September 23, 2010

The First Atmospheric River of the Season

Some of the most important wintertime weather features of our region are the plumes of moisture that stream northeastward out of the tropics and subtropics. In the discipline these plumes are often called "atmospheric rivers" and the atmospheric river that is often discussed in the media is the "pineapple express." This weekend the first major atmospheric river of the season will strike our region, specifically central and northern Vancouver Island and adjacent portions of British Columbia.

Here is a recent computer forecast of the amount of water vapor in the atmosphere (the fancy name is "column-integrated water vapor"--throw that around and you will impress your friends!) for 11 AM on Saturday. The blues are high values-see the atmospheric river?

The plumes of atmospheric moisture associated with these rivers is usually associated with warm temperatures--in fact it HAS to be that way, because only warm air can hold large amounts of water vapor. When this warm juicy air strikes our mountains it is forced to rise--the result being large amounts of precipitation. Want to see what the models are going for? Here is the forecast 24-h rainfall ending 5 PM on Sunday. The reds are FIVE TO TEN INCHES OF RAIN! There is even a white area, where more than 10 inches is predicted.

Fortunately for us, the U.S. side of the border will only get a weakened share of this wet bounty--after a generally dry Saturday the front that is associated with this precipitation will move through rapidly, with only modest showers over western Washington.

Do we expect a lot of strong atmospheric rivers this year? Will one hit the weakened Howard Hanson Dam? What I can tell you is that the upcoming winter will be a La Nina period and generally the strongest atmospheric rivers and floods are during neutral years (neither La Nina or El Nino). So although we expect this fall and winter to be wetter than normal (due to La Nina), there is less chance for a mega-rain/flooding event. Yes, it could happen, but it is less likely.


  1. Very interesting post. But would I be wrong if I said that if an express event occured during La Nina, it most likely be during the early fall.

    If La Nina provides an active western pacific, warmer water in middle, and a stronger jet to carry that moisture pool across the ocean and high pressure inland, to me it seems like a perfect scenario.

    That would be a great research project.

  2. Here in Port Angeles, Vancouver Island is only 18 miles away, so it sounds like we better be prepared for some rain, and possibly a lot of rain!

  3. The way our weather has been all year I would hardly expect it to be a "normal" La Nina year! I think we should all prepare for anything and everything!

  4. So I guess I am a bit confused... where in western washington will get all this rain?

  5. So where in western washington where is all the rain going to fall?

  6. Cliff, I'm still waiting to learn what causes these jets. They seem to run counter to generalized atmospheric convection; namely the trade winds.

  7. Fellow Washingtonians, if you'll re-read Cliff's words and graphics, you will see that the plume will hit "Northern and Central Vancouver Island and adjacent areas of BC." Therefore, it will largely miss Washington as well as Victoria, BC, leaving us with more moderate rain amounts in the north and even drier in Seattle and southwards.

  8. I thought this winter was el niƱo, and last winter was la Nina.

  9. For those interested in the simple explanation of atmospheric rivers see this 1 page Science background article

    Quote: "Weather forecasters have long recognized the importance of narrow streams of poleward-bound air. A glance at satellite images of the wintertime North Pacific Ocean shows great,
    comma-shaped storms marching eastward, their tails arcing back southwestward toward Hawaii and beyond. These storms are redressing the imbalance between the warm tropics and cold
    poles by creating an atmospheric conveyor belt. Cold air sweeps broadly southward behind the cold front that runs along the tail, and warm air is driven poleward along and just ahead of the
    front. It is this warm and inevitably moist stream paralleling the front that has come to be known as an atmospheric river."

    The additional feature is the cold front associated with the cyclone should dip south of extratropical/tropical boundary so that the flow along the front can couple up with the tropical moisture.

    My hand-waving view is that you get this structure (the "hosepipe" or conveyor belt) between cyclones regularly but you only get an atmospheric river when the "hosepipe" is attached to a working faucet (the warm, wet tropical air)

    There are several (technical) papers and recording of presentations on the WX enthusiast might find interesting (particularly the ones about the Russian River flooding). The trick is to skip bits you don't understand then come back to them later (after reading around more).

    Western Wa is (probably) not going to get the rain this weekend. It's going into Vancouver Island and BC (see the plots). This is just an amazing example (white rainfall on the plot!) of what an atmospheric river can do. Of course if the river should move a little south ...

  10. These currents are what is usually known as the "Pineapple Express" here in BC, correct?

  11. Thank you Cliff for the information about flooding. I had wondered about how a La Nina winter would influence flood risks. Now I know!

  12. That is amazing to learn. My name is Peter. I am trying to learn about meteorology. I love it! Do you think that these atmospheric rivers will hit New Mexico where I live? I would love to know the answer! Thanks!


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