December 2023 is now a wrap!  I’m not going to do a year-end review of 2023 tonight, but maybe later this week….

Highlights:

1.  December was extremely warm and quite wet! In fact the warmest at PDX in over 70 years.

2. We’re going to see a pattern change to cooler temperatures and lower snow levels, beginning this weekend and continuing well into the 2nd week of January.

….Let’s take a look at December first.  There was a lot of talk during the month about how this might end up being a record warm December. Well…not quite, but extremely close! We were only 0.3°F shy of the record (1950), and tied the record for 50°-plus days (20 days, also in 1950).

Notice that the warmth was not equally distributed throughout the month. The first week was historically warm, with a new all-time record high temperature for the month of December (previous record was 65°). Then we were near normal to a few degrees above, for the rest of the month.  The week right around Christmas was the only time it really felt like normal December weather in the valleys. There were enough inversions and east wind to keep us seasonably cool for several days.

The month was nearly 3″ above normal for rainfall, but more than 5″ fell in the first week alone under multiple atmospheric rivers.  Then we eased back into a ‘normal-ish’ winter precip regime, with periodic rain systems separated by a few dry days in between. That was a fortunate thing; had we gotten another 5 or 6 inches in the 2nd week of December on top of all the rain the week before?  We’d have been looking at serious flooding & mudslide problems similar to 2015.

Moving on…today was a nice start to 2024, with partly sunny conditions and highs in the upper 40s. Tomorrow through Friday should be about as typical as it gets for January:  some rain showers (nothing too dramatic) with temps near normal. But then we see a shift in the upper pattern beginning Saturday the 6th:  cooler systems start coming in from the northwest as a trough forms in the Western U.S.  The 12z ECMWF for Sunday morning clearly shows the shift afoot:

In fact, the 18z GFS is showing even deeper troughing over the West at the same time:

As of right now there’s no clear signal for a full-blown cold blast in our region, though a few recent model maps have hinted at modified arctic air coming down at least into eastern Oregon and Washington. But for several days models have been pretty consistent on bringing in cooler airmasses, the kind that can bring snow down to 1,500 or 2,000 feet. Check out the 18z GFS (sorry!) ensemble chart, with a pronounced cooling trend beginning the 6th and continuing 5-7 days, possibly longer. Ignore the crazy wiggling lines on the rightmost one-third of the chart; it’s la-la land and this is the “Drunk Uncle” run, after all…

The 12z ECMWF is similar:

As of now it’s too early to tell if any of the bitter Canadian air will dive into our area. But I think it’s clear that we have an extended period of cooler weather systems and mountain snowfall coming up beginning this Saturday. It will probably be the best Cascade snowfall so far this winter:  good news for snow sporting enthusiasts!

This does NOT mean we are in for chilly weather from now through next March…the seasonal outlook is still very warm. But even a very mild winter usually has at least a brief period where colder air slips in for a few days. If that’s the case, then we would expect mild temps to return later in January.

For now, though…it’s time to celebrate the first real “wintry” regime of the season (whether or not it gets cold enough for valley snow).

Happy New Year!

Karl