An Unusual Heavy Rain Event on Sunday

Heavy rain will hit portions of western Washington on Sunday, bringing flooding to some rivers on the west side of the Cascades.   But it will be an usual event for reasons I will describe below.

Normally, our heavy rains events are associated with atmospheric rivers bringing warmth and moisture northward from the subtropics on southerly or southwesterly flow.  The following image shows the total atmospheric water vapor content for such a typical event, with the plume of moisture heading from the south-southwest.

But the forecast moisture distribution on Sunday is very different, with water vapor streaming in from the west (see image)

The reason for this is that we have an area of high pressure south of us...a ridge.. that is pulling moisture northward in the central Pacific and then swinging it around the high...and then directly into us.

So you may ask...so who cares about the direction of the moisture?   It turns out it does make a difference, because the winds bringing in the moisture will be different.  Instead of the normal southerly or southwesterly flow, there will be westerly (from the west) flow. 

This is important.   The Cascades are roughly a north-south barrier and upslope flow, and resulting precipitation, can be greater for westerly than southerly flow.   In the business we call this more orographic enhancement.

Let me show you the forecast precipitation this weekend from the UW WRF model that is run in my department.

The 24h total ending 4 AM Saturday is wet enough, with 1-2 inches over the western slopes of the central and north Cascades.  That is orographic enhancement in action. Note the profound rain shadow in the lee (east) of the Cascades---this is also the result of the strong westerly flow.  And yes, a mini rainshdow to the SE of the Olympics.

 The next 24 hours has the same pattern, but weaker precipitation with unusual amounts of precipitation over NW Washington.

 But the next 24h is another story, with heavy precipitation (black color, 2-5 inches) over the western slopes of the Cascades.

 You want to be impressed?...here is the total of all these periods...the 72 precipitation ending 4 AM Monday.  Yikes!  5-10 inches of rain over the lower slopes of the central and northern Washington Cascades.

I am afraid that some flooding is inevitable from such rainfall, and the NOAA/NWS river forecast center is going for flood stage over several western WA rivers draining the central and northern
Cascades (see below)

Is this heavy precipitation good news for skiers?  Not necessarily.  The air mass is relatively warm, with a snow level around 7000 ft, so there will be rain at lower elevations, with snow over the high terrain.   Here is the snowfall prediction for the next 72 hr...you can see what I mean.

Last weekend, the Snoqualmie Summit snow report talked about "slop".  God knows what they will call it on Sunday.  Saturated slop, perhaps.