The Heaviest Precipitation of the Year Ahead in Some Locations?

If you ask most Washington State residents when the heaviest precipitation of the year generally occurs, they will tell you during the late fall and winter.  

It turns out that this is true for many, but not all, Washington State locations.    For some locations in eastern Washington, JUNE is the wettest month or at least equal to December.   

For example, consider Northport, a town in northeast Washington on the Columbia River (red marker on the map)

At this location, June has as much precipitation as December!  At some locations in northwest Washington and southeast BC, June is clearly the wettest month

How can this be?

One reason is that thunderstorms and convective showers are increasing rapidly this time of the year, with maximum thunderstorm activity in late June (see map below)

But there is another reason.....and we will experience this feature this week.

During the cool season, the westerly (from the west) jet stream is strong, and eastern Washington is in the rainshadow (see map of annual precipitation below)

But during May and June, the westerly jet stream weakens, and sometimes a trough of low pressure slides south of Washington, with moist southeasterly flow moving northward up its eastern side into eastern Washington.  No rainshadow!

This is exactly what is predicted to occur on Monday and Tuesday (see upper level map (500 hPa, about 18,000 ft) for early Tuesday).

Eastern (and western) Washington needs to be prepared for substantial spring rains.

The total through Wednesday morning is substantial in the west, and much of eastern Washington gets moistened.

Another low-pressure trough will move in later in the week, with the accumulated precipitation total through Sunday at 5 PM shown below.  Very, very substantial amounts in eastern Washington and Oregon.  Less irrigation will be needed next week.

 Enough precipitation over the Olympics and eastern Washington that I suspect the drought talk should fade.

 

A Wet Memorial Day

 If you are planning an outdoor excursion on Memorial Day (Monday), I have a suggestion:  bring an umbrella and wear a warm jacket.

Consider the latest forecasts.  The total precipitation through Sunday at 5 PM shows only a little drizzle on the windward side of some local terrain.

But on Monday, a strong upper-level trough will move in (see upper level map below, with blue indicating lower pressure than normal...troughing), producing much colder than normal temperatures (highs in th 50s) and substantial precipitation.

Here is the forecast precipitation total through Tuesday morning:  western Oregon and Washington will be thoroughly watered down, with some rain extended over the Cascades into Yakima and Kittitas counties. 

More precipitation is predicted for later in the week.  In fact, the regional totals through early June are scary wet (see below).  Perhaps this will comfort folks worried about drought. 😊

It is important to note that Memorial Day weekend is infamous for wet conditions in our region.  It illustrates this soggy fact, below is a plot of the historical precipitation at SeaTec from May 23 through May 28.

Most years have some precipitation over that period...and occasionally it gets quite wet (almost 2 inches).   This year, we will enjoy about a half inch over that period, so we will be a little wetter than normal.     That's fine...we can use it.