Strong Atmospheric River Brings Unusual March Rain

Saturday brought record-breaking daily precipitation to a number of western Washington and Cascade locations, with some 24-h totals getting above 4 inches.  As I will show, several NW rivers have hit flood stage and slope failures are being observed in vulnerable locations (like the infamous rail stretch from Edmonds to Everett.

Yesterday was a beautiful example of the precipitation effects of atmospheric rivers: relatively narrow currents of high water vapor content that can stretch thousands of miles northward out of the tropics and subtropics.

Let me illustrate with two images:  the first, remotely sensed total water vapor content observed by U.S. weather satellites.  This product, produced by NOAA NESDIS, shows very high values of water vapor in the tropics, where warm air can hold lots of water vapor, and a narrow--but juicy--extrusion of high values stretching from near Hawaii to our shores.  Yes...this is also called the pineapple express.

 A simulation of a similar quantity by the UW WRF model shows this narrow moisture plume quite clearly.

This moist plume is also quite warm and is associated with high freezing levels, thus the rain in the mountains.  Winds are quite strong aloft and from the southwest.  Let be show you this, by displaying a plot of the radiosonde sounding at Quillayute, on the northern Washington coast. Temperature is red, dew point is dotted blue and heights are shown by pressure (500 is roughly 18,000 ft, 850 is 5500 ft).

The winds are blocked by the Olympics at low levels (that is why they are southerly down low), but a few thousand feet up they are southwesterly and strong.  Temperature and dew point are on top of each other to about 18,000 ft--that means the relative humidity is 100%--the air is saturated.   When such air hits the regional terrain it dumps huge amounts of precipitation--the precipitation rate is far less offshore.  The storm total precipitation product for the coastal radar (Langley) shows the huge precipitation enhancement of the coastal terrain (particularly the Olympics).  Around an inch offshore, but 3-6 inches over the mountains.

Here are the regional and local 24-h precipitation totals ending 9 AM on Sunday.  Some lower-elevation locations on the SW Olympic peninsula got well over 4 inches.  And there was a strong rainshadow to the lee (NE) of the Olympics were only a few hundredths of an inch fell.  Same thing on the eastern side of the Cascades.  You got to love an area where there is roughly a 100 to 1 ratio of precipitation within 100 miles during a single day.  

 

With the heavy snow pack of last week melting quickly at lower elevations and torrential precipitation, the local rivers are reaching bankful or flooding;  take a look at the current flooding map provided by the NW River Forecast Center.  Moderate floods (blue color) are fairly unusual in March.

Showers will continue today and into Monday, but then high pressure will build in, bringing sun and temperatures in the upper 50s to perhaps 60F.  Be prepared for springtime warmth.