September 30th, 2026: Monsoon season wrap-up | UA WRF Weather Forecast Discussions

Previous two days of weather

The HRRR-UA WRF 12Z forecast from Monday morning (left) versus the observed 48-hour total accumulated precipitation from MRMS (right) shows the forecast captured precipitation coverage well, though totals were lower than predicted. Still, there were some >3.5″ totals recorded in the mountains of eastern Arizona and New Mexico.

HRRR-UA WRF 12Z 48-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) for September 28th and 29th.

The forecast captured the rain bands predicted in the convergence between Hurricane Polo and Odalys outflow and the upper-level low-pressure system, but it overdid cell organization, which may have led to the overestimated total precipitation shown above.

HRRR-UA WRF 12Z simulated radar reflectivity forecast (left) and corresponding observed radar from MRMS (right) for September 28th at 5:30 pm to September 29th at 4:30 pm.

The Pima County Flood Control network shows significant precipitation totals across the region, with greater than 1 inch on surrounding mountains and less than an inch in the Tucson metro area.

72-hour precipitation totals from Pima County Regional Flood Control District gauges, Alert Map.

Summary

For a visual summary of the 2026 season, the NWS Tucson has some great graphics on their website: https://www.weather.gov/twc/Monsoon

A particularly interesting graphic shows that, while total rainfall amounts were about normal for the various locations they typically report on, the total number of lightning strikes for Arizona was much below normal. This may signal weaker total monsoon activity, since strong updrafts correlate with lightning strikes.

Lightning chart showing the number of (cloud-to-ground) lightning strikes in the selected region. Lightning strikes are a unique way to track the monsoon, as the data do not suffer from issues more common to rain gauges (low spatial coverage across the region outside population centers) and can therefore provide a more complete picture of thunderstorm activity. The bar chart displays total strikes to date for each year, as well as the total for the entire season ending September 30th. Data from VAISALA NLDN.

Despite the possibility of reduced updraft intensity over the season, mountainous regions received more than 3 times (blue shading) the 1991-2020 season average, whereas the lower deserts were below average, as shown in the plot below made by Mike Crimmins. A broader subtropical high (monsoon ridge) can weaken steering flow, suggesting a possible seasonal trend in which thunderstorms formed but couldn’t move away from the mountains.

Percent of average precipitation: Percent of average is calculated by dividing the total accumulated precipitation by the long-term (1991-2020) mean and indicates how much totals have deviated from average precipitation for the June 15th to present period. 100% is equal to the average for the time period shown on the map. (Seasonal Average Total Precipitation)

The North American Monsoon season calendar end date is upon us. It’s been an interesting season, and thank you for reading! While our monsoon season discussions are done for another year, the NWS discussions remain a valuable resource to consult for a detailed explanation of the current outlook.

Until next season…