Excessive Heat Warning begins today; temperatures could reach triple digits around Seattle


SEATTLE -- Hot weather in August is not unusual, but the heat coming this week is set to push the upper limits of how toasty Seattle can get.

The National Weather Service is predicting record-setting heat Wednesday and Thursday, when temperatures at Sea-Tac Airport are forecast to reach 99 degrees.

An Excessive Heat Warning is set to go into effect at 2 p.m. Tuesday and remain in effect until 9 p.m. Friday for much of the interior of Western Washington:

A huge ridge of high pressure is building over the Pacific Coast this week, bringing in a very hot air mass from the Desert Southwest. Meanwhile, a thermal trough will build into the western half of the state by midweek.

This is the classic heat wave pattern for the Northwest and the combination of the searing heat in the ridge, plus the added boost of a little east wind that warms further as it sinks down the Cascades -- and coming at statistically the hottest part of the year is the "perfect storm" of conditions to maximize the region's heat potential.

MORE | Seattle Cooling Centers | Renton

Highs Tuesday will get well into the 80s, with low-mid 90s likely on Wednesday.

The furnace goes full blast on Thursday with widespread upper 90s at least, and potential for areas from Seattle south and east to reach 100 degrees or more. Multiple raw computer forecasts are predicting a high of about 100 degrees at Sea-Tac Airport on Thursday, which would be just the third ever recorded triple digit reading at Sea-Tac (July 20, 1994 (100) and July 29, 2009 (103) the other two), and fourth in Seattle's history. (The Downtown Federal Building registered a 100 degree high on July 16, 1941.)

But even if the all-time records survive and/or we don't reach 100, we'll for sure set some new daily record highs in Seattle. Tuesday's is 92, Wednesday's is (only!) 89, Thursday's is 90 (probably gone by lunch) and Friday's is 95 (the first real challenge).

A small silver lining is that it is a dry, desert heat -- no mugginess here. But the light offshore winds will inhibit cooling at night with lows only dropping into the mid-upper 60s -- which means temperatures remaining in the 90s and 80s well into the night.

Where to find relief

If you're looking for relief during the day-- there is no place where it'll be cool, but going to the coast or getting near the Strait of Juan de Fuca will help some. Latest projections show the thermal trough won't make it all the way north through the western half of the state, which in turn means the offshore wind wouldn't be terribly strong in the far North Sound and may even allow for a weak afternoon seabreeze. The air mass itself will remain very warm, but highs along the coast, the San Juans and the northern Olympic Peninsula are expected to "only" top out in the mid-upper 80s, with temperatures closer to just 90 in Skagit and Whatcom Counties. Thus, no heat warnings in effect there.

Projections show some cooling under way late Friday as the ridge weakens a little and the thermal trough moves inland, but not early enough in the I-5 corridor to thwart the peak heating of the day. Thus, some projections are again putting Seattle at or near 100 degrees again on Friday afternoon.

Even with the thermal trough going away and easing back on the warming east winds, the air mass will remain quite hot into the weekend and early next week with highs still expected to reach into the 80s on multiple days.

How does this compare with previous record heat waves?

Long-term heat waves are difficult to achieve in Western Washington because it takes a special set up to hold the ocean breezes back, which naturally want to flow inland with the general west-to-east flow of the weather. Many times it's not so much a persistent east wind but a ridge of high pressure that creates such a hot air mass that we'll get into the 90s without any warming help from the east winds (although usually there will be 1-3 days amid that stretch where the east wind boosts temperatures even further.)

The "big kahuna" heat wave that holds all of the modern records was just over 8 years ago in late July of 2009. That featured an 8-day stretch of heat that went: 89, 94, 97, 103, 96, 84, 90 and 89. Days 1-7 and 2-8 of that stretch constitute the top two warmest weeks in Sea-Tac History. The other long stretch of heat was August 6-12 in 1981 that had a stretch of 86, 91, 93, 99, 98, 93 and 86.

A 17-day stretch in August 1977 had 13 days that were 85 degrees or warmer, peaking with a four-day stretch in the 90s between August 9-12.

And most recently, a heat wave in 2015 didn't reach lofty numbers, but did manage to set the record for consecutive days above 90 degrees at five; no day in the streak getting warmer than 93.

This current set up isn't quite the same as the 2009 setup which netted the all-time records of 103-105 around the Puget Sound region. That pattern had tapped into a bit of humid air, which kept nighttime lows near 70 -- also all-time records. That gave a better springboard for the heat on that July 29th day to reach 103 in Seattle. With drier air, overnight lows this time around are expected to be able to drop a little bit down to the mid-upper 60s -- still quite mild, but not 70. However, even if the 103 record stays safe, this heat wave could make a name for itself by hitting 100 on back-to-back days.

Meanwhile, the dry streak keeps churning on...

Probably lost in the heat headlines is the fact that rain remains completely absent from the forecast. Monday marks Day 44 with no measurable rain in Seattle. We're now in 4th place with clear sailing ahead to reach the 51 day record on Monday.

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