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...

Tuesday's Tricky Snowfall. Yes, it will probably snow.

The South Coast of B.C. is one of the most difficult places in the world to forecast snow—it’s a delicate balancing act. You need arctic air and you need moisture.

Often, only one of those ingredients is in place, or one is extremely marginal like what is expected on Tuesday. Arctic air is very marginal, as most of it is bottled up in the Interior and away from the coast.

It’s no illusion that our computer models don’t have the appropriate resolution to effectively differentiate such a tight snow accumulation gradient along coastal communities.

Have you ever seen snow vary like this? My guess is yes, if you’ve spent several winters in the Lower Mainland you understand the extreme variation I’m referencing:

Source: Reddit

Source: Reddit

There are a billion things wrong with forecasting using a single, low resolution deterministic computer model. Look at this…

Courtesy: WeatherBell

Courtesy: WeatherBell

This is the equivalent of weather throw-up.

These automatic snowfall outputs are widely shared online, but there’s a problem—it assumes a 10:1 snow ratio which is a crude average that will not apply on Tuesday.

30 mm of liquid water equivalent (imagine melting down the snow) could give you roughly 30 cm.  Tuesday, we can expect a snow ratio near 5:1 which would only give us 15 cm of snowfall for similar amount of liquid water equivalent...

This is the typical back-breaking snow Vancouverites and local hospitals brace for.

Another product that we can use as a guide is one unique deterministic model that has superior resolution. Then, we’ll finish this post with the ensemble approach and some snow forecast amounts.

Courtesy: Weatherbell

Courtesy: Weatherbell

The model above looks fairly reasonable. If you have plans to travel Tuesday across the Malahat and north on Eastern Vancouver Island please check webcams ahead of time and consider postponing travel.

This model also attempts to resolve some of the higher terrain across the Lower Mainland, but when precipitation intensity is highest Tuesday the snow level will likely fall to near or temporarily at sea level. When precipitation eases, it will change back to a wintry-mix or mainly light rain and drizzle for lower elevations.

Okay, the grand finale:
 

WPCProb.PNG

This is far from perfect, but it does have its merits.  

Anywhere there’s a colour you can expect some local snowfall accumulation near to these regions.

This would also likely extend up to Campbell River and the Comox Valley.

Snowfall Forecasts (use as a rough guide…this forecast is only low-to-medium confidence).

Vancouver Island

·       Campbell River: Trace-5cm   

·       Comox Valley: Trace-5cm (higher amounts in Cumberland)

·       Nanaimo: Trace-5cm (higher amounts above 200 m)

·       Cowichan: 2-5 cm (10-20 cm for Lake Cowichan)

·       Malahat Summit: 5-15 cm

·       Victoria (Saanich and YYJ): Trace-2cm   

·       Downtown Victoria: No accumulation expected

Lower Mainland

·       Downtown Vancouver: 0-2 cm

·       YVR: Trace amounts in highest precipitation rates (otherwise no accumulation expected)

·       Surrey: Locally 2-4 cm over higher terrain  

·       Tri-Cities: 2-5 cm (locally higher amounts possible)

·       North Vancouver: 2-5 cm over higher elevations; there is potential to overachieve here

·       Abbotsford: Trace-5 cm