Heavy Rain and Super Rain Shadow

There was some impressive rain totals today for June, with the southwest side of the Olympics getting 2-3 inches so far, with 1-2 inches on the western side of the Cascades (see plot of 24h rain ending 7 PM Wed).   But just as impressive was the profound rain shadowing on the northeast side of the Olympics and the eastern slopes of the Cascades.   Amazingly, there were only a few hundredths of an inch of precipitation around Sequim and .02 inches at Friday Harbor in the San Juans.  

Take a look at a blow-up precipitation plot for a better view. One location near Sequim had only .01 inches.  Hundreds of times that rainfall fell 50 miles away on the other side of the Olympics.

Why such a difference?   We had a very moist airflow coming into our region, driven by strong winds.   Here is plot of the winds with height at Quillayute, on the NW Washington coast. Winds near crest level were southerly to southwesterly at 30-40 knots (the heights are in pressure, 850 is about 5000 ft).  Quite unusual for this time of the year.

With such south/southwesterly winds, the moist air was forced up on the southerly/SW side of the Olympics (producing heavy precipitation) and then descended (causing drying) on the north and northeast side of the barrier.

The rain shadow was very obvious in the National Weather Service weather radars (see example below) as a "hole" in the precipitation shield.

And the rain shadow was also apparent in the visible satellite imagery this afternoon (see example below)

The heavy rain is causing some local rivers to surge to near record levels for the date--for example, the Wynoochee River near Montesano.

The situation will dry out into Saturday, but rain will return Saturday night into Sunday morning.   Nice for our plants and water bills.

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