Light northwest flow is persisting over the South Coast today with an arctic air mass and relative higher pressure over the interior of British Columbia. A very moist warm front moving towards the South Coast this evening/overnight, with weak outflow conditions will be present for regions of the south Coast including the Fraser River gap. This may produce a classic overrunning snowfall event for the South Coast
Snow forecasts are extremely challenging for the South Coast, for a number of reasons and will test even the most seasoned forecasters.
I'll highlight a couple:
1) Snow forecasts are very sensitive to precipitation amounts. For example, you would have a tough time perceiving the difference between 5 mm and 10 mm of rain, but these precipitation amounts can easily turn a mundane 5 cm into a more treacherous and significant 10 cm snowfall event, depending on snow density
- We have medium-to high confidence we can predict the amount of precipitation for this event, so this isn't where the majority of our uncertainty is coming from
- In other words, getting the precipitation amounts exactly right is near impossible. That's why you often see such a range when it comes to forecast snowfall (ex. 5-10 cm), as topography can play a significant role as well
- Snow varies in density, so 10:1 may not be the ideal snow ratio (for this event 5:1 may be more apt)
- Again we know this will be a wet marginal snow event, so a lower snow ratio will be used by forecasters to try to create the most accurate forecast possible
For those who need a refresher on what a rank 6 snowfall would entail:
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 wouldn't necessarily run to the store to buy milk and bread
- Substantial transit delays may be possible for higher elevations including transportion to SFU
- Snowfall warnings are likely issued or will be issued by Environment Canada
2) Temperatures are often very close to freezing (and often slightly above) with our typical snowfall events
- Very critical with snowfall forecasting. Then there's evaporation and cooling processes that can drop the temperature close to freezing, even if precipitation starts when it's 2 or 3 degrees outside.
- it's also important to watch the wet-bulb zero temperature during snow events.
Watch how the sounding changes over time:
- Here is where we're running into serious problems when it comes to our snow forecast; many regions along the South Coast over the next 24 hours will receive VERY little snow due to too much rain/snow mixing along the coast and the warmer waters of the Strait of Georgia interacting with the region; typically this is the last defences for the region to avoid a snow event
Current water temperatures for the region with Georgia hovering around 6C.
- When the SE component/winds pick up; precipitation will likely change to rain for the immediate coastline for east Vancouver Island, but will remain as wet snow inland
A big player to help us cool the atmosphere will be diabatic processes such as evaporative cooling; which is contrasted by adiabatic theory or parcel theory where absolutely no energy exchange occurs within the parcel.
For a snow event to occur in the Lower Mainland an energy exchange isn't optional, but absolutely required to allow the wet bulb zero temperature to approach zero and initiate an accumulating snowfall.
Precipitation will likely begin as rain or a rain snow mix for most regions; and as the snowfall interacts with temperatures initially above zero this evening the snow flake will melt. Meteorologists will use profilers to determine the elevation of this melting by looking for bright banding on soundings.
Now, melting snowfall requires an immense about of energy and an energy exchange is taking place between the atmosphere and the hydrometers. In layman's terms, think of the rain zapping energy out of the atmosphere, and in turn a falling temperatures will occur.
Another common process is evaporative cooling, where snow flakes fall into very dry air, and as they evaporate they decrease the temperature of the surrounding air. This won't be the major player for this snow event as the atmosphere is relatively saturated at this point, although some drier air will be infiltrating the coast through some valleys and inlets.
Forecaster's run the risk of being too specific with snow events such as what's about to transpire; consequently,I believe a forecast of 0-20 cm is appropriate for the Lower Mainland. Take a look at the potential snowfall gradient shaping up, looking at advanced probabilistic guidance. Below is the probability of receiving in excess of 10 cm of snow, depending on your location.
As expected, lower amounts for Delta and White Rock and higher amounts for West Vancouver, Coquitlam, Maple Ridge Mission and Abbotsford.
To account for a little bit more uncertianty and what we could expect if the system and snow overperforms a little bit.
Let's take a look at the 75 percentile for snow:
Weak Outflow? Blocked Flow? A Barrier Jet?
- Some insight from meteorologist Chris Doyle
This includes the barrier jet and blocked flow which will also play a significant role in this upcoming event; the warmer air advection from the south and the blocked flow travelling west will create a tricky situation for the South Coast. At this time, it's tough to predict the exact strength of these processes and if the blocked flow dominates; the system may produce a more widespread snowfall
GEMREG: (10:1)- Note the lack of projected snowfall along Georgia Strait (much more mixing will be occuring)
A higher resolution and more zoomed in cropping reveals a little bit more and you can clearly see elevation begins to come into play:
There's higher elevation or spine that bisects a portion of the Lower Mainland
Want proof? Here's a representation of Vancouver with an 80 metre sea level rise:
Rough snowfall forecasts by region
1) Vancouver Island
- Campbell River: Trace-5 cm (locally 10-20 cm inland (West of HWY 19 towards Campbell River Airport)
- Comox Valley: Trace-5 cm with locally higher amounts at higher elevations (Ryan Road etc.)
- Cumberland: 5-15 cm
- Parksville: 2-8 cm
- Port Alberni: 2-10 cm
- Nanaimo: Trace-5 cm (with locally higher amounts at higher elevations)
- Lake Cowichan: 10-20 cm
2) Sunshine Coast
- Powell River: 2-5 cm locally higher amounts away from Georgia Strait
- Sechelt: Trace-2 cm
3) Lower Mainland
- North Vancouver: 5-15 cm; (lower amounts closer to sea level)
- Abbotsford: 8-15 cm
- Maple Ridge: 5-10 cm
- Surrey-White Rock: Trace-5 cm for White Rock with locally higher amounts along the Spine and North/East. Surrey locally 5-10 cm +
- Port Moody: 5-8 cm+(elevation dependent)
- YVR; Trace-5 cm
- Vancouver Proper: Trace to locally 10 cm at higher elevations (SE from UBC towards Surrey)
Snow Levels/Mountain Snow
- Although snow levels will rise considerably for regions in Washington State the mountains of British Columbia will likely receive gobs and gobs of snow during the next 48-72 hours
- Some of the heaviest snows of the season will be falling on the ski slopes and over the next 72 hours some mountain resorts could be facing near 75-100 cm of fresh powder
A possible update will be needed for this evening. Use the hashtag #BCstorm to report weather observations and tune to the Weather Network for live coverage.