When Did The "Monsoon" End Over Southern Arizona?

It is fairly obvious that the "monsoon" season has ended across southern Arizona, but when exactly did the synoptic pattern change? I've gone back and examined the upper-air charts and the Tucson soundings to address this question. To me, it appears that the "monsoon" season ended around 5 September. Another question is: What signifies the end of the "monsoon?" From my perspective, the end occurs when the subtropical anticyclone at 500 mb mb retreats southward so that it is no longer over, north, or east of Arizona - and remains so for an extended period (that is, a change that is not a transient shift of a couple of days). Admittedly, a messy definition, but one that is based upon the synoptic circulation patterns, rather than Td or PW thresholds.

The morning Tucson sounding on the 6th of September (above) shows deep, westerly flow prevailing though most of the troposphere  - a situation which has continued through this weekend (i.e., for 13 consecutive days).

The concurrent 500 mb chart (above) on the 6th indicates the center of the anticyclone was over northeastern Sonora. The center of the anticyclone has continued to remain over northern Mexico (or far south Texas, or west of Baja) ever since the morning of the 6th. Thus, a significant change in the synoptic pattern occurred.

However, moisture values (see the above 14 day times series of PW) have gradually diminished. This is often the case, since residual mT air can linger over the deserts. Additionally, the diurnal circulations over northern Mexico and southern Arizona can result in moisture sloshing back and forth, as long as the mT air remains over the northern GoC. There was an active period of storms from the 9th to the 15th - these were transitional storms in the westerlies that were feed by the instability (CAPE) resulting from residual moisture (PW values around 30 mm) and cool (-10C) middle-level temperatures. This situation is also not unusual, since the transition to Fall can produce conditions in Arizona that are similar to the Plains' severe thunderstorm environment.