KING

SEATTLE – The heat is on. Over the next three days, record-breaking temperatures are expected to hit the Northwest.

Western Washington was in the upper 80s to low 90s, and even higher in Eastern Washington and Oregon.

"It is going to be a scorcher today, tomorrow and probably Saturday as well," said KING 5 meteorologist Rich Marriott. "Things will moderate as we head into Sunday."

Sea-Tac Airport hit a new record of 89 degrees as of 5:30 p.m. Thursday. Chehalis hit 94, Issaquah 92, Bellevue 91 and Belfair 90 degrees.

The heat wave could make it difficult for some people to breathe. A smog watch is in effect for King, Kitsap, Pierce and Snohomish counties. There is also an air stagnation advisory over parts of Whatcom and Skagit counties, Hood Canal, the Chehalis Valley and the west slopes of the north and central Cascades. Air quality is expected to be unhealthy for sensitive groups Thursday and Friday.

If you're especially sensitive to pollution, you should limit your time outdoors. The Clean Air Agency is asking drivers to carpool or ride the bus if possible. Air quality should improve overnight Saturday into Sunday.

According to KING 5 meteorologist Jeff Renner, what's driving temperatures up is not only a ridge of high pressure off the coast, but also a thermal trough, which is a zone of very hot, low-pressure air moving northward from the interior valley of California toward Washington.

The warmest air will shift east late Saturday, and offshore winds will bring cooler air on Sunday, bringing the temperatures down in the low 80s.

The National Weather Service warns that isolated lightning is expected anywhere in Western Washington from early Saturday evening through Sunday.

A red flag warning for the southern Cascades has been extended through noon Saturday

Eastern Washington

The forecast for the Spokane area has temperatures soaring to the mid- to upper 90s for Friday through Sunday.

The Grand Coulee Dam hit 101 degrees Thursday.

Any thundershowers are most likely this weekend over the Cascades and Eastern Washington.

"Spokane could see a few thunderstorms Sunday night into Monday - not good news," said NWCN forecaster Nick Allard.

The beginning of next week should cool down a bit when temperatures should settle into the 80s.

Excessive heat warning in Oregon

Oregonians are also bracing for a heat wave that could bring three straight days of 100-plus degree weather to the usually temperate Willamette Valley.

The National Weather Service issued an Excessive Heat Warning and urged people to be aware of heat-related illnesses.

The high temperatures are also fanning fears in about lightning-caused wildfires in Central, Southern and Eastern Oregon.

A Red Flag Warning was put in effect from 12 p.m. Thursday through 9 a.m. Saturday for the Cascades, Mount Hood and Suislaw forests. This means very dry and hot conditions with extreme fire danger.

Firefighters are concerned about the warming temperatures and about what one official called "squirrelly winds" flowing around Mount Hood.

The heat wave is expected to break by Sunday, cooling things down by about 15 or 20 degrees.

The Associated Press contributed to this report.