Midweek rain could give way to record-breaking temperatures by Saturday

Friday would be a good day to come down with a scratchy throat or some mystery ailment — a vitamin D deficiency, perhaps? — to take full advantage of what could be record-breaking temperatures as Seattle gets its first dose of summer.

Even if you're too busy or too ethical to play hooky from the weekday grind, Saturday is shaping up to be a sizzler — and could even become the earliest day on record to exceed the 90-degree mark.

"It's a two-day heat wave," said Johnny Burg, a meteorologist with the National Weather Service in Seattle.

The mid-May warm-up is a nice break from the rain forecast today and Wednesday and sure to delight local sun worshippers. But there could be avalanche problems in the mountains, so you really should postpone that ice-climbing trip up Mount Rainier.

By Sunday, the clouds will return with normal temperatures for this time of year and daytime highs in the low- to mid-60s.

A ridge of high pressure moving up from California should settle over Washington's coast this Wednesday afternoon, and people there will experience their own little heat wave on Thursday and Friday, with temperatures in the upper 70s and mid-80s, Burg said. As the high-pressure system moves up the coast, it will head inland into British Columbia.

Meanwhile, an "inverted trough" of warm air is expected to set up over our coast, which will help "bring warmer air from down south," Burg explained. At the same time, winds from B.C. will blow toward us. Instead of coming off the ocean, those winds will travel across land — and because land warms up more quickly than water does, the air will hold that heat and keep it closer to the surface.

For the Seattle area, that means Thursday will start out cool but will heat up by afternoon, with temperatures likely to reach the upper 70s. Record-breaking daytime highs are possible on Friday, when we could get hotter than the 84-degree record for May 16, set at Seattle-Tacoma International Airport in 1985.

But Saturday is the day forecasters will watch most intently. The mercury rose to 92 degrees on May 20, 1963 — and May 20 has remained the earliest day of any year for Seattle to get temperatures of 90 degrees or higher since record keeping began at Sea-Tac airport, Burg said. A reading of 90 degrees or higher on Saturday would bust that nearly 45-year-old record by three days, he said.

It's still too early to tell how the heat-up will play out in the mountains, said Kenny Kramer of the Northwest Weather and Avalanche Center. But he said avalanches "are definitely something to be concerned about."

Last winter followed a typical La Niña pattern, with colder-than-normal temperatures and greater-than-normal snowfall, especially in the Cascades, Kramer said. Until a couple of weeks ago, the snowpack was still building, not decreasing as it typically does in spring. Now, across the Cascade Range, the snowpack is 140 to 200 percent of what it normally is, he said.

The snowpack hasn't undergone a typical spring thaw-freeze cycle — in which it melts during the day and refreezes overnight — so there's a good chance our upcoming heat wave could trigger slides of heavy, wet snow, Kramer said.

"But it won't be uniform," he said. Conditions depend on how the snowpack formed and changed over the winter in different locations.

If you're heading across one of the passes to Eastern Washington this weekend — or if you're planning a backcountry ski trip or mountain climb — Kramer suggests checking avalanche conditions before heading out. Visit www.nwac.us or call the avalanche hotline at 206-526-6677.

Sara Jean Green: 206-515-5654 or sgreen@seattletimes.com