"Average" Rainfall in Arizona

 NOAA National Water Prediction Service reports daily, weekly, monthly, seasonal, and yearly accumulated precipitation and the totals can compared to "normal".  Their data access website states that normal precipitation is defined as the 30-year PRISM normal from 1981-2010.  The Parameter-elevation Regressions on Independent Slopes Model (PRISM) climate mapping system developed at Oregon State University are considered the most detailed, highest-equality spatial climate datasets currently available.

But is the average still the same in the 16 years since 2010?

Methods

I downloaded total accumulated precipitation for water years ending September 30 for the last 10 years.  Unfortunately, the files for many of the years were corrupted and I was only able to look at the three years from October 2022-Oct 2025.  

I clipped the rasters to the Southwest US and took the mean for each grid cell.

Results

Over the last three years, some areas of the Southwest have had 50% of normal precipitation, and other areas have had 150% of normal. 

NOAA NWPS % of normal 2023-2025

Color key:

Zooming in to the area around Prescott, Arizona:

NOAA NWPS % of normal 2023-2025

Conclusions

Most of the area around Prescott is between 85-95% of normal (medium orange), confirming the presence of long-term drought.  Areas less than 75% of normal (dark orange/red) include much of the Bradshaw mountains around Prescott, the Dugas area along I-17, the Mogollon Rim - east of Payson and from Sedona to Ash Fork, and the south slopes of the San Francisco mountains around Flagstaff.

However, there are significant areas between 115 and 125% of normal (light green), including some areas above 125% of normal (dark green).  These above-average areas include the Juniper mountains and Upper Verde River, the Santa Maria river near I-93, a bit of Sedona and Schnebly Hill to Mormon Lake, and the north side of the San Francisco mountains. 

This shows how spatially heterogeneous average rainfall can be, and how a simple description of a state as being in drought or not can be misleading. For example, the excellent Drought Aware web app from Esri shows broad swaths of Arizona currently in drought, but my analysis shows that there is likely much more spatial variability.