Greetings all!  I hope your Thanksgiving was safe and peaceful. For the first time in my life, I didn’t spend the holiday in The Dalles with family due to COVID and their vulnerable immune status. Just stayed in East Portland/West Gresham/Whatever, and had a dinner with my roommate and a very close friend.

We’ve had some days in the past week with quite a bit of fog in the lowlands. This photo was taken by Justin Schepige in the King Valley area near Philomath, Saturday morning. Notice how the skies are clear above the 1,000-foot level, as they often are during Willamette Valley inversions.

Here in Portland we sometimes luck out during these inversion events, due to easterly breeze from the Columbia River Gorge.  Such was the case today:  gentle east wind kept the skies clear and allowed temps in Portland to reach 52 degrees F this afternoon (normal high is 49).  At this time of year, anything above 50 degrees with full sun and not-too-strong wind feels awesome.  I decided to seize the day and got together with fellow WeatherTogether blogger Charlie Phillips, to practice my hoop skills a bit.  But I definitely need a little more practice before my ground game can come along!

After a brief return to rain tomorrow, guess what?  We have a bunch more dry weather on the way!  But I can assure you it won’t be as nice as today.  More on the “less nice” factor below.

SUMMARY:

1.  We have a quick-moving front tomorrow morning. Rain will be quite heavy for a few hours, then we dry out again by afternoon.

2.  The first 7 days of December look VERY dry.  Instead of the usual parade of Pacific storms, we have an unusually strong upper-level ridge covering much of the Western U.S. and Canada.  This means mostly clear skies and above-normal temps in the middle to upper atmosphere.

3.  The lowlands will likely see some inversions, or “fake cold,” beginning Tuesday and continuing most of the time through at least the beginning of next week. That means fog, stagnant air, and relatively chilly temps for the lowlands once the inversion is in place.

4.  There’s a chance we could see some rain again next week (i.e. 8-9 days away).  Or we might stay dry and inversion-ish through most of the first two weeks of December – kind of like what happened in 2017.

THE PATTERN:

Inversions in the Pacific Northwest take two forms.  One is the summer marine layer.  It is caused when the strong sun heats the interior of the North American continent, and results in lower surface pressure over the land.  Cooler, higher pressure over the ocean begins to push onshore, causing a layer of cool air to ‘slide under the edge’ of the hot continental airmass.

But in late fall and winter, our inversions are instead caused by cool air pooling in the valleys and basins on long, clear nights.  The weak winter sun is unable to warm the surface effectively, so the layer of cool air remains and continues to accumulate as the pattern persists.  These inversions are marked by high pressure over the continent.  In the case of the Pacific Northwest, that high pressure is usually centered over the Columbia Basin.

The Columbia Basin is nearly a perfect geography for especially strong inversions.  It is a large basin surrounded on all sides by much higher terrain: Cascades to the west; Rockies to the north and east; and the Blues and Wallowas to the south.

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Tomorrow’s system will actually wipe out the high pressure in the Basin, but only for a day.  A system from the Pacific aims for Vancouver Island, but it drags a fairly wet front onshore tomorrow morning as well. It will be a quick dump of rain; Portland will likely see 1/3 to 1/2 inch from this:

By Tuesday we dry out again as the upper ridge reasserts itself. High pressure will very quickly begin to develop over the Columbia Basin. By 10am Wednesday the upper atmosphere looks like this:

But the lower levels of the atmosphere look like this:

The cold pool of air creates very high pressure east of the mountains. That, in turn, drives a very strong east wind through the Columbia River Gorge.  Gusts on Wednesday could reach 60-80 mph on Wednesday in the really windy spots (i.e. Corbett and Crown Point).

This ridgy pattern with warm air aloft and “fake cold ” in many lowland locations, will persist through next weekend. Perhaps the cold pool weakens slightly later in the week, but I wouldn’t get my hopes up for many “warm” days in the lowlands, by that I mean temps in the 50s.  Here are the two most recent ensemble runs for the GFS model:

One thing to bear in mind with these inversions and east wind?  The longer the high pressure and wind persist, the more chilly air they will bring into our area. Wednesday’s Gorge wind will be strongest in the eastern metro (35-50 mph gusts) , while areas further away from the Gorge only see 25-35 mph for peak gusts. But if you maintain that wind for several days, the colder air from the Gorge will gradually bleed out and cover all of NW Oregon. We could well begin this pattern next Tuesday/Wednesday with quite a few 50s across our corner of the state, especially on the coast.  Current models suggest that the wind dies down as we approach the weekend. But if it doesn’t, temps across NW Oregon may gradually trend downward, with low/mid 40s in Portland during the day.  Nights away from the immediate Gorge wind would also get colder, likely well down into the 20s for many places.

To sum up:  After tomorrow’s rain we have a fairly long dry stretch at what is normally the wettest time of year. Rain might make a comeback about a week from now, or we might be in for a two-week stint of mostly dry conditions. The longer we stay dry, the stronger the inversions and low-level cold will tend to get.

Stay warm and enjoy the sunshine!  -Karl