Potent Atmospheric River Will Reach the Northwest Tomorrow

If this blog had music, you would hear the JAWS theme in the background (click here if you want the audio).

A very potent Westerly Atmospheric River (WAR) event is coming, with heavy rain and flooding.  Another drought-buster event that will fill our reservoirs.  And there is more:  plenty of wind and high surf on our ocean beaches.

A recent infrared satellite image shows a plume of high clouds associated with the approaching moisture and strong winds (see below).

 And the latest UW WRF forecast for Thursday night at 7 PM shows the high moisture value (red colors) aimed right for us.  It looks like a fist.  This is an example of an atmospheric river, a plume of large moisture values, but it is not a pineapple express--those events bring moisture from the southwest near Hawaii. In this case moisture is streaming northward over the western Pacific and then heading nearly due east towards us, Westerly Atmospheric River.

This moisture plume is associated with strong westerly wind sand when the moist air is forced to rise by our regional terrain the result will be very heavy precipitation.  To illustrate, here is the Washington State 48h precipitation ending 4 AM on Saturday.  Broad areas with 5-10 inches and some exceeding 10 inches. That is serious folks.

The following  72 hours brings more, with the Oregon Cascades getting hit hard as the atmospheric river moves south.  Even California and its thirsty reservoirs will get a piece of this action.

The NOAA/NWS River Forecast Center is predicting flooding on many western WA rivers, several reaching moderate flood levels (see below).  So be careful in the normal flood-prone spots.

Now I know my snow loving friends are getting nervous about this atmospheric river event.  

Atmospheric rivers usually bring warm rain through elevation, which is not exactly good for snow.   That is true...but this case is not a pineapple express so temps will be a bit cooler and the period of warmth will be relatively short (about 36h).   The snowpack will be knocked back a bit at lower elevations, but some of the rain will freeze in the snow.  And higher elevations will have a net snow gain.   Let me show you.

The UW WRF model suggests that  next 72 still bring some snow to the higher elevations (see below)

But the exciting thing is that after the atmospheric river passes through on Monday, we moved into colder air with lots of shower activity....and large amounts of snow (see below for snowfall for 72 hr ending 4 PM Wednesday). Feet of snow over the entire Cascade Range.  Ski season begins.

Finally, the strong winds working on a long fetch over the ocean, coupled with some big storms offshore, will bring big waves to the coast...some getting up to 20 feet.  The NOAA Wavewatch III model suggests that later on Friday there could be 7 meter (around 23 ft) waves hitting the central WA coast.  Good wave watching at Westport.