June 13, 2009

Convection over the Mountains

During the past week it has been completely dry over the western WA lowlands, but not over the crest and eastern slopes of the Cascades, where convection (cumulus, cumulus congestus, and cumulonimbus--thunderstorms) have developed each afternoon. A satellite picture from Friday afternoon shows an example (see image). I have also include radar images from the NWS radar at Camano Island and Spokane. You can see some convective cells over the Cascades and immediately to its east.

So what is going on? During the past several weeks we have had a persistent flow pattern with low pressure to our south and easterly flow (from the east) approaching the Cascades. This air has been only marginally stable, which means it has the potential for convection with sufficient surface heating or mountain lift. Anyway, as this air has started to rise on the eastern Cascade slopes cumulus convection has developed, increasing in strength over the crest. Then as the cells descent the western slopes they tend to weaken...since sinking causes warming and drying that works against convection (see my book for more on this).
There is another way that mountains encourage convection. When slopes are heated, upslope flow often occurs and for a mountain crest upslope flow occurs on both sides. The upward currents join together at the crest with strong upward motion resulting...such upward motion can initiate convection.

Want to see a nice time lapse of convection forming over the Olympics by this mechanism?..check out http://www.drdale.com/lapse/lapse090610.mov
..from a HD cam near Silverdale (Dale Ireland)


  1. -

    hello Cliff, ..

    I was surprised where and with reading what you've written above, that you've not mentioned "orographic" lifting. Is this because, in this particular instance - where considering the general circumstance that you've covered, there had been none significant. ?

  2. This comment has been removed by the author.

  3. I spent most of Saturday up the Skagit River Valley. We noticed the convective action and building clouds fairly early in the day. Got a few sprinkles up by Diablo Lake. Around 17:00 we finally got good thunder and lightning when I was near Baker Lake, just north of the town of Concrete. These conditions are so predictable once you know what to look for and it is really fun to watch them develop. I am sure that there are other folks out there complaining about their ruined picnic.

    Needless to say one should plan to play golf, climb trees, ascend mountains, go sailing, etc. on days when thunder and lightning are not likely or have a reasonable plan to seek shelter when the storm finally approaches. Really sad that three folks died on the water during the marine push we had earlier in the week.

  4. richard583...I didn't use the term "orographic lifting" explicitly but that is in my description. Certainly the lifting of easterly flow as it approaches the Cascades is important...cliff

  5. Dale Ireland's time lapse footage liked to in the post is great. An added bonus is the submarine that's escorted through the shot just after 8am.

  6. Cliff,
    The 2:30 sunday afternoon long-term forecast discussion by the NWS talks about a "synoptic scale change" that is in the models for next weekend. If you have a chance could you talk about what this change would be in this particular case and what causes these larger scale changes this time of year?


  7. return reply

    hello again Cliff, ..

    Of interest perhaps, more incidentally, relative to my question posed above: In fact I live in Paradise, CA - N. Central Sierra foothills; .... 1750 ft. above and to the east of Chico, CA, and "up the hill" - also, from "Oroville", to the south. (aver. rainfall annually, 50 plus inches.)

  8. Checking out the congested cumulus (wink) is one of my favorite activities, but getting to watch timelapse video of it is even better. What a fantastic video--peaceful yet energizing, breathtaking and moving. Thanks for sharing it (and the author for letting it be shared).

  9. Spent yesterday on eastern slopes. Wonderful convection, giant cumulus with upper portions tilted to prevailing winds like giant viagra wind vanes. Thunder haunting us all day long and fierce downpours nailing us on Manastash ridge, I didn't think the PNW capable of! There is a lot of drama over there right now for those interested.

  10. That was a pretty cool time-lapse of those cumulus-cumulus congestus clouds building up.

    Know this is out in the long range weather, but me and my buddy from work will be doing a 4/ 5 day camping-hiking trip over in the Stehekin village-Bonanza peak area(eastern side of the Cascades) the first week of October. So maybe we`ll see some interesting weather then but also hope for nice weather as well.

  11. Beautiful time-lapse photography! I'm a photographer myself, and would love to learn a couple of the technical aspects required. 1. What is an entry level time controller I can use with my Nikon Digital SLR. 2. What are ideal frame-rates? ie..what dale used?
    3. What software compiles the series of stills into a video.
    I'm mostly on Mac, but have access to PC's. Comments are welcome, or e-mail straight to kenrieman'at'gmail'dot'com


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