April 10, 2023

Super Rainshadow

 The Pacific Northwest is home to some of the best rainshadows in the world:  areas of dry conditions downstream of terrain barriers.

And on Sunday we were treated to one of the best examples of such features, in the lee (northeast) of the Olympic Mountains.

As many of you know, when air approaches a barrier, it is forced to rise, producing clouds and precipitation (see figure below), and when it sinks on the downstream side, the air descends, warms by compression, and drys out.  

This is where the rainshadow is found.


On Sunday, strong south/southwesterly flow was approaching the Olympics (see a map of winds around 11 AM Sunday).  Thus, the sinking air was on the northeast side of the Olympics.


The visible satellite picture at this time clearly showed a nearly cloud-free area in the rainshadow areas.  This is sometimes called the "Blue Hole."


The radar imagery at that time showed it was a rain-free zone:


Do you want to be impressed?  

Here are the precipitation totals for Sunday.  Around 3 inches on the windward, southwest side of the Olympics, but only 0.01 inch over portions of Whidbey Island.  Mama Mia!  That's a rain shadow.


The air passing over the Olympics not only produces a rainshadow, but also affects pressure and wind.

On the upstream side, the air cools as it rises, which in turn causes low-level pressure to increase (the windward high) since cold air is denser/heavier than warm air.

On the downstream (rainshadow) side, the air sinks and warms, causing low pressure, since warm air is less dense than cold air.  

Below is the forecast of sea level pressure on Sunday morning (the lines are isobars of sea level pressure). You can see both the windward high and the lee low.  Sometimes these pressure features cause strong winds, as air accelerates from high to low pressure.


And, of course, the Olympic features are not the only Northwest rainshadow.  We have dozens of them downstream of local mountains.  We live in a rainshadow-rich region.


7 comments:

  1. Cliff, why does the SW Kitsap area get so much rain? I'd say it is beside the Olympics, but not upwind of them. In fact when the wind is from the West, the area would be downwind of the Staircase area.

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    Replies
    1. The reason is that the low level winds approaching that part of the Olympics is typically southeasterly, and thus forced to rise by the Olympics in that region....this enhances precipitation rate.

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  2. It's interesting that the graphic depicting precip amounts predicted by the UW WRF-GFS model run shown in the previous blog post indicated strong rain-shadowing/subsidence northeast of the Olympics but failed to capture its extent. My location in Bellingham looked to be in the band of color indicating 0.5-0.75" but the total amount I've measured is <0.3" and the current forecast as well as radar imagery indicate that the event has now come to end here.

    BLI received just 0.22" between Sunday afternoon and this afternoon and its total year-to-date precip is still the lowest on record while its total water year-to-date precip is <0.1" above the record lowest.

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  3. Sunday was a weather adventure. Start by Hood Canal in Central/SW Kitsap, catch the Kingston boat to Edmonds, proceed up to Bellingham and return trip back to Kitsap. Went from pouring, to sprinkles to just cloudy to WIND and then the whole mirror image. Pouring when the truck got parked back in Seabeck.

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  4. All's I know is that it poured much of Sunday, Monday and today wasn't too bad, dry for part of the day, but cloudy though and not terribly warm.

    It was wet here in Tacoma, and in Olympia where I was Sunday with family.

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  5. Enjoyed a great hike in the Olympics. We hiked up an easy facing slope to an exposed ridge in the rain shadow. The ridge had high wind with moderate rain and views of heavy rain to the southwest. Dropped back down on the east side of the ridge and only had a few showers and light winds. It was very cool to see the rain shadow effect up close.

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  6. I measured a morning minimum temp of 31.9F at my location in Bellingham on 4/12, the 74th freeze since the first of the season on 11/3/22 - the most freezes I've recorded during a season by a substantial margin.

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