The Mid-March Precipitation Transition over the Northwest

Residents of the Northwest are a savvy group regarding precipitation, considering their considerable experience with the wet stuff.   

And the really precipitation-aware know about an interesting subtlety:  the late-winter precipitation plateau and the mid to late March transition over western Washington.  And the eastern Washington precipitation leveling during spring.  And more.

To illustrate, consider the climatology precipitation at Seattle Tacoma Airport, showing the probability of experiencing 0.01 or 0.10 inches of precipitation in a day.

The wettest period at SeaTac is from mid-November through early January, when the incoming storms off the Pacific are strongest and more frequent.  Then the probability of precipitation declines a bit in January, followed by little change through mid-March.  Subsequently, from the last few weeks of March through early May, precipitation significantly declines.   I call this the "spring drying."   

Interestingly, there is a bit of an uptick in Seattle's precipitation frequency in late May--at the beginning of the June Gloom period.  There are two reasons for this.  First, high pressure building over the eastern Pacific starts pushing more moisture into the region during that period, resulting in light rain.  Also, we can get some convection and thunderstorms during that period, as the stronger sun drives convective showers.  

This late-spring moist period is followed by the "big drying" in July that takes us to the desert-like conditions of late July and early August.

What about the rest of Washington State? 

Richland, in the arid Columbia Basin, has its peak probability of precipitation during late November--in the middle of the Northwest storm season.  In December precipitation starts to decline, with minor ups and downs, until early April.

But then something interesting happens.  The precipitation stops declining and levels off until early June.   Why?

The answer:  spring is shower and thunderstorm season over the Columbia Basin, as the increasing sun causes the atmosphere to destabilize--to start convecting.  Weak disturbances off the Pacific help provide some lift and initiate these showers. 

In mid-June, the ground has dried up substantially (less moisture available) and the disturbances of the Pacific become infrequent and weak.  The result is a sharp drop in precipitation during mid-June at Richland and vicinity.

The situation at Spokane is similar in Richland, except the winter peak is a bit later (very late December).

The subtleties of Northwest weather are always fascinating.  You start with variations in the storms and moisture coming off the Pacific.  Add to that changes in vertical stability and thunderstorms/convection as the temperature change with height changes by season.   Varying winds during the year alter the effects of terrain (windward enhancement/lee rain shadowing).  The precipitation story is complex and nuanced.