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For Bumbershoot weekend, as Labor Day weekend is known in Seattle, the weather looks very fine, low 80s with some clouds wandering aimlessly, as they will do. Monday may or may not require bumbershoots, with some showers or just the warm-and-muggy routine we’ve gotten into.
But the big weather news is that Seattle is about to, in KOMO’s Scott Sistek’s words, “obliterate its record for all-time warmest nighttime lows in a month.” All-time in this context means the last 122 years that we have records for. Ridges of high pressure have attenuated the effects of cooling marine flow, plus some itinerant low pressure systems have been hoovering up warm, moist air from the south, resulting in muggy August nights.
UW meteorologist Cliff Mass explains that the mugginess is another way of talking about dew points. “[A] plot of dew point for the past six months, show that we are now ‘enjoying’ the highest values so far this year,” he writes. With little marine air making it in from the coast, or hot, dry winds from eastern Washington, we’ve been having a sous-vide summer.
Mass also points you to a UW study (that he has some questions about) indicating that these nighttime “heat waves” are growing more frequent. The atmospheric mechanism they found operating will now sound familiar: “The records show that nighttime heat waves happen during high humidity, where water vapor in the air serves as a blanket to trap heat.”
The researchers predicted that climate change would inaugurate not just more frequent nighttime heat waves, but keep them around longer as well.
