First, some images that need little explanation - some areas around Yuma got a year's worth of rain in a couple of hours - that's what the high variability of desert regions can bring. Somerton is only a bit southwest of the airport, which had only 0.91".
Below is a snip of the observations from the airport during the event. Note that it reached 109 F just before the storms.
The WRF-NAM yesterday morning forecast an interesting intrusion, almost sea breeze like, into southwestern Arizona of very high CAPE air from the northern GoC. Forecast above was valid at 12:30 pm MST. Usually, the high CAPE air within the GoC marine layer mixes out rapidly when that air moves over land - so the persistence of the high CAPE air in the forecast indicated something unusual was happening.
Below is an upper air sounding from the Army Proving Ground near Yuma taken at 23 UTC, right before the event began. CAPE was only around 1000 J/kg, but with almost no CIN, and winds below 500 mb were very light. The military mostly shuts down routine operations on weekends and Holidays - so, even though there are frequent soundings from the Proving Ground during the week, there were no soundings from Labor Day for comparison to yesterday - so it goes.
At bottom is a regional radar plot from about 7:00 pm, showing the storms over Yuma. Although there was some southwesterly steering flow above 500 mb, these were apparently balanced by cell propagation toward the southwest, so that the storms were basically stationary.
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