Heavy to excessive monsoon rainfall forecast over the Desert Southwest, isolated flash flooding risk
Heavy to excessive rainfall from monsoon thunderstorms may bring isolated flash and urban flooding to the Desert Southwest into Sunday, September 28, 2025. A plume of anomalously high monsoonal moisture streaming northward on the eastern flank of a slow-moving upper-level low will fuel several rounds of widespread showers and thunderstorms, with greatest impacts expected across southeastern Arizona and southwestern New Mexico.
Satellite image acquired at 23:40 UTC on September 24, 2025. Credit: NOAA/GOES-West, RAMMB/CIRA, The Watchers
- A plume of anomalously high monsoonal moisture streaming northward on the eastern flank of a slow-moving cutoff low will fuel repeated showers and thunderstorms across the Desert Southwest through September 28.
- Heavy rainfall rates may trigger isolated flash flooding and urban flooding, with greatest risk in slot canyons, burn scars, and steep terrain. The flash flood threat is expected to increase on September 26–27 as soils become saturated from repeated storms.
- ASOS stations in southern Arizona and New Mexico are forecast to receive 38–64 mm (1.5–2.5 inches) of rainfall during the next 10 days, equal to 10–20% of annual totals. A Flash Flood Watch is in effect for southeastern Arizona from September 26–28.
- The NWS WPC has issued Excessive Rainfall Outlooks for September 25–29, with marginal risk (≥5%; level 1 of 4) across southern Arizona and New Mexico and slight risk (≥15%; level 2 of 4) near the Arizona–New Mexico border.
A plume of anomalously high monsoonal moisture is forecast to stream northward along the eastern flank of a slow-moving upper-level low, fueling multiple rounds of showers and thunderstorms in the Desert Southwest through September 28.
Heavy rainfall rates brought by the storms may trigger isolated flash flooding and urban flooding, particularly in terrain features such as slot canyons, burn scars, and steep slopes.
Forecast discussions from the NWS describe a closed low located near the California coast, drifting southeastward and gradually becoming detached from the main flow.
According to the NWS, the flash flood threat will increase on September 26 and 27 as soils become saturated from repeated thunderstorm activity but showers and storms are expected to continue into September 28 before the system weakens and shifts eastward.
Model forecasts initialized at 00:00 UTC on September 25 show wide disagreement in rainfall totals. The GFS projects ~57 mm (2.25 inches) for southeastern Arizona’s Upper San Pedro watershed, representing more than 10% of the region’s annual precipitation, while ECMWF guidance shows only ~12 mm (0.48 inches), under 5% of annual rainfall.
Point forecasts from ASOS stations in southern Arizona and New Mexico suggest 38–64 mm (1.5–2.5 inches) of rainfall over the next 10 days, equal to 10–20% of annual precipitation at some locations.
In response, the NWS Tucson Forecast Office has issued a Flash Flood Watch for southeastern Arizona from September 26 through September 28.
The NWS Weather Prediction Center (WPC) has also issued multiple Excessive Rainfall Outlooks (EROs) for the period between 05:00 PST on September 25 and 05:00 PST on September 29, with marginal risk (≥5%; level 1 of 4) EROs valid over southern Arizona and New Mexico and slight risk (≥15%; level 2 of 4) EROs targeted over the Arizona–New Mexico border and southern New Mexico, highlighting the potential for flash flooding on multiple days over the region.
This Desert Southwest event is part of a larger Pacific jet stream pattern shift. Over the same period, a strong atmospheric river is forecast to impact southern British Columbia, delivering more than 100 mm (4 inches) of rainfall to western Vancouver Island and the Coast Mountains between September 26 and 29.
References:
1 Quick Look at Heavy Precipitation Forecast over the Desert Southwest – CW3E – September 24, 2025
2 Short Range Public Discussion – NWS/WPC – September 25, 2025
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