First, at 8 AM (1500 UTC) there were no lines over the Cascades, but a band of high clouds was moving over western Washington. You can see snow over the Cascades and the volcanic peaks (like Rainier) are evident as isolated areas of snow.
But then something strange happened. As the high clouds moved over the Cascades (10 AM), a strange line formed, extending to the east-northeast of Mt. Rainier, with weaker lines associated with Mt. Baker and Mt. Adams.
By 11:45 AM (1845 UTC), the Rainier and Mt. Adam's lines were half way across eastern Washington.
But then something strange happened. As the high clouds moved eastward the strange lines disappeared!
Want a better view? Here is an image from the high-resolution MODIS Terra satellite around noon.
And here is a MODIS close up of the lines extending from Rainier and Adams.
So what is it? Alien spaceship tracks. Government topic secret program? Chemtrails?
Nope...there is another answer.
The winds aloft yesterday were generally from the west at roughly 10,000 ft (700 hPa ) and west-southwest higher (see radiosonde sounding at 5 PM yesterday). In the upper troposphere (from 15,000 ft to 30,000 ft) the air was near saturation in this sounding (the temperature--red--and dew point --blue--were close together).
A band of high clouds moved over the region during mid-day. These thin high clouds indicated that the atmosphere had reached saturation at some upper levels, producing extensive ice clouds. But something supercharged the high clouds, making them far whiter downstream of the mountains.
The probable cause? Vertically propagating mountain waves! Some mountain waves stay low and produce mountain wave clouds over and immediately downstream of mountains near crest level. But when the conditions are right, the mountains produce waves that move (or propagate) upwards, with their maximum upward velocity well above and somewhat downstream of the mountain (see figure). In such a wave, the cloud can start downstream of the mountain peak and extend downstream a considerable way.
So we started with a thin veil of clouds...a layer of ice crystals. Then extra upward motion produces enhanced cooling and production of even more ice crystals. The cloud became whiter. These ice crystals were not evaporating downstream since the air was generally saturated. So a long line formed downstream of the mountain.
Later, when drier air moved in, the lift of the mountain waves were not enough to produce ice crystals and the air cleared.
Could we model this using computer simulation? You bet. Here is the UW WRF cloud fields at noon on Monday. Look carefully and you can see enhanced lines embedded in a general mass of upper clouds. We have come a long way!