Radar loops show this very nicely. I will have to make a movie of it since it is indeed rare (maybe 1 or 2 nocturnal events in the summertime). I saw very little on the surface chart, perhaps local flows were more dominant given the proximity of the hills around here. Area soundings showed 2C decreases at 500 hPa from 00 to 12 UTC, and a 3C increase at 700 hPa at Spokane. We happen to be on the cold side of the jet and it did not appear to be related to any jet streak circulation especially with anticyclonic curvature.
The dew point has been higher than usual, especially with the missed rain of the other day. It rained south of here 2 days ago, but yesterday was dry and the dew points dipped down into the mid-50s only, then came back up to 60. Of course our weather has been marked decidedly by the regular passage of midlatitude cyclones, which I must say has had a profound effect on Seattle, but not apparently us.
Day 2 update: more TS obs ! 1230-4am, and again this am from 9-11am!
They seem to initiate off the the hills to the south of the basin. Looks similar to the cap rock area.
This afternoon there is a TS playground out by Yakima which is awesome from my westward facing balcony. Just a wall of cumulonimbus: