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