Mountains are largely irrelevant at night, and don't seem to be much of a factor with very long distance daytime (1000 plus miles). So I suspect the long distance daytime is a skywave phenomenon - although much weaker than nighttime skywave. That would also account for the deep fades associated with 1000 mile daytime reception.
When I lived in Utah during the 70s and 80s, the mountains did not appear to impede skywave reception. Back then, WHAM and the Chicago clears were regular catches in Utah -even with the towering Wasatch Mountains to the east. Now for FM, it's a completly different story. If you travel west of SLC, the FMs are clear to about 150 miles because most of the FM and TV antennas are on Farnsworth peak, which is about 5000' above the valley floor. However, if you travel east to the other side of the mountains, the multipath is horrific. If fact, if you are not within line of sight of the FM tower, you can have a strong 60-80dbu signal that is unlistenable due to the multipath. I recall the frustration in trying to clear up a strong, but distorted signal by moving the antenna. Many of the SLC FMs rely on boosters and translators.
Ground conductivity in West Texas is very good, not so good in Dallas. Although for really long distance reception my biggest problem in Dallas is loop saturation.
During a trip to Wichita Falls, WBAP was like a local during the day. KOMA sounded good, albiet weaker than WBAP>
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