XTREME Orographic Weekend

Yes, I know, I didn’t spell EXTREME right, but it’s a more dramatic headline that way isn’t it?

The past 36 hours has been a “textbook case” of what we call the “Orographic Effect” caused by “Orographic Lift”.  No, not orthopedic lift, like shoes, but OROGRAPHIC.  Orographic lift means lifting related to topography.

We happen to have a large mountain range oriented north/south through Oregon and Washington (Cascades), and a decent chunk of the year we get westerly wind flow between 2,000 and 10,000′.  When that air is forced to rise over a mountain range, it cools and has to drop its load of moisture in the form or rain or snow.  Then on the back side of the mountain range the opposite occurs.  The air warms and dries as it moves downhill.  Thus enhanced rain on the “up wind” side of the mountains and less rain on the “downwind” side.


You can see the strong westerly wind on the Troutdale wind profiler over the past 48 hours.  Time goes from right to left (most current is left side).  Sea level is the bottom, and the chart goes up over 15,000′.  I’ve highlighted the main elevations we care about for that lifting over the Cascades.  In winter it’ll be even strong than these 25-35 mph speeds for an even stronger effect.  On the flip side we don’t have such a warm/humid airmass in the winter.


When there is plenty of moisture, as there was this weekend, the difference in rain totals can be HUGE.  Take a look at the numbers from west of the Cascades



Yes, you are reading those numbers correctly! 6 TIMES AS MUCH RAIN fell on the west slopes of the Cascades compared to the west metro area.  Notice rain in this pattern picks up as you move east across the metro area.  The air stream is already “feeling” the lifting over the Cascades piling up in front of it as it passes over the metro area.

The heaviest totals are generally just west of the Cascade crest, which in this case is the Hood River/Multnomah/Clackamas county line.  That extends from Mt. Hood north to the Gorge.  Portland’s water supply (Bull Run Watershed) was wisely chosen to come from this area over and west of the crest.  It’s extremely rare that water is in short supply for a whole year in this area.  Even most dry winters will end up with a wetter spring for recharging the reservoirs/lakes.

Once that air moves east, the opposite occurs.  The air moves downhill and dries out.  Check out Parkdale and Hood River’s totals…yes, only a quarter inch or less!


There was about 10 times as much rain just 15 miles west of Parkdale!  Anywhere east of the Cascade crest is in what we call a RAIN SHADOW.  The mountains are “shadowing” those areas from the rain.  This explains the progression in tree types from fir to pine to oaks to no trees and just sagebrush as you drive from Cascade Locks to The Dalles through the Gorge.  The same thing happens when driving through the Cascades except you go up and down in that case.

I can anticipate a couple of questions:

  1.  If the wind blows from east to west (opposite direction) is it wetter EASTSIDE?  The answer is YES, if there is moisture available.  But that doesn’t happen often.  There would have to moisture coming from the east/northeast/southeast.  Not much of a moisture source out there.
  2. What if the Cascades were oriented east-west through Oregon instead of north-south?  The answer is that westerly flow wouldn’t cause the big change in rainfall.  In that case it would need to be northerly or southerly wind to make the big increase/decrease in rain.

There you go…now you know why it poured in the mountains but it was just a typical showery day west metro on Saturday.

Looking ahead, this upcoming week will be pretty slow with a weak upper-level low spinning nearby through Thursday.  Just a chance of a shower here and there.  Friday and Saturday it MAY be wetter, but I’m not sold on that for now.

Chief Meteorologist Mark Nelsen

46 Responses to XTREME Orographic Weekend

  1. Lee Wilson says:

    Who dropped the ball on the winds?
    We’re getting great winds.
    Gusts up to 20 mph.

    Which is the speed my turbine produces 200 watts.

    The winds are from the South and at times from the south east.

  2. Lee Wilson says:

    Who dropped the ball on the winds?
    We’re getting great winds.
    Gusts up to 20 mph.

    Which is the speed my turbine produces 200 watts.

    The winds are from the South and at times from the south east.

  3. runrain says:

    Heat advisories issued for the SF Bay Area this weekend. Looks like they will get their hottest temps of the year, or close to it. It is strange to be at Fisherman’s Wharf or in the Marina when the temp is near 90!

    • W7ENK says:

      I was there end of June 2013 when they were getting temps into the 90s, the fog was pushed back right up to the immediate beaches but the city was roasting. Didn’t phase me one bit, I actually enjoyed it!

  4. Rain started here about 5am- should see maybe half an inch today.I’ve noticed quite a discrepancy in forecasts for my region in the next few days: One says unsettled mainly cloudy conditions with coolish temperatures, the other indicates sunnier warmer weather (up to mid-70’s on Sunday). So we’ll see which one wins out.

  5. Umpire says:

    My sister lives in Salem. Texted that the storm last night went on for nearly an hour, and power went out three times. We saw a few flashes from Delta Park, in North Portland nearly to the Columbia.

  6. 44 degrees F this morning here. Coldest morning of the season so far. Could see thunderheads over the Cascades yesterday but none visited the lowlands anywhere near me.

    • runrain says:

      Saw quite a few flashes on the southern horizon last night from Happy Valley. Was interesting to see a starry night sky almost to the horizon except for a fringe of clouds along the southern edge where the storms were.

  7. chiefWright (Marquam 350') says:

    Yay, finally! A little black raincloud drifting over Mt. Angel is giving me a lovely little donner & blitzen show!
    Perhaps a dozen or so flashes thus far. Most of it looks to be C-C; maybe one or two C-G strikes to show up on the map.

  8. Goducks09 says:

    Lots of flashes looking due south from downtown!

  9. Hal in Aims says:

    distant rumbles in the direction of Mt. Hood………

  10. W7ENK says:

    Any word on the thunderboomers popping up near Mt. St. Helens and down over the Coast Range between Tillamöök and Lincoln City?

    Quite a few lightning strikes with some of those, they’re mostly moving to the SSW. If they keep up, the cells over Gifford-Pinchot could end up down here in Portland Metro before sunset…

  11. 45F here this morning under clear skies. It’s my lowest temperature for astronomical summer- fittingly on the last day.

  12. Boydo3 N. Albany says:

    Monday looks toasty..

  13. Hmm. says:

    So far I’m at a whopping 0.64 inches of rain for the whole month. Everyone’s grass is a dirty yellow color. So much for a rainy northwest.

  14. Jesse-SW Portland Suburbs says:

    So much for that two straight weeks of warmth, eh Mark?

    • Mark Nelsen says:

      8 days was close enough among friends right?

    • High Desert Mat says:

      Or family?

    • Jesse-SW Portland Suburbs says:

      This is true, Mark. 😉

      Looking like a pretty dynamic period coming up. Cool to very warm again for a few days (low to mid 80s possible Sunday/Monday), then potentially shifting to very cool.

      In the short term, Friday could see PDX’s first sub-60 high since May with a dying warm front encountering a resident cool airmass.

  15. runrain says:

    Corvallis was mostly on and off mist on Saturday during the OSU game, but there were some moderate showers earlier during tailgate time.

    I see those tiny white flies are back again, although I wonder if this latest rain might have pounded them into submission.

    Looks like maybe pushing 80 late weekend – early next week?

    Finally, hope the rain gives the Willamette a boost. I have never seen beaches downtown like I saw last week!

    • Paul D says:

      Mark put that 80 in there just to tease people. He’ll drop it a couple degrees each day….. 🙂

    • Boydo3 N. Albany says:

      Euro looks quite warm for the first part of next week. Mid eighties are possible. But by the end of the week we will be saying bye bye to summer…..maybe

    • Paul D says:

      Looks like we can’t put a fork in summer yet.

  16. W7ENK says:

    Just crossed the threshold into La Niña territory last week, SSTs in ENSO 3-4 region are down to 0.52C

    Now, will it hold there for 5 consecutive months to declare an official La Niña??


  17. W7ENK says:

    The rain began falling around 5:20a at my place in Milwaukie on Saturday. It was weird to hear the rain falling and the crickets still chirping, especially so early in the morning. It rained pretty much all day, had 0.25″ by 10am, up to 0.50″ by 1pm, finished out the day with 0.76″ when all was said and done… Déjà vu all over again!!

    Another 0.09″ yesterday, but my area was missed by the heavy showers that came through in the afternoon. Mt. Angel, not so much. A storm cell blasted through the Oktoberfest around 4:30p dumping (probably) a quarter inch of rain in about 10 minutes! There may have even been some small hail with it, but I was hunkered down inside the Mt. Angel Sausage Company’s back room with a beer in hand watching through the window. Again, I was surprised there wasn’t a flash of lightning and a rumble of thunder.

  18. Tyler Mode in Battle Ground says:

    I ended up with 0.91″ at my house in Battle Ground, while Vancouver was less than 1/2″.

    • Mike says:

      Yes it’s nice to get some action in the weather, I feel so much better. Weird Huh? Just made different I suppose.
      Take a look at the north hemisphere water vapor picture, The amount of east/ west flow over the U.S. looks tremendous and very unusual. Note the degree of spinning going on with that low just south of the allution Islands.

    • I’m due west of the Olympic Range and had only .07″ through nightfall Sunday (and it all fell on Saturday).

      .22″ overnight, almost all since midnight, and may well get significantly more today as there’s a chance of thunderstorms in my forecast.

    • Ack! Due east of the Olympic Range.

  19. I’m curious if the word “orographic” is in anyway related to “orthogonal.” Obviously when the flow is orthogonal to the Cascade crestline, orographic mechanisms are strongest.

    Sometimes in snowstorms with easterly flow through the Gorge, The Dalles is just as ‘wet’ as Hood River in terms of total snowfall. Apparently the low-level cold air bumping into the east slopes of the Cascades, somehow squeezes extra moisture out of the incoming storm.
    An east-west oriented Cascades? The Pacific Northwest would probably divide more into warm/cold climate regions, instead of warm/dry. If we assume the mountains cut across the northern reaches of Oregon: then you’d have a very chilly, grey landscape in northern maritime regions. But MUCH cooler summers in Eastern Washington since there are no mountains blocking the Pacific air. Also the arctic blasts would likely just spread out over the north side, while southern locations (aka Roseburg and Medford) would be EXTREMELY mild in the wintertime.

    • Dallin Burnett says:

      Hmm, I’m not sure about southern Oregon being that mild. It would still be vulnerable to arctic blasts from eastern Idaho and without N-S Cascades, the region would be even more vulnerable than it is now.

      Eastern OR and WA would still definitely be milder and wetter than they are now, and so would southern Idaho and parts of CA, NV, and UT. Rather than the climate abruptly transitioning from oceanic to semi-arid across the Cascades, there would be a gradual change from oceanic to humid continental conditions as one moves towards the east.

    • Michael Goss says:

      Nope, the “oro-” in “orographic” is from the Ancient Greek “óros”, which means “mountain.” It is related to the English word “raise”. The prefix “ortho-” is also from Ancient Greek, in this case “orthós,” with several meanings including “straight” and “right angle”. The origin of this word (before Ancient Greek) is less certain, but it may be related to English “arduous”.

    • W7ENK says:

      Dallin, I think his hypothetical is based off the “what if” the Cascades ran from the Coast to the Rockies, which would be more than plausible given their actual length from N-S.

  20. Anonymous says:

    I was driving east on I84 about 5 pm today and got into probably the heaviest rain I have ever driven in. Interesting I had been at the coast where it as dry and sunny.

  21. Scooter says:

    For sure, where I’m at it rained continuously for almost 36 straight hours. I have seen this so many times in the past where Portland might even be sunny and those of us in the foothills just continue to keep getting soaked. It’s the stuff that makes weather so interesting. Early tomorrow I am headed over to Fossil for a week long camping trip and it will be interesting to see the differences in rainfall. No hurry, no worry!

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