Weather Blog

Seattle set to obliterate all-time record for warmest nights

Seattle set to obliterate all-time record for warmest nights
Photo of lightning over Seattle on Aug. 10 by Kenneth Malone. It's the warm and muggy nights that have not only brought a few nights of thunderstorms to the area but also primed Seattle to shatter its record for warmest August nights.

Seattle is about to not set, not break, not even shatter, but obliterate its record for all-time warmest nighttime lows in a month -- and this counts the city's entire 122-year records, including the Downtown Federal Building.

Through midday on Thursday, the average low temperature in Seattle for August stood at exactly 60 degrees. It's notable because 1) Seattle has never in its history had a month with an average low temperature at 60 degrees or warmer and 2) It's never really been that close. For Sea-Tac Airport, where the numbers count now, the current warmest average low was 58.4 degrees in 1967. (Warmest overall is 59.1 in July of 1941 at the Federal Building).

Seattle needs to average 61.9 degrees for its average low over the 29th-31st to hold this 60 degree monthly average. So far on the 29th as of this writing, the morning low was a whopping 66 degrees (another obliteration alert: The record warmest low for Aug. 29 is 60) but there is still a chance the temp will drop below 66 before 1 a.m. PDT (midnight Pacific Standard Time -- where climate is recorded) so Thursday's low is not set in stone yet. But if the 66 holds, Friday and Saturday's lows just need to be 60 degrees or warmer to set this 60-degree monthly mark, and forecasted lows are right at 60 for both nights.

Either way, the all-time warmest month ever is essentially guaranteed. Even if Thursday ends up 64 and we *hit our record lows on Friday and Saturday*, which has zero chance of happening, we'd still break the all-time Sea-Tac record by a full half degree. So the only drama is what number we will write in the record books when all is said and done.

Month has been a bit...unusual

Most of our warm overnight lows come on days when we have our searing heat waves -- the kind of days when hits 90 or so and the east winds keep pushing warm air in overnight. Lows on those nights are routinely in the mid 60s.

But what if I told you this month our warmest days have had the cooler nights? Our three days over 85 have had lows in the mid-upper 50s. While some of our days with highs in the low 70s have had lows in the low 60s.

It's because it's been an unusual summertime pattern. The predominant pattern has been one of two things: A ridge of high pressure offshore and another one further inland that has keep the differences in pressure quite light across our region, and in turn kept our marine breezes that cool us off and keep dew points low very weak. That's also why it's been so sunny and warm -- little cooling marine breeze -- but allowed to cool off a night on the warm days -- no east wind to act as a warm blanket.

The other pattern has been occasional upper areas of low pressure that spin offshore pulling in warm, moist air from the south and some rain and/or thunderstorms to add even more moisture to the mix for muggy conditions.

It's those muggy conditions and thunderstormy evenings that have prevented overnight lows from dropping too far overnight. If the dew point is bouncing around 59-61, temperatures won't drop below that, it'll just become foggy.

Put it all together, and you have 15 of 29 days in August with a low at 60 or greater and eight others were at 59. Average lows are around 55-57.