Significant moisture surge into southern Arizona

The moisture surge discussed yesterday has occurred and been quite spectacular across southern Arizona.

First, there has been a classic and very pronounced surge of low-level moisture northward up the Gulf of California. Pressures at Yuma dropped quite low yesterday afternoon and the moisture surge was observed near dark between 8 and 9 pm there. The passage of the surge was quite spectacular as indicated by the changes in surface conditions from 8 to 9 pm. It has continued through the night. The NWS Doppler radar data show that the surge extends upward to just above 3000 ft MSL at Yuma this morning.

However, across southeast and south central Arizona moisture has also increased dramatically over the night due to both outflows from northern Mexico and the moist flow behind the back-door front, also mentioned yesterday. The change in the soundings at TUS from yesterday to this morning is also quite spectacular. Strong moisture increases are apparent up to 600 mb. This influx of moisture occurred during the late night and early morning hours and was more gradual than the surge observed at Yuma. Surface observations from Douglas to Tucson to Sasabe to Sells all showed significant dewpoint increases during hte night.

The morning sounding at Tucson indicates that signifcant CAPE may be present by afternoon. If the low-level moisture continues to increase, potential CAPE by afternoon could become quite large for out here. Temperatures at 500 mb have fallen and there is some steering flow from the north-northeast. Thus, it appears that an interesting and stormy afternoon is possible over southeastern Arizona.

In the longer term, Tropical Storm John developed yesterday afternoon. The NHC expects it to reach hurricane strength this morning. The forecast by NHC anticipates that John will rapidly intensify into a major hurricane and track to the northwest along a track that would, if it verifies, bring another major push of moisture into the Southwest by Friday and Saturday. All told a very interesting week and Labor Day weekend shaping up!