The stormy weather that may have played a role in the deaths of three boaters last week is a common occurrence that boaters need to be aware of, experts say.

The region was hit with what’s known as an onshore or marine push, a shift from above normal temperatures to cooler air moving in off the ocean. Marine pushes happen frequently in spring and summer and are often accompanied by strong winds. Forecasters knew one was moving into the region late Thursday, but its ferocity took them by surprise.

“We didn’t expect that it was going to be as powerful as it was,” said meteorologist and University of Washington professor Cliff Mass. “Within 15 minutes, we went from warm, nice conditions to winds blowing 20, 30, 40 miles an hour and temperatures 15 degrees cooler.”

The push churned up waters on Lake Washington, where a boater drowned late Thursday night, before moving north toward Chuckanut Bay, where a sailboat overturned in the early hours of Friday morning, leaving two sailors missing and presumed dead.

On Thursday afternoon, the National Oceanic and Atmospheric Administration forecast called for sustained winds of 10 to 20 knots. Around 8:30 p.m., a forecaster at NOAA’s weather office in Seattle’s Magnuson Park upgraded the forecast to winds of 15 to 25 knots and issued a small craft advisory.

Around the same time, Mass stood nearby on a hill in the park, watching whitecaps starting to form on Lake Washington. Within minutes, the sky darkened and the visibility dropped dramatically.

“It was like a dust storm,” he said. “No one should have been out there in a small boat.”

A couple of hours later, two men left Ivar’s Salmon House on Lake Union in Seattle, got in a 16-foot runabout and headed east toward a marina in Kirkland. At some point during the trip, the passenger in the boat, 56-year-old Kirkland resident August Reyes, fell or jumped in the water. The skipper grabbed a boat cushion and went in after Reyes, trying to save him.

But Reyes was quickly swept away in the choppy water. His body was found the following day in Lake Washington, floating about 20 feet offshore.

The marine push then moved north, where an 18-foot sailboat with six people aboard capsized on Chuckanut Bay, about two and a half miles south of Bellingham. The boat headed out about 8:30 p.m. from Bellingham Bay, authorities said, and capsized around 2 a.m. Friday in seas of one to two feet.

Three women and a man managed to swim ashore in the 49-degree water but the other two sailors, Sean Mansfield and Gunther Frank, have not been found.

Authorities have not conclusively determined what role weather might have played in the two boating accidents. Still, Mass said boaters need to be aware of marine pushes, which usually hit about 7 to 10 p.m. after a period of especially warm weather—exactly the type of weather likely to entice boaters out onto the water. 

“Every boater should know about these things,” he said. “We have a few every summer.” 

Marine pushes are typically gradual, with winds building over several hours. But Thursday’s push was complicated by thunderstorms occurring simultaneously in southwestern Washington. Like a marine push, a thunderstorm sends cold air to the ground, causing strong winds.

Brad Colman, the meteorologist in charge of NOAA’s National Weather Service office in Seattle, said the two converging events resulted in an uncommonly strong push.

“The thing that made this so unusual was the abruptness and the gustiness of it,” he said.

NOAA revised its forecast three times Thursday night, Colman said, in an effort to get the most up-to-date information to people. The two boating accidents, he said, “are a tragedy, and if we could have done something, obviously we would have.” 

Mass urged boaters to check NOAA’s marine forecasts before heading out and check websites or listen to weather forecasts that provide information on differences in sea level pressure. A good indication of how strong a marine push will be is the pressure difference between Seattle and Hoquiam, Mass said—if the difference is four millibars or greater, expect wild conditions on the water.

“People have got to be ready for those nights,” Mass said. “On the night of a push, if they’re not ready to handle winds of 20 to 30 knots, they shouldn’t be out there.”