The residents of the Northwest are a rainfall-savvy group, considering their considerable experience with wet things.
And those who really know about precipitation know about an interesting subtlety: the late winter precipitation plateau and mid-to-late March transition in western Washington. And rainfall east of Washington flattens out during the spring. And more.
To illustrate, consider climate precipitation at Seattle Tacoma Airport, which shows the probability of 0.01 or 0.10 inches of precipitation in a single day.
The wettest period at SeaTac is from mid-November to early January, when extra-Pacific storms are strongest and more frequent. Thereafter, the probability of precipitation decreased slightly in January, followed by little change until mid-March. Then, from the last few weeks of March to the beginning of May, rainfall drops dramatically. I call this “spring drying.”
Interestingly, there was a slight increase in rain frequency in Seattle in late May – early June Gloom stage = Stage. There are two reasons for this. First, high pressure building in the eastern Pacific began to push more moisture into the region during that time, resulting in light rain. Also, we may get some convection and thunderstorms during that time, as the stronger sun leads to convective showers.
This period of late spring wetness was followed by a “major dry spell” in July that brought us to desert-like conditions in late July and early August.
What about the rest of Washington State?
Richland, in the arid Columbia River Basin, has the highest probability of rain in late November – the middle of the NW hurricane season. In December, rainfall begins to decrease, with minor ups and downs, until early April.
But then an interesting thing happened. Rainfall stopped falling and leveled off until early June. Why?
The answer: spring is the season of showers and thunderstorms over the Columbia River basin, because sunrise causes the atmosphere to destabilize – convection begins. Weak disturbances beyond the Pacific help create some lift and initiate these showers.
By mid-June, the ground has dried up considerably (with less moisture) and the Pacific disturbances have become infrequent and weaker. As a result, rainfall fell sharply in mid-June in Richland and surrounding areas.
The situation in Spokane is similar to that in Richland, except the winter peaks a bit later (very late in December).
The subtleties of Northwest weather always have a strange attraction. You start with the variability of storms and humidity from the Pacific. Add to that a steady vertical shift and thunderstorms/convection as the temperature varies with the seasonal variation in altitude. Wind variation during the year alters the influence of topography (wind strengthening / rain cover). The story of rainfall is complex and nuanced.
