We "only" reached 70 degrees today. At 6 degrees below average for a high temperature, it's fairly cool, but not extraordinarily so. The last time we didn't hit 70 was on July 24th, meaning it's been nearly a month of consecutive 70 degree-or-higher days, with some days much higher (Sea-Tac hit 96 on August 11). That's pretty impressive.
We may have kept our streak going, but setting a new record for consecutive days above 70 is not going to be easy. The record is 61 days (thanks Scott Sistek), so we'd have to keep our streak going into the end of September to set a new one. It'd be pretty amazing if we broke that record, but the chances of doing such a thing are next to nil.
Why did we have such a downward shift in temperatures today? It's not like we had any massive rainstorm come through the area or anything. Besides, even on August 13, our rainiest day of the summer (0.85 inches at Sea-Tac), the temperature got up to 74 degrees. What made this day different.
The answer, my friends, lies in the all-too-familiar phenomenon we call the marine push. Also known as our "natural air conditioning," it occurs when pressure over land is lower than that over the ocean, and cool, moist maritime air flows into Western Washington. Strong marine pushes generally are associated with thick stratiform cloud decks extending into Western Washington all the way to the foothills of the Cascades. As the day goes on, the sun often "burns through" these clouds, but if the push is thick enough, the sun won't be able to disseminate the stratus clouds, and the surface won't heat up as much as a result. Those are the types of pushes that give you particularly cool days, sometimes as much as 10 degrees below average.
Let's take a look at the marine push event that occurred today. All of these images were gotten from the UW weather loops website. It's a great website with satellite, radar, and model loops and time series of general observations throughout our area. They even have lightning strike data available. In addition, they have an archive of tons of previous satellite and radar images, so you can look at all your favorite storms from the past. I've relived many a storm through this awesome feature.
You can see that it took a while for the clouds to clear out. At 10:00 a.m., they were widespread throughout Western Washington and spread far into the Cascade foothills. They gradually declined in extent and depth, and were gone completely by 3:00 p.m. It wasn't the strongest marine push of all time, but it was enough to cause a cloudy morning.
This coming week looks to feature a decrease in the magnitude of these pushes, and by early next week, they may not be happening at all. However, these pushes will return later in the week, and we will cool down again as a result.