Thanks for your comments, Todd. Interesting.
As you stated, the lower frequencies in the band do traveler farther on groundwave, highly dependent on the ground conductivity. A great example I know of is traveling east from Denver across Kansas. This area has some of the best ground conductivity in the country, at 15-30 milli-siemens per meter. 50 KW KOA-850 in Denver and even 5 KW KHOW-630 are easily receivable in Hays, Kansas at 309 miles. They do however peter out by Salina, Kansas at 400 miles. This can be extended a little, maybe another 100 miles, with a loop.
One of the characteristics of winter daytime skywave are the long deep fades, sometimes 20-30 minutes in length as mid-morning approaches noon. I find that patience is important. Just sit on a frequency and wait. The fade ups do not last long, sometimes only a minute or two. If you are in luck, you'll be able to ID.
One must be careful to avoid anything near sunrise or sunset due to the transitional enhancements at those times. Skywave is skywave of course, but during the period of 3 hours after sunrise and about 2 hours before sunset the upper region of the MW band is noticeably enhanced in strength. The lower end has some enhancement, about 3 dB, where the upper end can be upwards of about 15 dB. I find that 10:30 AM local time to about 2-3 PM to be fairly safe in calling it daytime skywave. Of course it depends on direction of the transmitter from the receiving location too.
Skywave is always there, just a little stronger in mid-winter. Yay!