Inversion creates temperature extremes

While the Puget Sound lowlands shiver, temperatures at higher elevations across Western Washington have soared into summerlike territory for about a week, thanks to a persistent atmospheric inversion.

Saturday’s high at Paradise, on Mount Rainier, was a toasty 64 degrees, more typical of mid-July than January. Hilltops southeast of Olympia peaked at 72 degrees Sunday, while the capital city itself barely budged the needle into the high 30s.

Visitors to Snoqualmie Pass over the weekend frolicked in the sun in shirt sleeves, while at Crystal Mountain Ski Area conditions were like spring skiing.

“It’s amazing,” said National Weather Service meteorologist Andy Haner. “This is as extreme as it gets.”

On his blog Sunday, University of Washington atmospheric-sciences Professor Cliff Mass joked: “Instead of going to Hawaii, rent a helicopter.”

As little as a thousand feet of elevation gain has been enough to leave the cold behind in many places. Sunday morning, the chilly blanket that shrouded Seattle was only 600 feet thick, Mass reported.

“Usually, the atmospheric temperature gets colder as you go up in elevation,” Haner said. “In an inversion, things flip over.”

For those with cabin fever, Haner recommends a short drive east on Interstate 90. “You get to milepost 35 and you will suddenly pop out into beautiful, mild, sunny weather.”

The inversion owes its existence to a strong high-pressure system that has been parked over the region for several days, Haner explained.

A similar system in the summer would result in a heat wave. But during the winter, the long nights allow more heat to dissipate to the cloudless sky. The heavy, cold air is trapped near the ground.

But aloft, the high-pressure system causes the air to sink and warm.

Inversions are very stable, Haner said. “It will stay in place until something comes along and mixes it up.”

Inversions also trap pollutants, which is why the Puget Sound Clean Air Agency imposed burn bans that are still in effect in King, Pierce and Snohomish counties.

On Sunday afternoon, a slight east wind helped clear much of the fog from the Interstate 5 corridor between Seattle and Everett. Sunday’s high temperature at Seattle-Tacoma International Airport reached 38 degrees, while parts of Seattle cracked 40. Tacoma and other points south remained shrouded throughout the day, Haner said.

The pattern should start to break down by Tuesday afternoon, as a series of fronts sweep through the area, with rain likely. In lowland areas where the inversion has kept temperatures in the low to mid-30s, the storm front will actually warm things up into the mid-40s.