January 26, 2011

Tale of Two Half Months and the Banana Belt

The weather during the last two weeks has been warmer than normal over much of the region, in contrast to the cold weather of the first half of the month. Take a look at the temperatures for the past four weeks at Seattle-Tacoma Airport (the normal highs and lows are shown too).

During the first two weeks the high temperatures only reached normal values (around 45F) for a short two-day period. Then the switch was flipped and temperatures surged above normal, with very few days dropping to the normal lows. The weird thing is that average temperature for the whole month will end up near normal! A good example of how you got to be careful with weather statistics.

As shown below, a figure giving the cumulative precipitation of rain at Sea-Tac, precipitation for the month will be near normal, but in reality most of the month was fairly dry and the rain was concentrated in the middle.
Now today there was an interesting hot spot on the southern Oregon coast where the area near and just north of Brookings on the Oregon/CA border surged into the 70s! Specifically, Brookings hit 73F, North Bend reached 70F, and another station (FPRO3 north of Brookings) hit 76F. Here is a plot of temperatures, winds, pressure and other parameters at 2 PM. The surface air temperature is in the number in the upper right next to each circle (click to get a big version).
Now why so warm? This was a modest example of the Brookings Effect and the reason the southern Oregon coast is often called the Banana Belt of Oregon. Every month of the year has seen temperatures in this coastal zone hit the mid-70s and more.

The secret is to have mild air and offshore (easterly) flow.But there is more to this story! There is something unique about the southern Oregon coast, something that is obvious from a terrain map (see the one below):

For most of western Oregon the Willamette Valley provides a break between the coastal mountains and the Cascades, but not the southern portion....high terrain (known as the Siskiyous or Klamath Mountains) extend from the coast well inland.

Now if you get a situation with easterly flow--like we have today, the air descends rapidly to the Oregon coast. When air descends it is compressed since pressure is higher near the surface that aloft. Compressing air causes it to warm (like in your bike tire pump). Thus, the tongue of coastal warming.

So forget Hawaii and head down I5 to the southern Oregon coast. But hurry. The situation is changing soon and rain will return on Friday to much of the area. But the showers will only be temporary--by the end of the weekend our persistent ridge will rebuild and dry, mild conditions will return. La Nina lowland snowstorm? Very unlikely during the next seven days.

I understand there is wager between the well know local writers Knute Berger and Eli Sanders with Joni Balter of the Seattle Times regarding snow this winter (http://crosscut.com/blog/crosscut/20067/What-if-Seattle-s-rain-were-snow-/). Perhaps this game of chance should be open to the rest of us....


  1. Not to whine or anything, but I really just want some winter. This 55 degree stuff is frustrating.
    I know, I know, just wait till June, then we will have winter...

    I can't really even go to Mt. Hood for some winter because it's warm up there, too!

    Just 3 weeks of nothing above 35 degrees. A few flakes- maybe even a few inches of white. That's all I ask.

    Ok, I'm done. Thanks for listening.

  2. There's obviously nothing scientific behind this statement, but it would surprise me not to see another pattern shift back to a colder, wetter pattern sometime in the next month or two. As many of us will remember... we had snow with a hard freeze in March 1989. Heaps of snow (in Olympia where I lived at the time) in mid Feb 1990. A couple small snow events in Feb 95? A quick dump in late Feb 2001 but it didn't get cold. A week-long period of flirting with Arctic air and snow in various places (though it might have missed Seattle, I can't remember) in mid March 2002. And it must have been 2003 when Seattle had a little snow in Feb and/or March as well. In Feb 2006 we all thought winter was over then it got cold in the middle of the month, with a unusual blast of cold straight from the east but no snow. And I'm probably forgetting at least one or two other recent events.

    All this to say... it ain't over until the fat lady sings... or something like that.

  3. Cliff,

    You say, "The weird thing is that average temperature for the whole month will end up near normal! A good example of how you got to be careful with weather statistics."

    I think that what you are calling "normal" is actually the average over some number of years. An average is useful only when it is accompanied by a measure of variability or diversity such as standard deviation.

  4. Snapdragon - asking for the same thing here. June won't be winter though, it will be too warm...! Though it would be pretty cool to somehow see snow in June, haha. Friends of mine all over east of the Rockies go on about the snow they keep getting and say they don't mean to rub it in.

  5. Fogmageddon this morning. I hate the fog. Can't see anything.


  6. I see that the 8-14 day outlook (http://www.cpc.ncep.noaa.gov/products/predictions/814day/) seems to ever-so-slightly be teasing us again with the possibility of (only) a beginning of a retrogression of the longwave pattern. Seems like we heard this idea in the NWS Discussions a few times a week or so ago, but of course it didn't happen.

    Will be looking with interest at the Feb outlook released on Monday. Hope hope hope for some real winter . . .

  7. I remember seeing a large healthy Date Palm tree growing in Coos Bay, Oregon. That tells you something. It's 65 degrees in Florence, Or as I write this. Boy, would I love to be romping on the dunes right now.

  8. Get ready for February. I have a feeling its gonna be colder then normal with a few inches of snow....

  9. My concern is that we will have a "late bloomer" La Nina which will make April wet and cool. Whining skiers will fade and whining gardeners will ascend. Even I may begin to take this weather pattern personal. Sequim has had heavy wet fog all day and I have all the lights on. My guess is that Hurricane Ridge was the place to be today. Cheers.

  10. PBS has a show about that "banana belt" section of Oregon. Check out "A Wild American Forest," http://www.aptv.org/schedule/showinfo.asp?ID=299826. In the description: "The beautiful, scenic Klamath-Siskiyou eco-region, straddling the border between California and Oregon, is one of the world’s most important temperate forest regions."

  11. Michael DeMarco:

    yesterday was not that great looking up at hurricane ridge.. this camera helps me keep tabs on it:


  12. Whats your take on the snow chances Saturday night?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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