Hope you all had a very happy Thanksgiving!  The weather sure has been quiet and beautiful. Yesterday Tuesday was the 6th consecutive day of mostly sunny skies for the majority of our region. Today the clouds moved back in on the west side.  Daytime temps during this dry stretch have been near normal for most of us (upper 40s / low 50s), but nights have been very cold…about the coldest we can get without any arctic air at our doorstep. 

Even PDX had six mornings in the 27-30° range, beginning last Friday. Vancouver Airport was in the 20s every night and dropped as low as 23.  Also lots of low 20s and even a few upper teens in the Willamette and Rogue valleys. Even California’s Sacramento Valley saw some subfreezing temps, despite being near 60° during the day.  And the high deserts of Central Oregon?  Lots of teens and single digits at night!

This has meant lots of windshield scraping for those of us who have early morning commutes.  Also…the nighttime chill seems to have finally finished off the last of the autumn colors, which peaked flamboyantly in early November this year.   Here is my roommate’s favorite winter cover crop, taking a nap during the frost and then waking up as soon as it thaws out.  

But why did it get so cold this past week?  The obvious answer is:  long nights and clear skies. That allows radiational cooling to dig in at the surface, even if the airmass overhead is relatively mild.  In addition, back on Thursday-Saturday we had some coolish-dry flow from the NNE.  That lowered dewpoints across the region and limited fog formation for a few days, allowing temperatures to plummet at night more than if we had widespread dense fog throughout the event.  Fog and low clouds did eventually develop this time, especially in the southern Willamette Valley and the Columbia Basin. 

These cold nights have largely erased the running warm anomalies for the month of November in Portland. Combine with the normal September and relatively warm/dry October, and Fall 2023 should end up slightly warmer and drier than normal.  Probably the biggest anomaly of the season were all those extremely warm nights in early/mid October; that had the effect of delaying our foliage peak by nearly two weeks.

Anyway…looking ahead, today was the transition day between our glorious late November dry spell and a very active pattern for early December. Tomorrow we just have a few light showers coming in during the morning and midday.  The showers will probably begin as wet snow in the Columbia River Gorge, east of Cascade Locks…the low-level cold air is deeper and colder over there.  I’m not expecting any travel problems on I-84 in the Gorge, though higher elevations may see minor impacts.  Here in Portland and Vancouver it won’t be cold enough for snow, but it will be a very cold and wintry rain thanks to cool air at the surface. 

Friday and Saturday should be the first big snowstorm for the Cascade passes, as a strong and cool system moves in from the North Pacific. Some models are suggesting 2 feet or more at pass elevation, perhaps even 3 ft. for the upper ski resorts. Midday Friday to midday Saturday seems to be the prime time for heavy mountain snow.

Skiers and snowboarders shouldn’t get too excited, though. Sunday through Tuesday next week looks very wet and quite warm, as a heavy atmospheric river sets up along the PNW and pulls up subtropical moisture from near the Hawaiian Islands. That will melt a significant amount of the freshly fallen snow, and any unmelted snow will be saturated with rainwater, greatly reducing its ski quality. 

Here is the 10-day ECMWF snow total map.  Don’t let those heavy totals fool you; I bet at least half of the snow below 6,000 feet is wiped out by the warm AR early next week. 

At lower elevations, the biggest threat will be the risk of flooding.   The 12z ECMWF shows about 6” of rain in the western valleys by 4am next Wednesday morning!

It’s always hard to pinpoint rain totals during AR events. The band of heavy rain is often only 100-150 miles wide and can oscillate from north to south. Rather than equally widespread flooding, more likely we some areas get saturated with very heavy rain while others just get “normal wet” conditions with no flooding. We will just have to wait and see.  Keep posted to local weather groups and TV over the next couple days as more details unfold.

Between the cold nights we just endured, and the upcoming heavy snow and rain, I think it’s safe to say that the season is turning from autumn to winter. Friday is December 1…the first day of meteorological winter. Now is the time to make sure your vehicles and home are properly prepared for the season change!  

Karl