Huge June Precipitation Contrasts Across the Region

If you think rainfall at your location is representative of the region....think again.

This June there were huge precipitation contrasts.

Consider the accumulated rainfall from June 1 to yesterday (June 26 is shown below).

Wow.   The windward (western) side of the Olympics received 5-7 inches so far in June.  In contrast,  only 1-1.5 inches fell over Puget Sound and under a half-inch dampened Sequim, northwest of the Olympics.  

Over TEN INCHES at some locations on the western slopes of the Cascades, declining to under a half inch along the eastern slopes of the Cascade and into the Columbia Basin.

A closer look at the contrasts from Seattle to the Cascades is shown below.  From 0.79 inches over western Seattle to 11.30 inches in the terrain.   

The bounty of western Cascade rain has not only dampened some hikes but have led to healthy river levels.  Below is the latest USGS river level plots.  Most western Washington rivers are near normal (light green) and some above normal (dark blue).   In contrast, some rivers on the eastern Cascade slopes are below normal.

The western side of Washington and particularly the western slopes have been wetter than normal while the eastern slopes of the Cascades have been relatively dry.   Why?

This month the winds approaching the region have been stronger than normal.  Stronger-than-normal winds from the west.  Such a wind field enhances upslope flow and thus precipitation on the western sides of mountain.

In contrast, enhanced winds from the west produce stronger descent and rain shadowing on the eastern side of barriers---producing a strong rainshadow.  

What set up the strong westerly flow this this month?   

Below are the heights at 850 hPa, which you can think of as pressure around 5000 ft.  Lower values over the Gulf of Alaska and higher values to the south.  This pressure pattern produces west-southwesterly winds approaching our coast (white arrow).

How was the situation this month different from normal?  That is shown below (the anomaly of 850 hPa height--think pressure at 5000 ft) .    This month we have had a stronger than normal low over the Gulf of Alaska, which helped enhance the southwesterly flow and brings more moisture into the region.