With a sunny weekend in store and temperatures more like April than February, spring fever could reach, well, fever pitch over the next few days.

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An outbreak is sweeping the Puget Sound region with no cure in sight.

Symptoms include daffodils, lawns that need mowing and the throaty trill of tree frogs.

With a sunny weekend in store and temperatures more like April than February, experts warn spring fever could reach, well, fever pitch over the next few days.

“It’s like a time machine,” said University of Washington meteorologist Cliff Mass. “Basically, we’ve been advanced by a month or two in temperature.”

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On the heels of the region’s warmest January on record, February has so far continued the trend of above-normal temperatures. Friday’s high at Sea-Tac reached 59, 9 degrees higher than normal. Saturday’s forecast is similar.

The result of all the unseasonable warmth has been a springlike surge in the natural world.

“I never remember having to cut my grass this early before,” Mass said.

Workers at Roozengaarde Tulips near Mount Vernon started picking daffodils Feb. 8.

That’s several weeks earlier than normal and a full month ahead of last year, when chilly conditions prevailed, said Brent Roozen.

“When it gets warm, the flowers really start to push,” he said.

If the trend continues, Roozen expects his tulips will begin unfurling two weeks ahead of schedule.

In Seattle, daffodils in full bloom dotted some hillsides in January. Dandelions started rearing their heads early this month.

“Even the weeds are getting in on it,” said Randall Hitchin. As plant collections manager for the UW Botanic Gardens, Hitchin has been taking note of several impatient bloomers.

“Everybody is saying, isn’t this a little bit early?”

Some plants, like flowering cherries, respond more extravagantly to temperature than others, said UW botanist Takato Imaizumi. Scientists have succeeded in unraveling the ways plants sense changes in light, but are still baffled when it comes to heat.

One theory suggests warming temperatures might increase the fluidity of plant cell membranes, in the same way heat softens butter. But other botanists suspect rising temperatures may unfold tightly wound molecules inside the cells.

One thing that’s clear is that many plants are most sensitive to nighttime temperatures, Imaizumi said.

And one of the most striking things about this year has been warm nights, with temperatures up to 13 degrees above normal.

“A lot of nights haven’t been getting cold,” Mass said. “It’s pretty extraordinary.”

Weather records make it easy to compare temperatures past and present. But recollections of those first blooms of spring can be fallible. The USA National Phenology Network is seeking to quantify data on phenology, or the timing of biological cycles, in response to changes in climate and weather.

The program is just getting started, but last year volunteer observers noted the first yellow sprays of forsythia in Washington the week of March 6. This year, blooms were popping out in some Seattle neighborhoods in early February.

Animals are taking notice, too.

King County ecologist Jo Wilhelm was surprised to hear Pacific tree frogs croaking in January at wetlands she monitors between Redmond and Duvall. “They usually start up in mid- to late February,” she said.

Birds are always flexible when it comes to courtship and mating, said UW biologist John Marzluff. They’re most strongly keyed in to changing light levels, but species like juncos and chickadees do seem to be rushing into romance more quickly this year.

“Their hormones are raging with the increasing daylight,” Marzluff said. “So if temperature and food are warm and abundant, they are able to start breeding.” With an early start, some small songbirds are able to raise multiple broods in a single year.

The region’s human population is feeling the fever, too.

King County Master Gardener Joyce Harms is so emboldened as to imagine planting tomatoes before mid-May. “Typically, if you plant before then they sit there and shiver, and it doesn’t do you any good,” she said.

Harms has enough experience with fickle Northwest weather to temper her optimism. The season’s final frost usually strikes in mid-April, she pointed out.

“We’re not out of the woods.”

But National Weather Service meteorologist Johnny Burg said there’s no sign the El Niño pattern driving the region’s early spring weather will dissipate anytime soon.

And a massive high-pressure ridge to the north will keep skies here sunny at least through the early part of next week, Burg said.

“It’s going to stay warm for a while.”

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

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