Why spring in the Pacific Northwest starts a month earlier than other parts of the U.S. | KNKX

Why spring in the Pacific Northwest starts a month earlier than other parts of the U.S.

Feb 21, 2020

The mornings have been cold and crisp under clear skies this week in many parts of the Pacific Northwest, with lows around freezing in places. But powerful sunbeams have pushed afternoon highs into the pleasant realm of the 50s.

KNKX weather expert Cliff Mass says this is in keeping with what he calls “meteorological spring,” when the weather stops delivering storms and cold temperatures that are the hallmarks of winter. Instead, trees and shrubs start sprouting green buds, crocuses push through the soil and people begin shedding layers of clothing needed in colder weather as they enjoy warmer temperatures.

“There’s a distinction between astronomical spring and when metrological spring starts,” Mass says.

He says the more common idea about when winter ends is by the calendar, on March 20 or 21, the day known as the vernal equinox, when the length of daytime and nighttime hours (or light and dark) are as equal as they get.

“That’s when the sun is crossing the equator from south to north and the day is pretty much the same, everywhere in the world, approximately 12 hours,” Mass says.

Mereological spring is different. It’s based not on the calendar, but on what is happening in our skies and atmosphere.

“That’s when the weather really starts to turn,” Mass says. “When the chances of big storms really drop off, the chance of heavy rain with atmospheric rivers declines, when the chance of getting heavy snowfall or even moderate snowfall goes almost to zero.”

And Mass says that tends to happen here around the third weekend in February – a full month ahead of many other places in the U.S., such as the East Coast, where major storms continue to hit through March.

Mass says the reason for our early spring weather has to do with our geography. You can listen above to hear more details in our discussion.

CLIFF’S FORECAST

“Pretty good, except for one day: Sunday,” he says. “Definitely an improving trend.”

Friday: sunny and cold in the morning with temperatures zooming up into the lower 50s. “A really nice day.”

Saturday: a transition day, with a weather system approaching. Sprinkles on the coast and in Northwest Washington, but mostly dry in Western Washington. Light showers in the evening.

Sunday: a deep low pressure center approaches from the ocean, crossing southern British Columbia. Substantial winds (30-50 mph in places) with possible power outages. Rain in the lowlands and snow in the mountains. The system moves through in the evening.

Monday-Wednesday: “Quite favorable.” Cloudy skies. Very low chance of rain (10-15 percent), highs in the low to mid-50s. Continuing sunshine.

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.