Ashland and Medford often have poor air quality (and fog) in the winter because they lie in a topographic "bowl" that allows pollutants to concentrate (see map). When an inversion builds over the basin, the build up is enhanced since inversions are very stable layers that act as a "lid."
And, in fact, on Thursday morning there was an inversion above Medford, as shown by the radiosonde launch at 5 AM (see graphic, red is temperature, blue is dewpoint). An inversion is where temperature increases with height. The numbers at the right are pressure (in hPa): 700 is about 10,000 ft, 850 is about 5000 ft.
A webcam from Oregon DOT along I5 in Ashland shows that the murk continued into this evening.
Here in Washington, the Colockum Tarps fire is still growing and spreading west and southward, approaching the wind turbines on Whiskey Dick Mountain east of Ellensburg (see the amazing picture below)
There have been a lot of thunderstorms in eastern Washington and over the eastern slopes of the Cascades, as well as over eastern Oregon. The winds from the thunderstorms helped reduce the air pollution in some areas, but the massive amount of lightning on Thursday could well have started more fires. Time will tell.