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).
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...
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:
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:
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...