In much of the U.S. it is called Indian Summer. In France the term is St. Martin's Summer, while in eastern Europe Old Ladies Summer and Gypsy Summer are often used. All of these names describe a warm period in the autumn that follows a period of cool weather-- in fact, some demand a killing frost before the warmth. Well, we don't get too many killing frosts west of the Cascades in autumn, so the term Indian Summer here is usually reserved for periods in mid-fall when the temperature rise to summer-like levels*. And we have real shot of this on Tuesday of next week.
Well, let me first document that we have had some cool weather--in fact the last two weeks have generally been cooler than normal, and last night there was a real chill in the air (I know about this personally, I was out searching for my lost dog in Mountlake Terrace). Here are the temps at Sea-Tac airport compared to the normal highs and lows:
Today (Friday) there will be considerable clouds and some light sprinkles, particularly in the central Cascades.
Saturday will be sunny and warmer...very nice...but you won't mistake it for summer--Indian or otherwise.
Sunday a weak trough will bring cooling and lots of clouds (maybe a bit of sun)
But Monday starts the warm-up and Tuesday it will be in full bloom. Want to see some weather maps? (of course you do!) Here is the upper level (500 mb) chart for Tuesday morning at 5 AM. Wow...big ridge over us...and a trough over CA that is working its way north.
Here is the sea level pressure, surface winds, and lower atmosphere temperatures at the same time. Good offshore flow with higher pressure east of the Cascades and lower pressure west of the mountains. You will notice a low pressure area near the Oregon/CA border.
With offshore flow, there will be virtually no clouds and air will be compressed as it sinks from higher elevations. Here are the predicted surface (really 2-meter) air temperatures at 2 PM on Tuesday. Temperatures above 70F over land, particularly on the eastside of western Washington and along the coast, downstream of the Olympics. Expect the warmest temperatures in the foothills...like North Bend and Issaquah.
So get your bathing suit ready, Tuesday will be your day. I suspect this will be our last excursion into the 70s this year...better enjoy it.
* Sequim dropped below freezing on Wednesday night.