Start of spring brings showers, sun breaks and most intense Puget Sound convergence zones | KNKX

Start of spring brings showers, sun breaks and most intense Puget Sound convergence zones

Mar 20, 2020

The official start of spring comes a little early this year, in tandem with the vernal equinox that showed up in most U.S. calendars on March 19. Although meteorological spring began in the Pacific Northwest about a month ago with signs of warmer weather, KNKX weather expert Cliff Mass says around now is when certain typical features of spring become evident.

These include puffy cumulus clouds in many sizes, rain showers and sun breaks, sometimes even thunder and lightning as the Puget Sound convergence zone gets extra power. At the root of all of these is instability — forces of nature colliding in ways that manifest in these features, with lots of movement. Mass says a key to understanding what’s going on is the concept of convection. And it’s a concept most people can observe in their everyday lives.

“If you ever make hot cereal — oatmeal let’s say. You put the oatmeal in there and you heat up the bottom of the pot,” Mass says.

As the pot warms and the oatmeal cooks, careful observation reveals that some of the cereal goes up, some goes down.

“And that’s what we call convection,” he says.

This is driven by the difference in temperature, between the hot bottom of the pot and the colder materials above it. The same thing happens in the atmosphere in spring, when the sun is strengthening and warms the surface of the earth and ocean, while the air aloft remains relatively cold.

“So we can get a big change in temperature with height — and that tends to cause instability,” Mass says. And that, in turn, creates many typical weather patterns of spring, including our famous showers and sun breaks.

Perhaps the most unique spring phenomenon for Western Washington is the pronounced occurrence of the Puget Sound convergence zone. This is when winds from the west that hit the Olympic Mountains split and then come back together on the other side, producing a narrow, intense band of rain somewhere over Puget Sound, typically north of Seattle. In spring, this can be especially intense.

“We can get these towering cumulus clouds, that are driven by the instability, right over Puget Sound,” Mass says. “So the best convergence zones with the big towering clouds, heavy precipitation, sometimes a little hail and often some lightning — those are associated with convergence zones interacting with relatively instable air, during the spring.”

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.