“Thermal Winter” Begins With Exceptionally Warm Pattern

“Thermal Winter” Begins With Exceptionally Warm Pattern

It begins on December 20/21 and ends March 19/20, if you follow the conventional calendar.
It begins on November 6/7 and ends February 3/4, if you go strictly by sun angle.
It begins on December 1 and ends February 28/29, if you’re into weather and climate data.

…But if you REALLY ask an expert on seasonal weather in the Pacific Northwest…..they will tell you that when it comes to normal temperatures in the lowlands, winter begins around the 20th of November and ends near the 19th-20th of February. Or as I like to think of it, the zodiacal months of Sagittarius, Capricorn and Aquarius!

Consider the following:

1. At DLS airport near The Dalles…the normal high temperature drops below 50 degrees F on November 17, and stays below 50 through February 20. More importantly, the MEAN temperature is below 41 degrees F (5 degrees C) from November 22 – February 18.

2. In the Columbia River Gorge & Columbia Basin, nearly all of our “real” cold continental airmasses – the kind that produce snow, ice and daytime temps near or below freezing – occur between mid-November and the end of February. (In exceptional years, an arctic airmass can drop south in early November or early March, as happened in 1955 and 1989 respectively.)

3. Our “fake cold” inversion season generally runs from about November 1 until some point midway through February. (This is driven mainly by weak sun angle, and thus is “centered” quite neatly around the winter solstice.)

So even though our wall calendars love to insinuate the notion that each of the four seasons is supposed to lag the solar cycle by 6 or 7 weeks? In the case of PNW winter, the lag is only about 2 weeks. Our official calendars miss the mark by a full month!

In summertime the normal temps line up quite neatly with the calendar cusps…thermal summer is only a few days earlier than astronomical summer, with September and June roughly the same temps. And in between? Four months of spring and two months of fall!

Today was November 19…and the first sub-40 high temperature in The Dalles this season. We only managed to make 39 today, due to an inversion that developed last night. Our low-level chilly airmass is going to quickly get wiped out, however, the next 24-36 hours. Remember how I said we’re entering what is normally the coldest quarter of the year? Well it sure won’t start out “normal” this year!

Instead, we have an incredibly warm and muggy pattern for this time of year, on tap for Tuesday through Thursday. Also quite a bit of rain expected at times. The culprit is an “atmospheric river” of moisture, similar to the Pineapple Express pattern where mild temps and heavy mountain rain are transported northward from near the Hawaiian Islands. But most atmospheric rivers are merely warm-temperate to subtropical in origin. Instead, this Tuesday we will see an unusual AR, which comes more directly from the south and taps into EQUATORIAL moisture:

With that kind of tropical energy on tap, one would naturally assume that the temps for the Pacific Northwest could get extremely warm Tuesday and Wednesday. Indeed, that’s exactly what’s going to happen, especially east of the Cascades where there will be more sunbreaks. In late November, a warm ridge directly over the top of us usually means Fake Cold and inversions for the Columbia Basin & Gorge. But when we’re on the northwestern edge of the ridge instead? THAT is the one scenario when unseasonable warmth can reach the lowlands.

Indeed, places slightly to our east, like Pendleton, may be the warmest spots of all. The 12z GFS operational surface map shows PDT flirting with 70 degrees by 1pm Wednesday afternoon:

Wow! I don’t think I’ve ever heard of 70-degree temps east of the Cascade crest, this late in the year. I see the official forecasts for The Dalles are going conservative and staying closer to 60. That’s normally smart thinking for our climate in late November…but still, I bet we at least see a 64 or 65 out of this if there’s any sunbreaks at all. Which is 17-18 degrees above normal!

Looking ahead….there is still no clear signal of a chilly/cold pattern on the horizon. We cool off somewhat for Black Friday and beyond, but stay pretty much average to a few degrees above through the rest of the holiday weekend.

Some model runs have tried to hint at some moderately interesting changes (i.e. colder), beginning about the middle of NEXT week. But we’re not going to hire any ‘Wishcasting Fortune-Tellers’ just quite yet!

Enjoy the wet and warmth and HAPPY THANKSGIVING!

2 thoughts on ““Thermal Winter” Begins With Exceptionally Warm Pattern

  1. Great post Karl. I especially liked your reasoning for the “real” winter for the Pacific Northwest.

    I’ve always wondered what November 1955 was like… I remember from when I was a kid seeing super low record lows on the KOMO newscasts during mid November and being in disbelief. Here’s the temps at Sea-Tac from that stretch of November 1955.

    11/11: 38/15
    11/12: 21/13
    11/13: 23/14
    11/14: 23/9
    11/15: 24/6
    11/16: 31/21
    11/17: 31/23

    The reanalysis data from NCEP has the textbook arctic outbreak pattern for the PNW – I’m still trying to figure out why this air mass was so cold though. Sea-Tac got 1.4 inches of snow on the 11th and didn’t get any more until 4.6 inches from an overrunning event on the 17th. The lack of snow makes this event all the more impressive.

    500mb reanalysis plot: http://s3-us-west-2.amazonaws.com/weathertogether.net/wp-content/uploads/sites/5/2017/11/USA_500mb_Nov1955.gif

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