A month ago, a sunny day in the 70s would be considered normal.
Today's it's considered record-breaking.
In what some might call the "better late than never" scenario, the weather pattern Tuesday has the most classic warm-weather set up for this region than Western Washington has seen all year long -- even better than the long, warm stretch of 80s in September!
A rare thermal trough has pushed into the area Tuesday, not only keeping the skies (mostly) clear but also giving a boost to the temperatures.
The trough creates an east wind that not only keeps the cooler, marine air at bay, but also warms and dries as it sinks down the western slopes of the Cascade Mountains. This double whammy usually conspires to give Seattle and environs their warmest temperatures of the season.
Had today been August 18th, we'd be talking about a sweltering day in the 90s. However, it is mid-October, and aside from just the general cooling due to the season change, the longer nights and less intense sun conspire to create inversions that can toss a monkey wrench into the equation.
Cold air has a greater density than warm air, so it will pool at the lower levels like a sludge that is difficult to budge. When the nights are clear and long, it allows nighttime temperatures to drop quite far -- into the 30s and low 40s Tuesday morning. When the sun comes up and begins to warm the Earth, the warm air being not as dense as the cold air will sit on top of the cold air dome, creating what's called an inversion. Normally the temperature cools as you go up in altitude in the lower atmosphere, but on days like today, it can be colder at the surface than higher up.
To wit: At 8 a.m. Tuesday morning, it was 45 degrees in Seattle and 36 in Tacoma and Olympia in the fog. However, 10,000 feet up at Camp Muir on Mt. Rainier, it was 43 degrees at the same time!
So the inversion will keep a lid, so to speak, on the temperatures unless something comes along to break the inversion.
Enter, the east wind.
The thermal trough has created quite a pressure difference Tuesday to where the pressure in Western Washington is much lower than in Eastern Washington -- in fact, it's by far the largest differential of the year -- even greater than what few warm days we had this summer (you want to know why it didn't get to 90 this year? We never had a decent east wind -- in fact, we didn't have much of any east wind. All our warm days were nearly exclusively created by just a warm air mass moving in from the south, with no east wind to give it the extra oompf.)
That east wind, being warm and dry, can do a good job of acting like a blender and mixing up the cold air at the surface, stirring the pot and getting warm air to blend in and scour out the cold air. I suspect we'll see some rapid warming through the day as this mixing process takes place. It will also be fairly breezy out toward the mountains with gusts to 25-35 mph possible.
By the time all is said and done, Seattle should climb into the upper 60s -- maybe even touch or break the record high of 70 for the day. (As I alluded to earlier. The normal high a month ago on Sept. 18 is 70. It's amazing how fast fall arrives around here!) Some spots in the mountains near this warm east wind who are above the inversion could see low-mid 70s in some spots. Even the coast will be warm today -- especially the north coast as the Olympic Mountains also partake in this warming process with east winds.
But be sure to get out and enjoy today, because also unlike summer, these warm events don't last near as long in the autumn. Cooler weather returns Wednesday with even a chance of showers late, and thus this likely spells the end of anything close to 70 degrees until next spring (or summer?)