The National Weather Service and Environment Canada have both released discussions and special weather statements entertaining the idea of a high impact snow event this weekend for portions the Pacific Northwest. Area's north of Seattle have the highest chance of seeing accumulating snow, but fine-tuned details of who sees the most snow may not be known until early Saturday, if at all, as models try to come to a consensus.
what we know
On Saturday an upper level trough will sag down from central British Columbia, as shown below on the 500 mb GFS chart:
what we don't know (it's a lot by the way...)
1) Exactly how cold the air mass will be
- Models show varying strength of NE outflow out of Fraser River Gap and Central BC. If this is even a little weaker than expected, a cold rain will anticipated, with only snow at higher elevations...This will lead to a busted snowfall forecast
2) How much precipitation will fall/where
- All depends how this upper level feature/weak system develops and if the spin and lift will be strong enough to develop off Vancouver Island, as forecast by the GFS below. This isn't a typical strong surface low aiming at the coast, which will help limit the amount of warm air advection with the event off the Pacific, since it's coming from the NW
A simple explanation of vorticity is a counter-clockwise or clockwise spin, with counter-clockwise spin representing lift and possible precipitation (shown over Vancouver Island Saturday night as positive vorticity in greens).
This frontal forcing will be fed by the NW jet stream, which will be enhanced this upcoming weekend with a jet streak (area of faster moving air) embedded in a jet stream.
- Saturday PM to Sunday PM (very tentative)
First look at accumulation (Gasp!).
Take these with a grain of salt...
- Vancouver Island: 5-20 cm
- Vancouver: 2-10cm
A lot of uncertainty, hopefully this range will decrease a little bit closer to this event.