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....