It’s been a fun ride, if you don’t mind driving in a little snow and/or didn’t need to go anywhere the past week. We’ve seen several little (or big depending on location) snow events. Of course the largest and most widespread was Monday night through Tuesday morning. Officially PDX picked up 2.5″, which now means it’s the snowiest December since 2008 in the metro area. We were overdue for a cold & snowy spell in December, they’ve been very mild lately. Although we still haven’t seen a regionwide outbreak of cold arctic air.
That cold modified arctic airmass has once again surged south and through the Gorge, into northern Oregon. At noon we’re barely above freezing in Portland with a chilly east wind blowing on the east side of town.
That cold east wind isn’t too strong, just about 5 millibars from The Dalles to Portland pressure differential. Weak sauce… right now things look like this as sea level (black lines, not the colors). Cold high pressure sending easterly wind into NW Oregon/SW Washington and a weak area of low pressure forming near Haida Gwaii
That low slides down the coastline, like previous ones have, but this time it goes inland NORTH of us. Looks like this around 10am Thursday. You like my 3rd grade drawing skills?
We get a relatively strong southerly wind later tonight through the first half of Thursday. That means the cold arctic air is replaced by a much milder airmass (temporarily). Snow levels lift up to around 2,000′ or so by midday. All models support the idea of a quick late night warmup from Portland south and all along the coastline. We’re going for a low of 23 in Portland tonight, but that’s before midnight. I expect temperatures to be around 33-34 by 7am across most of the metro area. Precipitation from this system starts as snow which will stick in some spots from the Portland metro area north. South of Portland I think sticking snow is unlikely (including Salem). The cold air sometimes lingers longer than expected in the Longview/Kalama/Woodland/St. Helens areas in this setup. I’ve seen it before. I think that’s where a quick 1-2″ is more likely.
After 10am or so precipitation turns to light showers, mainly rain in the lowest elevations. Most models look something like this
- It’s unlikely this will be a significant snow event in the lowest elevations from Portland/Vancouver southward, especially after what we’ve seen the past few days.
- Sticking snow begins between 5-8am from Portland/Vancouver area north, but for most of us I don’t expect much road accumulation due to rising temps. Snow will still be in the air for a few hours though; it may look quite snowy. I am forecasting NOTHING TO 1″ ACCUMULATION, BEST CHANCE FOR STICKING NORTH OF PORTLAND
METRO AM COMMUTE: Spots of leftover snow/ice on roads from overnight freezing, possibly a few spots of new snow, but not much. Temps rising from 32 around 6am.
METRO PM COMMUTE: Totally clear roads, except leftover snow/ice way up in hills, and possibly northern Clark/Cowlitz counties. Temps in the upper 30s.
COAST/SALEM/ALBANY: Likely clear roads & no snow tomorrow
GORGE: Snowy morning, 1-4″ new snow by midday. I-84 may be icy/snow first half of day
The low pressure system and upper level flow is perfect for heavy snow in Cascades and the Blue Mountains. Tomorrow is not a good day for travelling I-84 east of La Grande or through Cascades passes as you can see in the accumulation graphic above.
Just like Tuesday night and this morning, that chilly arctic air will attempt to surge south by Friday morning. But it won’t be quite as cold. Expect lots of clouds and temperatures only in the mid 30s Friday. New Year’s Eve should be just fine weatherwise, unless we get scattered icy spots on the roads. You can see that chilly high pressure back in control at 10am Friday with more of those nice hand-drawn features. BTW, I think in high school art class I got a C at best…really! Hmmm…
Friday/Saturday will mark the end of the cold northwesterly flow we’ve been in for about two weeks. Here’s the view overhead tomorrow
By Monday, we’ve transitioned back into an active westerly jet stream again with various model solutions for next week. All of them are wet.
Sometimes we get a big snow/ice event at the end of a cold spell. That’s when we get a deep upper-level trough offshore and low level offshore (easterly flow). That’s not happening this time. It’s a relatively quick change to southerly wind Saturday through Monday. When solid precipitation arrives later Sunday and Monday, most likely it’ll be rain. But this means even MORE snow in the Cascades. Snowpack is now well above average in many parts of Oregon. Very good news!
That’s a wrap for 2021. I’ll be off Thursday-Sunday so no blog posts until early next week. Stay safe in your travels.
Chief Meteorologist Mark Nelsen