March 01, 2012

New Weather Feature: The Strait of Juan De Fuca Cloud Line

Sometimes one observes a feature that is a bit unusual and one was obvious today--a line of clouds and precipitation down the Strait of Juan de Fuca that extended across northern Puget Sound.  Here is an image from the Camano Island radar at 6:54 AM this morning.  A nice precipitation line starting about 1/3 into the western side of Strait.

A visible satellite image shows the cloud line quite clearly. Over the ocean there was a field of convective clouds in the cool, unstable air offshore.  We call this open cellular convection because there is more clear (open) areas than clouds.  The whitish areas over the eastern slope of the Cascades is snow.

Now let me show you something that simply warms the heart of a numerical modeler such as myself:  a fifteen hour forecast for 7 AM this morning of cloud liquid water from our super-high resolution WRF modeling system (4/3 km resolution). got the line almost perfectly.  Sometimes it is clear we have made real progress...this is one of them.
 So what is going on here?  I haven't spent too much time analyzing the case yet, but as a start I checked out the low-level model winds (see below).   The winds offshore are from the northwest and are blocked by the Olympics.  Some of the air is deflected to the north by that barrier.  Over the western Strait the confluence of the northward-deflected air and air moving off Vancouver Is. causes convergence (piling up of air) that forces upward motion.  Which creates clouds and precipitation.  Anyway, I will be examining this case further after the Northwest Weather Workshop (which you can still register for....starts tomorrow...see link to the right).  Over a hundred people are registered now--a really interested mix of the weather community and interested laypeople.

Finally, we have some improving weather ahead.  A weak system later on Friday and then improvement on Saturday.  Sunday should be great....sunny, much warmer air...many will see the mid-50s.   A big ridge over us...see below.   Plan to get outdoors!

Dog Status
     Someone thought they saw our dog again in Mountlake Terrace (near Cedar Way and 236th Ave SW).  If you live around there, keep an eye out.   We and friends have walked  that area--no luck yet.


  1. Cliff, a similar thing was happening Sunday morning. I am a pilot for a local company and flew back and forth through a line of clouds,rain and snow that extended roughly from Penn Cove westward through the straight of Juan de Fuca. It was fairly stationary and seemed to last for several hours. As I recall the surface winds were similar to the ones shown on the chart in this post.

  2. Hi, Cliff. Sorry to put this here, but this blog hasn't been updating on my Google RSS feed since 30 Jan. The last story it shows is the "destruction of weather landmarks". I even deleted the gadget and then re-subscribed... no dice. Might want to check on that, in case I'm not the only one missing your posts!

  3. It sort of looks like a weak convergence generated by Vancouver Island.

    Speaking of which, we often have north or northwest winds on Sinclair Island while Guemes Island, a few miles to the south has southwest winds during weak onshore flow in the summer. Clouds are usually not associated with this fair weather phenomena. But winds blowing down the Strait of Georgia and the Strait of Juan de Fuca meet somewhere south of Sinclair Island. I travel between these islands all the time and not infrequently it is 5 or more degrees cooler on the west side of Guemes when this happens.


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