September Ends Cool; Some Mountain Snow Too

9:30pm Monday…

With a week to go, September is running near average temperature-wise.  That comes from a very warm start followed by the last 9 days of cool temps.  Across other parts of Oregon it’s a bit below average.


Looking ahead, it’s clear September will end very cool this weekend and early next week.  In fact it’ll be about the coolest it can get this time of year.

Right now the very weak late September jet stream is just to our north, sending plenty of clouds and a few sprinkles.  That general setup continues through Thursday.

Mark Jet Stream

There won’t be any weather systems nearby Wednesday so that should be a spectacular day like we saw this past Saturday.  But Friday through the weekend that strong ridge of high pressure offshore pushes west and north.  That forces a cold dip in the upper atmosphere south over the western USA.  This IS the same setup that sends cold arctic air down into the Pacific Northwest during the winter.

Mark Jet Stream3

But of course it’s late September so snow won’t fall down to sea-level, or anywhere close.

For the weather geeks, 850mb temperature is forecast by both ECMWF and GFS to fall to at least +1, and possibly as low as -2 over Portland/Salem.  The coldest on record over Salem anytime before October 7th is between -2 and -3.  So it doesn’t get much colder than this in late September.  What does that mean?  It means snow COULD fall as low as Government Camp (3,500′).  Of course that means precipitation has to show up.  The earliest fall snowfall at Government Camp was September 23rd, 1984 when 3″ fell.

ECMWF Snow Level From 850mb Temps LONG TERM

This setup is typically not a big rain/snow producer; because the air is coming down from dry land up north, not from the Pacific Ocean.  Take a look at three rain forecasts for Friday through next Monday.  Generally not much rain in the lowlands

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This general pattern is likely to stick around for awhile.  Both 6-10 day and 8-14 day outlooks from CPC show the same “western trough/eastern ridge” pattern continuing through the first week of October.


Mark CPC 6-14 Day Outlook wKML

Mark CPC 6-14 Day Outlook2 wKML

Chief Meteorologist Mark Nelsen

18 Responses to September Ends Cool; Some Mountain Snow Too

  1. Jason Hougak says:

    Historic winter storm barring down on Montana. The Montana Rockies could get 2-3’ with snow possible down to the valley floors.

  2. W7ENK says:

    FYI: First frost of the season possible in the Western Valleys this weekend. Time to think about bringing in any sensitive plants.

    • Larry says:

      oh sweet! First frost (possible) this weekend? Not often do Willamette valley citizens see frost in September. Mountain snow is just a bonus. Though, I wonder what the weather in October will be like…

      • I’d like a repeat of 1991. Bellingham got 2″ of snow on 28 Oct that year.

        • Larry says:

          I wonder what the earliest recorded snowfall in Oregon is???

        • Jason Hougak says:

          That totally depends where in Oregon it is. At certain elevations it can snow any time of the year.

        • Larry says:

          Let me be a bit more specific; Cascade Passes??

        • Passes can easily get snow in September, and have seen it multiple times.

        • W7ENK says:

          Snow down to 4,000′ isn’t necessarily out of the question for any month of the year. 15+ years ago, Klamath Falls (4,300′) had an average snowfall in their climatology for every month of the year except for August. I’ve been snowed on down there on June 15th, it accumulated to 3″ everywhere but the pavement while I was packing my car to move out of the OIT dorms and into my own apartment. It has snowed in K Falls, and at Government Camp on the Forth of July in years past.

          So now that begs the question: Was that a late season snowfall, or early season? Where do you define the cutoff? Is it the Solstice? The end of Summer? And by which definition of Summer, Meteorological or Astronomical?

          Your question has many very complicated answers.

        • Larry says:

          Well… someone had to ask the question. 🙂

        • Roland Derksen says:

          I do recall that event myself- we saw enough snow here to cover the ground.

  3. Roland Derksen says:

    I find it somewhat ironic that despite the warm “blob” off the coast we’re forecasted to get below normal temperatures in the next few days. I guess the jet stream is a more powerful factor.

    • MasterNate says:

      Ocean temps are not a factor when we get offshore winds. That’s when we usually get our extremes, either cold in the winter or blazing hot in the summer with the E/NE winds

  4. Jason Hougak says:

    Mark what are your thoughts on the solar minimum?
    This is an interesting read.

  5. Roland Derksen says:

    I’m hearing forecasted overnight lows here slipping to around 39-40F by the weekend. I have to check, but I think the last time we had a daily minimum anywhere near the freezing mark in September was back in 1983.

    • It definitely can freeze this early, though of course it is not normal. I was reviewing the records for KBLI and the earliest frost I can see was on the 21st in 1955. In 1983 there was a low of 32˚F on the 29th. Current forecast lows are in the 40’s for the weekend. We shall see.

  6. Larry Cloud says:

    Thanks for the update, Mark.

  7. Ken in Wood Village says:

    Thank you for the update Mark. Sounds good to me. Let’s hope for an exciting fall and winter season 🙂

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