Deluge

News flash:  It is 9 AM on Sunday and it should be generally dry for the next 4-5 hours.  Immediate response necessary (e.g., go outside).    There are a few residual showers, some of which are producing rainbows (see 9AM Seattle SpaceNeedle panocam).  Note the blue skies.

Yesterday was an amazingly wet day for April, and a "highlight" of one of the wettest Aprils on record.   A number of locations (like Seattle) had their wettest April 14th on record and April is now the 4th wettest on record and WE ARE ONLY THROUGH HALF THE MONTH.

The rain distribution yesterday was interesting, with the heaviest precipitation found in a southwest-northeast band from SW Washington across Seattle and into the central Cascades. 

Here are the 24h totals ending 11 PM Saturday (only showing the locations with at least 1.5 inches).   1-5-2 inches over Seattle, but 3-5 inches over SW Washington and nearly 5 inches on the western side of the Cascades NE of Seattle.  8 inches near Mt. Rainier.

The heaviest rain was during the late afternoon and early evening when local streets started to flood.

A radar-based estimate of 48-h rainfall ending 11 PM Saturday show the band of heavy precipitation across central Puget Sound, with 2-4 inches being prevalent.  And you will also notice the rainshadow over NW Washington--those lucky folks from Sequim and Port Townsend to Bellingham.

What was going on?   A relatively narrow atmospheric river of moisture that sat over us for over 12 hours.    You can see the culprit in an infrared satellite picture at 11 AM Saturday (below).

A plot of the moisture content of the atmosphere at 11 AM yesterday shows the feature, with the orange and red colors indicating high values of total moisture in the atmosphere.  Heading right towards us.

Another very useful tool can be created by multiplying the moisture content by the wind...something called Integrated Vapor Transport (IVT).   That is what really controls how much rain we get on our mountains.  IVT at the same time (11 AM) as above  shows significant values moving off the Pacific, with an configuration matching the heavy rainfall.

The forecast for the next 24 hours?  Good news for sodden Washington, bad news for Oregon.  The accumulation from 5 AM Sunday to 5 AM Monday indicates lots of rain in Oregon and northern CA, which is excellent--they need more water down there.  Washington has enough.  More than enough. Our snowpack is above normal.  Our reservoirs are full.  Our rivers are running high.  Our soil moisture is high.

I am heading outside for a run with my dog while I have a chance.

from Cliff Mass Weather and Climate Blog https://ift.tt/2vf2J1n