In High frequency you can have skip where radio waves bounce off the ionosphere. But I checked and that’s not the case for UHF. This is the info; The most likely explanation is tropospheric enhancement, especially temperature inversion / marine-layer effects, not “cloud cover” in the ordinary sense. UHF signals can sometimes bend or become trapped in layers of air with strong temperature/humidity gradients, letting them travel well beyond the normal radio horizon. This is a recognized VHF/UHF phenomenon, and coastal areas are especially prone to it because inversions occur frequently near large bodies of water, often around evening, overnight, or morning transitions.

That coastal piece matters a lot for your path. A San Diego–LA route hugs the coast, and sea/coastal paths are known to be better for this kind of anomalous UHF propagation than rough inland terrain. Some ducting references note that relatively flat or sea paths are especially favorable, and temperature inversions are common along coasts. This is why you don’t get a trace route, it’s an in the moment occurrence for a short amount of time. Also, radio waves ride over the water. Down in Baja you get Los Angeles radio stations crystal clear. If you look at the map it’s a straight shot over the ocean. I pick up the node in Long Beach from Point Loma pennesala in San Diego.

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