So far tonight I have gotten nearly a dozen emails with pictures of "strange triangular clouds" and a request to explain them. Let's see what I can do.
Here are some samples, the first sent by Eric Berman and the second by Jacob Bruce. Pretty amazing pictures.
This a line of cirrus clouds that were precipitating ice crystals. These appendages with falling ice crystals are known as fall streaks or mare's tails.
To get a better perspective, here is a close up view of a NOAA weather satellite image at 7 PM. You can see a line of clouds stretching east-west. That is what folks were viewing.
The high-resolution WRF model run at the UW had put a cloud line across the region at about the same time. To illustrate, here is a 15h forecast from the 4/3 km domain for 8 PM that is simulating what a infrared satellite would see. Not perfect, but the model wants to create a thin line of cloud across the region.
Looking at the UW weathercam, it does appear that these features developed in jet contrails.
A picture immediately before the clouds developed, did show some features that looked like contrails aloft. These are oriented north-south.
So we had an area aloft that was near saturation and the moisture from the jet engines pushed it over to saturation...giving a contrail. The atmosphere aloft was close enough to instability that small cirrocumulus elements developed and subsequently precipitated.
On the meteorological Richter scale this might be a 1, but it is an interesting diversion on the beautiful summer night.
Photo sent by Linda Thomas