of the Lower Mainland, especially north of the Fraser River. I even think a couple localized amounts to 20 cm is possible for higher terrain of North and West Vancouver by Saturday AM
  • The secondary trough develops over the region giving mostly snow across the lower mainland through Saturday morning. Precipitation is expected to be lighter across Vancouver Island

Saturday PM

  • Snow expected to ease early on Saturday, with a mostly dry day expected for the region with a new wave of wintry-mix expected before Sunday morning which could once again bring locally heavy snowfall accumulations

 

Image above, the probability of seeing greater than 10 cm of snow a good threshold for a significant snow event for the Lower Mainland…

Precipitation shadow for East Vancouver Island expected to bring lower totals, with most cities on the east island generally seeing under 5 cm. Just 1-2 cm of snow is possible for Victoria, as precipitation intensity and temperatures become too marginal to support a noteworthy snow event for Victoria.

The Vancouver Snow Lottery: The Arctic Front

The Tuesday system generally performed as expected, with a couple nice surprises. 

The biggest shock was the precipitation intensity near YVR/Bellingham that allowed several slushy centimeters to accumulate at sea level; really amazing stuff, which can be attributed to diabatic cooling (process where melting snow can decrease the temperature of the surrounding environment).

Next Challenge.

An arctic front. It's a lottery—not everyone will see snow with this event. 

Now, you wouldn't normally associate weather in Vancouver with an arctic front, but our ridge is positioned offshore to allow a sneaky trough to slide down the coast with the accompanied arctic front bringing cool, continental air towards the coast.

The temperature profile of the atmosphere suggests this will fall as snow, as we will be dealing with outflow conditions (air flowing towards the coast), rather than a moist, onshore flow. 

But, we have a problem.

Moisture—or rather lack of moisture. These are often called moisture-starved fronts...and for good reason.

Let's find a time period with this arctic front where there's the most moisture available...

 Courtesy: WeatherBell. Brown shades indicate the highest moisture content late Thursday evening...barely scraping together .5 inch (15 mm) of moisture in the entire column of the atmosphere. During atmospheric rivers, it's not uncommon to exceed 60 mm or over two inches of water in the atmosphere.

Courtesy: WeatherBell. Brown shades indicate the highest moisture content late Thursday evening...barely scraping together .5 inch (15 mm) of moisture in the entire column of the atmosphere. During atmospheric rivers, it's not uncommon to exceed 60 mm or over two inches of water in the atmosphere.

A weak trough will develop southwest of Vancouver Island Thursday evening...and if the Lower Mainland is to see sporadic snow-showers it will likely fall before Friday AM.

Through the day on Friday the outflow increases and the atmosphere rapidly dries, so our precipitation chances rapidly decreases through Friday afternoon. 

There is also a possibility we could see locally a few cm of snow in the Fraser Valley with some local features such as convergence (two wind directions approaching a single location).

Using the models below we can become detectives and determine which is the most likely solution. Play spot the difference:

NAM SNOW.png
HIRESSnowYVR.png

Both models have the general pattern.

With weak outflow conditions in place, there's a higher probability of dealing with locally heavy, but spotty snow-showers on eastern sections of Vancouver Island; consequently this is a higher confidence forecast.

This is due to some enhancement with the topography of the Vancouver Island forcing air to rise in this region. 

Models are split on precipitation modelled across the Lower Mainland. These arctic fronts are notoriously tricky to forecast, but I still suspect a couple snow-showers to develop across the LM and Fraser Valley giving a localized 1-3 cm here and there...especially through the overnight into Friday AM.

The Tri-Cities and the Fraser Valley may be a more likely location for some of these snow-showers to develop.

By Friday PM, outflow conditions strengthen. Check this out:

 Yellow arrow denotes the powerful outflow winds that will develop out of the Fraser River Gap.

Yellow arrow denotes the powerful outflow winds that will develop out of the Fraser River Gap.

Fraser River Outflow develops through Friday. Some gusts may exceed gale force over open waters northeast of Victoria. Ferry service from Tsawwassen (Vancouver) to Swartz Bay (Victoria) would be most affected by the narrow outflow winds, but expect most sailings to not have any issues with the larger vessels sailing this route. 

Fairly significant wind chill values will also develop throughout the Fraser Valley and near the Bellingham, Washington region as we head towards the weekend. 

I don't want to touch the potential yet for Christmas Eve and Christmas. We'll dive into that later on Friday...