Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast

Did you notice how dry the air was yesterday over large portions of western Washington?  

Strangely enough, you can blame a huge storm over the Pacific Ocea!

To show you what was going on, below is a plot of the minimum relative humidities west of the Cascades on Tuesday.  In some locations, relative humidity dropped to under 20%!

Did you notice how dry the air was yesterday over large portions of western Washington Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast

A relative humidity forecast for 2 PM Tuesday predicted very low values (below 20%, brown colors) in western Washington and southwest BC.   Low values over eastern WA as well.

Did you notice how dry the air was yesterday over large portions of western Washington Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast
But why?

Yesterday's visible satellite image around 2 PM was impressive, with a deep low-pressure area in the center of the swirl and an extensive band of frontal cloud extending around the low to northern Oregon and then south into central California.

Did you notice how dry the air was yesterday over large portions of western Washington Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast

Associated with this storm, there was a strong low-level wind circulation.   Below are the forecast winds and heights (like pressure) at roughly 5000 ft (850 hPa).  You can see the intense low center off the CA/OR border.  Wow.    

Now, look at the wind barbs.  They indicated very strong easterly (from the east) winds over Washington and British Columbia.

Such winds not only bring in dry air from the continental interior but as the air descends the western slopes of the Cascades and coastal mountains, it is warmed by compression.  Warm air can hold more moisture than cooler air, and this caused relative humidity to plummet.  

Another factor is that the descending air starts higher up, where the air generally has less moisture.

Did you notice how dry the air was yesterday over large portions of western Washington Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast

In fact, the warming due to sinking air was very evident in yesterday's high temperatures, with max temperatures rising into the lower to mid-60s over northwest Washington (see below)

Did you notice how dry the air was yesterday over large portions of western Washington Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast
The giant low is now weakening and slowly moving southward (see an infrared satellite picture below for 8 AM today), and the easterly (from the east) flow to its north is weakening.  We will have another dry/warm day north of Tacoma, while to the south cloud bands and moisture will continue to circle around the low.
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Did you notice how dry the air was yesterday over large portions of western Washington Dry Storm Hits Western Washington as a Major Cyclone Swirls Off the Coast

Needless to say, large amounts of moisture are now streaming from the Pacific into California.  If only they had built more reservoirs...
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