April is one of the most erratic, varied, and difficult-to-forecast months in the Pacific Northwest. Major windstorms, while rare, are not unheard of; just last April, the Portland metro area got walloped by a windstorm that brought the most power outages to the city since the Hanukkah Eve Storm of 2006. It’s not uncommon to even see brief spurts of snow in the overnight hours in a Puget Sound Convergence Zone or heavy shower, though this snow rarely sticks at sea level. And on the other side of the coin, our high sun angles open the potential for scorching heat waves if a ridge of high pressure builds over the area. Sea-Tac hit a staggering 89 degrees on April 18, 2016 and just hit 73 on March 12 this year; 20 degrees above average for that date and their earliest 70+ degree day ever.
But if there’s one thing that truly defines springtime weather around the Pacific Northwest, it has to be our months on end of “showers and sun breaks.” As we transition into spring, the surface warms much more dramatically than the upper atmosphere, creating a large temperature decrease with height. The faster an atmosphere cools with height, the more unstable it is and the easier it is for convective showers to form. Some of these showers can be quite strong: Vancouver, WA, experienced the state’s only major tornado on record with a F3 twister in April 1972, and on May 4th last year, portions of the Pacific Northwest experienced a bona-fide severe thunderstorm outbreak, with areas of Thurston County getting particularly hard-hit as a thunderstorm with 70 mph winds and torrential rains toppled trees and power lines.
Cumulus clouds and showers over Western Washington on the afternoon of 3/18/2018. These were formed as the surface warmed throughout the day, allowing air to rise and form clouds. Note the clear skies over the Pacific and much of Puget Sound, where surface temperatures are much cooler. Image credit: NASA
But more often than not, our showers are much more benign and are associated with the same mechanisms that our wintertime showers are: unstable, post-frontal flow. The primary difference is not the showers themselves, but the sun breaks that follow. With such high sun angles at this time of the year, the sunshine that returns after a shower has passed is far more dramatic than it is near the winter solstice.
If you like spring, I’ve got good news for you! Seattle hosts some of the longest “spring like” weather in the nation, with average temperatures not reaching the 70s until late June. This is due to our moderating westerly flow off the ~55 degree waters of the Pacific Ocean and Salish Sea. Our dark and dreary winters can sure test our grit, but our long springs and absolutely perfect summers more than make up for it.
Charlie Phillips, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist forecasting wind energy along the Columbia River Gorge. Check out his weather website at weathertogether.us.