The Heavy Rain/Flooding Event Begins

An unusual heavy rain and flooding event has now begun, and if the forecasts are correct, it will be an amazing example of the prowess of modern numerical weather prediction.

The 6 PM radar says it all..with a plume of moisture moving into our region from the Northwest.

Let me show you the latest forecasts for 24h total precipitation.   For the 24-h period ending 4 AM Sunday, there is heavy precipitation along the western slopes of the Cascades (as much as 2-5 inches, black color), with an extension over Whidbey Is. and Port Townsend.  I have never seen an extension of heavy rain away from the mountains like that.

 The next 24-h  (through Tuesday at 4 AM) is just crazy, with some locations getting  up to 5 inches of additional precipitation.

And believe it or not, even more the next 24 hr under sustained northwesterly flow.

The 48-h total ending 4 PM Monday is just amazing, with the highest amounts hitting 10 inches (in a very small area) Not the profound rainshadow to the SE of the Olympics.   They will get essentially nothing. And dry just to the east of the Cascade crest.  Such huge precipitation contrasts make Northwest meteorology fascinating.

As I noted in my earlier blog, the maximum precipitation is lower down the slopes than usual, with enhanced low-level blocking of approaching air by the Cascades causing air to rise sooner than normal--thus pushing the precipitation to the west.

The NOAA/NWS River Forecast Center in Portland has revved up their warnings and is going for moderate flooding (blue dots) on several rivers draining the western slopes of the Cascades. Flood stage will occur on several more.

Many of the locations will get very close to a major flood, as illustrated by the river flow prediction for the Snoqualmie River near Carnation. 

If you want to see something impressive go Snoqualmie Falls on Monday or Tuesday.   In any case, you can choose to be entirely dry or completely drenched on Sunday....

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.