Weather Blog

5 years ago today, Seattle baked like it never had before

Those of you who were around here 5 years ago today were likely trying to find some place -- any place -- for relief from the heat.

July 29, 2009 remains the hottest day ever recorded in modern Seattle history, be it Sea-Tac Airport or the Downtown Federal Building, with a reading of 103 degrees. Some spots, like Boeing Field and the NOAA station at Sand Point, were even hotter, reaching 105 degrees. Everett hit 100, Bellingham hit 96, and Vancouver, WA took the heat prize with a 108 degree reading!

100 degree readings are quite rare -- the only time Sea-Tac Airport had reached 100 before was July 20, 1994 -- and even then, technically, it was 99.9 degrees that was rounded up. The Downtown Federal Building managed to reach 100 twice (June 9, 1955 and July 16, 1941) and there is a report from July 1870 that shows Seattle hit over 100 a few times that month, including 104, but weather measuring standards may not have been equal to modern times.

The 103 degree reading also means Seattle's record high is warmer for July 29th than Spokane (99), Pullman (100) and Yakima (102)!

It was so hot in 2009, 14,000 people lost power in Monroe when sagging transmission lines drooped into trees, causing a fire that knocked out three substations. Two Seattle firefighters battling a blaze in West Seattle had to be treated for heat exhaustion.

What makes the pattern so rare is you need to have a super heated air mass and then an east wind to give temperatures a kick into a second gear. That general pattern plays out a few times each summer and gives us our handful of 90-95 degree days.

But in 2009 the ridge was stronger than usual, and the air mass was a little hotter than usual. The days leading up to the big spike were already in the mid 90s due to the hot air mass -- and that was without the east wind! We also had a rare moist northerly wind in the days leading up to the heat that increased the humidity and dew point. That was important as it aided in keeping nighttime temperatures warmer and gave us a better spring board once the sun came up. The morning of July 29th, 2009 also set a record for warmest morning on record at 71 degrees -- the first time ever Seattle had failed to cool below 70 on any given date.

So there we were baking in the mid 90s already with no east wind. On the 29th, a thermal trough finally developed and added in the downsloping heating effects of that east wind and temperatures zoomed like never before. Seattle was already in the 90s by mid-morning.

Seattle "cooled" to 96 the next day and then 84 the next, but jumped right back to 90 again a few days later. The 7 day lineup of 89, 94, 97, 103, 96, 84, 90 is the hottest week on record (and it still hit 89 again on Day 8!)

2014's summer (and 2013's too) has been marked with hot air masses, but not really any east wind events as luck would have it. July 1st's 94 degrees was really the only east wind date we've had this summer. All those other dates in the 80s and low 90s have been without the east wind otherwise we'd be talking upper 90s again.

And the forecast for this week is more of the same: Toasty warm due to a big ridge of high pressure pushing up heat from the Southwest, but no wind to give it an added boost. The result: Highs in the mid-upper 80s (maybe a low 90) but triple digits? Safely in hiding, hopefully for a long time.