It's a brand new year; we're 4 days into January 2021. What has changed over the past week? Not much; it's still mild and wet. We ended up with near average December rainfall in Portland. For the region as a whole it's been a very wet start this month. Astoria around 5" in just four days!
And 1.50-3.00" in the metro area
Not enough for flooding, but the ground is saturated. Precipitation the past month looks like a classic La Nina winter setup; a wetter than average northwestern USA, but very dry Southwest
Snowpack is running a bit below average over and west of the Cascades.
That's not due to lack of precipitation, but "warm storms". Or at least warmer than average. An example would be this past weekend. Just a few inches fell at Government Camp and snow depth hasn't changed. But 2 feet of snow fell up at Timberline Lodge (6,000') which stayed above the snow level most of the weekend. This is more of a typical wintertime El Nino setup...plenty of precipitation but mild storms. The last time I remember such a mild La Nina year was 2000-2001. At least in 2017-18 we had cold arctic air move south into the USA and give us a cold/icy/snowy Christmas. You see that trend continuing this first week of January. See forecast snow levels, based on 850mb temps off the ECMWF model
Some snow is again falling down to the passes right now, but a warmer system arrives Tuesday night and Wednesday morning. We do get a nice cold system Friday; should be a good snow producer for lower elevations in the mountains. You see another snow level spike the middle of the upcoming weekend. Although for now most of the precipitation with that system is forecast to fall in the cooler airmass with/behind the cold front.
What's ahead? More of the same for at least the next 10-15 days. Notice the ECMWF ensemble forecast high temps keep our highs near/above average. Ignore the last day, it stops in the early morning (when it's colder)
Almost constant westerly flow off the Pacific ocean is the culprit, in fact upper-level heights RISE a bit next week leading to even warmer temps! A break from the rain? No more than a day here and there; hardly any gaps on those same 15 day ensembles
You probably already know what that means for a lowland snow chance...essentially near zero through the first half of January, possibly all the way to around the 20th. It's somewhat startling to see not one ensemble member giving Portland significant (or any) snow in the middle of a La Nina winter.
The ECMWF ensemble 850mb temperature forecast shows almost no members below -6/-7, what we need to see to at least get snow down into the West Hills.
As for rain, intensity will probably back off in the next 10 days as upper-level heights rise. Notice a 3-4" forecast for Salem, that's just typical wet, nothing too heavy.
To wrap it up, the message remains the same as we head into the 2nd half of winter...mild and wet through the foreseeable future.