Today was very warm around western Washington, with some stations (like Sea Tac) breaking their all-time daily record. Now let me make one thing clear...breaking a daily record is not that unusual. We break a few of them every year for highs or lows. Breaking monthly or all-time records like last summer's 103F...THAT is impressive.
Here is a map showing the temperatures at 3 PM..near the time of highest temperatures (note: you can click on it to expand). You will note that some of the warmest temperatures (70-72F) are on the east side of the Puget Sound basin away from the water. You will also note some easterly flow.
Today was a good set up for warmth. First, we had only limited high clouds and sun is fairly strong now--the same sun as mid September. Second, we had an approaching trough that brought southerly flow and relatively warm lower-atmosphere temperatures into the region. To see this, take a look at the temperatures and winds at around 5000 ft for 10 AM Wed morning, shown below. The solid lines are the heights of the 850 mb pressure surface...think of it as like pressure, with the winds roughly parallel to the height lines. You can see the southerly winds bring warmer air (reddish colors) into our region. You will also notice the easterly and southeasterly flow over the Cascades. Easterly flow adds to the warming because air warms as it sinks and is compressed. That is why the warmest temps are over the Cascade foothills and the east side of the Sound.
But the warmth is over now. As seen in the satellite image below, a front is now entering the region and tomorrow will only reach the mid-fifties and there will be light rain. And we stay in a cooler, wetter pattern for a while, which will include some needed snow in the mountains.