We can now put a record-breaking hot monsoon in the books and move on to the expectation of cooler temperatures with the arrival of the fall season! The autumnal equinox for our local area officially kicks off at 11:50pm today. Weather-wise it’s shaping up to be a nice finish to the workweek (and summer season). That tune carries on past the weekend and into next week with near to slightly below normal temperatures for both maxes and mins (e.g., Phoenix: upper 90s and mid-70s, respectively). The wildcard in the short-term forecast (now through early Saturday) is a steady stream of higher precipitable water values in the atmosphere around an inch originating from the remnants of former Tropical Storm Kenneth. The moisture is being funneled primarily into central and northern AZ via an eastward advancing Pacific disturbance riding on the southern counterclockwise flow of a large low pressure circulation positioned over the northern Rockies. That’s the big picture viewpoint.
For us, much of the incoming moisture is restricted aloft (above 15,000 feet or so). With relatively dry surface dewpoints peaking only in the 40s and the limited time window (less than 24-hours) for the atmosphere to moisten top-down, measurable rainfall making it to the ground doesn’t appear likely at this point. Sprinkles, brief light rain, and perhaps a thundershower are possible, though, mainly in the higher terrain and foothills north and west of Phoenix overnight and lasting through Saturday morning (e.g., Wickenburg area). The best chances for rainfall are up in the Bradshaw Mountains and along the western Mogollon Rim.
By Sunday and Monday, high pressure builds over the Desert Southwest region. Consequently, precipitable water values drop well below an inch, skies are mostly clear, and afternoon temperatures trend up a couple degrees compared to Saturday. Strong winds throughout the forecast period are not anticipated (i.e., terrain-driven and generally less than 15-20mph).