Rainier Pattern Arrives Tuesday

When most Portlanders think of December, they think of Christmas trees, advent calendars, and cars with those big bows on them. Certainly not sunny skies, and much less 50+ degree temperatures to accompany them!

But that’s exactly what we’ve seen to begin December. There was no measurable precipitation for the first 5 days of December in Portland, and 3/5 of those days had temperatures of 50 degrees or greater. And although it rained on the 30th, even November 29th was nice, with a high of 52 under bright sunny skies. I’m not a weather stats guru – I just know that it’s unusual to have such nice weather at what is normally one of the stormiest and darkest times of the year. When we see calm weather this time of the year, it is usually accompanied by persistent east winds or inversions with a persistent layer of worsening smog. And while we did see strong east winds Tuesday and patchy morning fog in spots this past week (primarily over the Central/Southern Willamette Valley), the easterlies were far less prolonged – and the fog/smog less brooding -than is typical during our December dry spells.

Oh, and Thursday morning had this. Note the amazing shadow cast by Mt. Hood – these volcano sunsets/sunrises, while hardly exclusive to the Pacific Northwest, are just another thing that make our corner of the world so special.

Unfortunately, our sunny skies and dry weather meant yet another round of extreme Santa Ana winds for California. Check out this incredible, terrifying footage taken of the “Bond Fire” in Silverado Canyon on Thursday.

When fire fuels and slope align #bondfire pic.twitter.com/gCGy12qZM2

— Stuart Palley (@stuartpalley) December 3, 2020

Another round of Santa Ana winds for Southern California – and Diablo Winds for the Bay Area – will begin late tonight/early Monday and persist through Tuesday evening, with peak fire danger likely occurring Monday morning/midday for Northern California and Monday evening/early Tuesday morning for Southern California.

Unfortunately, rain doesn’t look likely for Southern California anytime soon. But fortunately for Pacific NW rain fans and winter sports enthusiasts, it does look like we’ll return to a cool, wet, and breezy pattern beginning Tuesday that should persist through the weekend and potentially beyond, giving us some much needed snow up in the mountains and at least making things a bit more interesting around here for us weather geeks. 🙂

Forecast:

We’ve been in a very atypical weather pattern recently, with a big ridge of high pressure directing storms all the way north to the northern Alaskan Panhandle. But all of that will change this week, as this ridge flattens and moves to the east and we instead see a more typical La Nina pattern with a steady parade of cool, wet systems making landfall every day or two. Take a look at the spaghetti chart below… it doesn’t get much more predictably rainy than that!

The first of these systems will arrive Tuesday morning. As is often the case with the first system after a long dry spell, it won’t be particularly intense, as it will become weakened and directed to the north as it tries to punch through the outgoing ridge of high pressure. It should, however, be a slow-mover, so I wouldn’t be surprised if a steady, light-to-moderate rain sticks around through the entire day.

Wednesday should be a break between storms, but a stronger front should come through on Thursday, with showers persisting into Friday. This front will be cooler and should produce “advisory-level” snowfall amounts in the Cascades – nothing heavy enough to warrant a “Winter Storm Warning,” but definitely significant enough to affect travel and bring some fresh snow to the resorts.

But it’s the Friday/Saturday period that intrigues me the most. Some models show a pretty darn wet system moving inland. Unfortunately, this system has more of a southwesterly trajectory and may be a bit warmer than the other systems, and this can be seen in the rise in the temperature around the 12th shown in the spaghetti plot above. It’s still pretty far in advance so the details will inevitably change as we get closer. But it’s something to keep an eye on.

We’re really getting out into “extended” territory with next week’s forecast, but most models show a classic La Nina pattern, with a flat ridge of high pressure in the North Pacific and a trough over the Pacific NW. This means cooler/wetter-than-average weather, with lots of snow in the mountains. We’ll have to see how this plays out – lord knows I’ve been burned on many an extended forecast before!

But I’m cautiously optimistic. La Nina’s effects become more pronounced during December and really become visible after January 1. And it’s not like our typical weather this time of the year is warm and dry either. 🙂

Thanks for reading, and I hope you are looking forward to the pattern change as much as I am!
Charlie

Featured image: rain falling on a field in southern Estonia. Credit: Aleksander Kaasik