Monsoon Moisture and Cooler Temperatures Expected in Arizona, Relief from Scorching Heat

The primary weather event is expected to unfold later on Sunday into the first half of Monday.Photo byNWS

The National Weather Service in Phoenix has issued an updated forecast indicating a shift in weather patterns for Arizona over the coming days. Following a period of breezy conditions, the region will experience a cooling trend through early next week, with temperatures expected to fall below normal by Monday.

Starting Saturday, the weather will take a notable turn as moisture levels increase across much of Arizona. This shift will bring chances for showers and thunderstorms, particularly focusing on south-central and eastern parts of the state. Dry and mild conditions are anticipated to return starting next Tuesday and are expected to persist throughout much of the following week.

The trough that has recently influenced the region is moving north, leaving behind a lighter westerly flow. This has led to very dry conditions with surface dew points in the 20s and 30s. However, this dry air will be countered by the arrival of moist low-level flow from the south, which will raise dew points to the 40s and possibly as high as 55 degrees near Yuma.

The influx of moisture is being bolstered by Tropical Cyclone Ileana, which has formed south of Baja California. This system is expected to contribute tropical moisture to Arizona starting tonight and through the weekend. Although the moisture will primarily affect the lower and mid levels of the atmosphere, dry air will remain above approximately 700 mb.

As the moisture moves into Arizona, a weak upper-level shortwave trough is forecast to move through southern California into northern Arizona. This will likely lead to some scattered showers and potentially a few elevated thunderstorms from Saturday night into Sunday morning over south-central Arizona.

The primary weather event is expected to unfold later on Sunday into the first half of Monday. A stronger Pacific trough will advance southeastward into northern California by Sunday afternoon, continuing its track through the Great Basin on Monday. This system will enhance moisture advection into Arizona, with ensemble mean precipitable water amounts increasing to around 1.5 inches, about 150% of the normal for this time of year.

With rising low-level mixing ratios and cooler air aloft, the forecast indicates a likelihood of showers and thunderstorms, particularly over higher terrain areas. Storms may move quickly due to a strong southwesterly steering flow, potentially limiting the duration of heavy rainfall. However, some storms may cause localized flooding if they train over the same areas. The combination of instability and bulk shear suggests that a few strong to severe storms are also possible.