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My experience is that during the hot summer months, southwest winds develop about every afternoon as the hot air rises up the Pine Valley Mountains and is pulled through the Virgin River Gorge. I've seen it happen for years, but typically it is an afternoon and early evening wind. Lately, there have been more storms in the area that have caused significant microburst wind activity. I would presume this is what you are referring to and I think you can blame a lot of what you are seeing on this.
The air heats up with the rising sun, and it is especially true on the slopes of the Pine Valley Mountains. The air pushes up along the hills and over the tops of the peaks. This air is replaced by air coming from the southwest and is likely funneled from and through the gap in the Beaver Dam Mountains and the Virgin River Gorge. Some of the wind is also moving up into the Zion N.P. area as the air rises there too. In the evening when the sun goes down, the air on the Pine Valley Mountains is cooler and heavier and it returns down the mountain side and the strong afternoon/evening winds quickly die off. However, if there are storms in the area, frequently these winds will continue well into the night. See if you are recognizing the winds long after sunset and this likely is coming from the microbursts from storms in a 150 to 200 mile radius of SW Utah. If the winds die off after sunset, then it is the diurnal (daily) wind patterns you see in the hot summer in St. George.
This doesn't happen as much in the other seasons because there isn't as much heat generated by the sun.