Wow... there is a cool layer about 600 m (around 2000 ft) thick, topped by an inversion of around 12 C (about 22F). Yes, we are talking about warming by 22F in about 200 m (650 ft). Some folks had some interesting hikes today I bet.
Why so strong an inversion? Turns out we had some strong southeasterly flow aloft that not only brought in some warm air from the interior, but had some downslope warming as well. Warming aloft strengthens the inversion.
Here is a time-height cross section of winds above Seattle during the past day (time increases to the left). You can see northerly winds at low level (in the cold air) and easterly winds above.
Although inversions are fun, thunderstorms really get folks excited, and lightning and thunder will be back tomorrow (Sunday)...and particularly after 6 PM.
Later tomorrow, a sharp upper trough will be moving into our area from the southwest (see graphic)
At the same time, there will be substantial potential instability in the lower atmosphere (high values of CAPE: Convective Available Potential Energy)....see graphic.
Tomorrow, we can look at the forecasts of the HRRR: High Resolution Rapid Refresh model to get a better short-term forecast.