Heavy Rain, Flooding Rivers, and Profound Rain Shadows

As advertised, a moderate atmospheric river event occurred on Saturday bringing heavy rain and limited flooding to our region. 

The water vapor satellite imagery at 5 PM yesterday said it all....a huge plume of moisture stretching back thousands of miles into the central Pacific. (this type of satellite imagery highlights water vapor content in the middle to upper troposphere).

The 24-h precipitation totals ending 7 AM Sunday were impressive (see image, click on it to enlarge), with over four inches on the SW side of the Olympics and the western slopes of the Cascades and 1-2 inches over the south Sound lowlands.   Look closely and you will see only about 1/4 inch fell around Port Townsend on the NE side of the Olympic Peninsula...the rain shadow in action.

 Even heavier rain fell over the coast mountains and Cascades of western Oregon, with several locations receiving over 6 inches in that 24-h period.  Roughly 2 inches in Portland.

If you really want to appreciate the rainshadow, here is the 24-hour precipitation total from Seattle RainWatch, which calibrates radar totals using rain gauges.  .1-.2 inches over southern Whidbey Island, with 4-5 inches southeast of Puget Sound over the western slopes of the Cascades.

Even though the region has been dry this summer, such heavy amounts quickly fill major rivers.  Thus, several of the rivers draining the western Cascades are now or soon will be in flood stage (red and blue dots)

To illustrate, the Snoqualme River, near the town of Snoqualmie, is now at moderate flood stage.  Fortunately, it should decline rapidly today.

A plot of the winds and temperatures above SEA-TAC Airport for the past day, shows the surge of warm, southwesterly flow late Saturday (red is temperature in C, wind barbs are shown, time is in UTC (Z, GMT) on the bottom and the y coordinate is height in pressure (850 hPa is about 5000 ft).  This morning the temperatures cooled and the winds turned more westerly aloft.

As a result of the westerly winds aloft, Seattle and much of Puget Sound is in the rain shadow of the Olympic Mountains today, with lots of sun.  In contrast, the western slopes of the Cascades and Olympics are getting hit with moderate showers (see radar below).

A nice day around Seattle and Portland today, even with some moist, unstable flow reaching our coast.  You get to appreciate the rain shadows around here.

from Cliff Mass Weather and Climate Blog http://ift.tt/2yC3bHs