October 11, 2011

Tale of Two Radars: Rainshadow and Windward Enhancement

 Dog Alert!  See Message at the Bottom---Particularly If You Live in Mountlake Terrace.

With weather radars on both sides of the Olympics now, we can do something we have never done before:  examine simultaneously both the enhancement of precipitation on the windward side of the mountains and rain shadowing on the other.

Here is the storm-total precipitation from the new Langley Hill radar for the past two days.

I know it is hard to see the geography well.... but if you examine this closely you see values of over 2.5 inches on the southwest side of the Olympic, with lighter amounts offshore.  This is a good example of orographic enhancement, with the showers coming off the Pacific being enhanced from uplift on the western and southwestern slopes of the mountains as the air is forced to rise by the terrain.  The sharp cut-off of precipitation over the mountains is not real...it is due to mountain blockage effects.

What about the other side?   Here is the view from the Camano Island radar:

A complex pattern, but with a big hole of light precipitation to the northeast of the Olympics--this is the rainshadow, which extended from the north Sound to Bellingham. Rainshadows are caused by air descending and warming down mountain slopes on the lee side of a barrier.  The rainshadow was shifted away from Sequim and Victoria because the coastal winds have been more west-southwest than southwest or south-southwest.  And you will notice the precipitation really picked up over the western slopes of the Cascades as the air was forced to rise again--some amounts exceeded four inches.   Now these values are not well-calibrated, but the qualitative picture is certainly realistic.

For comparison, take a look at the visible satellite image this morning at 10:45 AM.  You can see the convective showers approaching the coast, the enhancement on the windward (western) side of the mountains, and if you look carefully you can spot some rainshadowing (really cloudshadowing here!) over northeastern Puget Sound.  Of course, the big kahuna rainshadow is to the east of the Cascades, where you could have escaped all the showers today.   And then air rises over the western side of the Rockies and clouds return from Spokane westward. 

At the UW we have software that combines the National Weather Service and Canadian radars--here is a view at 4:20 PM:

Really nice.  You can see the showers offshore, how they become more intense over land, how they die just east of the Cascade crest, are absent over eastern Washington, and start again...in a wimpy way... over Idaho.  And I haven't even mentioned the blustery winds of 20-35 mph all around the region today.

You got to love the weather around here...lots of variability!  Don't you feel sorry for people back east where the weather is so uniform and boring?

My Dog (see picture to the right) was seen today (10/12) near the intersection of 236th St SW and Cedar Way.  If you see her please call immediately (206 719 5603) 


  1. Very cool! The weather was pretty crazy today, you brushed on post frontal instability today in class, and it certainly was the day for it! I found this video from Scott Sistek's KOMO post, check it out! http://www.youtube.com/watch?v=5cJt-Ru0XKU&feature=player_embedded. If you look closely, you can see a cold-core funnel cloud at around 12:30 on the right side of the video.

  2. Today made me think of April - the sun would come out, then it would rain hard, then the sun would come out, and it'd all repeat again.

    I kinda wish it was April again... normally I don't mind our winters, but it seems like we just left the last one!

  3. We were in the "Sweet Spot" of that rain shadow the last 2 days. .02" on Monday and .08" on Tuesday. We had 33mph gust on Monday and 30mph on Tuesday. It really is fun to watch those 2 radars. I have them independent of each other side by side on my site.

  4. A related topic: where are the transitional zones at the margins of both KLGX and KATX coverages? I live in Olympia - KLGX is about 60 miles west; KATX is about 80 miles north. The Black Hills are in line between here and KLGX. KATX is pretty much straight up the Puget Sound basin, with the high ground on the Kitsap in the line of sight.

    Last night about 11:55pm I checked the KLGX loop and saw a very distinct precip cell with a return intensity of about 35~40dBZ moving in from over the Black Hills. I don't think at that time it showed as much of anything on KATX. Over the next fifteen minutes as the shower moved east, the KATX loop showed a smaller and less defined area of precip with return intensities of about 20~30 dBZ. By around 12:15am, KATX showed an increase in peak return intensity to about match that being seen at KLGX. However, the extent of the more intense return area for KATX was considerably smaller and less well-defined.

    It looks like the Olympia area might be in the transition zone between site coverages.

  5. i agree that the weather around here is fascinating, particularly because of all the microclimates, and local variability due to the terrain.

    as a native new englander, however, i must take exception to their weather being called boring. if anything, although it lacks specific sort of variability here, the day-to-day meteorological changes there tend to be much more dramatic. indeed, if you don't like the weather in new england, "just wait five minutes".

    having lived in both places, i do prefer the weather here for general comfort. however, i have also found it more boring, in a way, because several days or even a couple weeks can go by with the same pattern (sun, rain, foggy mornings, marine layers, daily highs and lows, humidity levels, etc...) prevailing. granted, during some of these scenarios, the variability within the state can still be high, but for any one observer in a particular place (and not somebody whose job it is to study weather maps all day! :-) ), the consistency of the pattern is unavoidable. growing up, i found the weather changes both frustraing (when good weather was so quickly replaced by bad), but thrilling. one of my favorite experiences will always be how it feels to have a hot, humid air mass from the gulf of mexico replaced over the course of a day by crisp dry air from canada. (and then back again two days later!) the changes just aren't as stark, swift, or unrelenting here.

  6. I love the new dual dual-polarization radar images, but... Could you explain how dbz compares to actual precipitation? The image frequently shows almost solid blue-green clutter (like overnight 11/13/11) over the whole Sound but the skies are clear.

  7. That poor doggie...I'm pulling for her!


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

A Drier Than Normal Spring over the Pacific Northwest

I had to water some new plants today because the last few weeks have been relatively dry.   So I was curious.....how dry has it been in our ...