September 16, 2026: Heavy Rainfall and Flash Flooding

September 16, 2026: Heavy Rainfall & Flash Flooding

Fig. 1: Left image: Radar reflectivity loop between 9:13 AM MST on September 16th and 1:51 AM MST on September 17 from the KIWA (Phoenix, AZ) radar. The green polygons indicate the Flash Flood Warnings issued by the NWS Phoenix, NWS Tucson, and NWS Flagstaff Weather Forecast Offices. Right image: Radar estimated total rainfall accumulation during the same time period.

During the late morning and afternoon hours of September 16, 2026, scattered showers and thunderstorms materialized across portions of south-central Arizona. Activity initially developed over Gila County before expanding across Maricopa and Pinal Counties, including the Phoenix metropolitan area (Fig. 1). The most substantial rainfall occurred from northern Pinal County through northern Maricopa County, where accumulations reached 2 to 4 inches, most of which fell within a two- to three-hour period (Fig. 1-2).

Fig. 2: Total rainfall amounts observed on September 16, 2026. (Click on image for higher resolution version)

The rapid rainfall accumulation led to significant runoff and inundation of streets, washes, and creeks, making many low-lying areas and unbridged low-water crossings impassable. Consequently, multiple swift water rescues were performed for motorists stranded in floodwaters (Fig. 3-5). One of the more notable water rescues was executed by Maricopa County Sheriff's Office who used a helicopter to rescue two individuals whose vehicle became submerged in floodwaters at a low-water crossing along State Route 238 between the city of Maricopa and the town of Gila Bend (Fig. 6).

Fig. 3-6: Images of flooding across Maricopa County. Image location and sources (Clockwise from top-left): Street flooding along Hualapai Drive in North Scottsdale (Kim Marie on Facebook); Truck stranded in floodwaters from Sonoran Wash along North Valley Parkway in North Phoenix (Berry Bipson on X); Vehicle stranded in floodwaters from Skunk Creek along Dove Valley Road in North Phoenix (Adam Klepp/ABC 15); Water rescue along State Route 238 (Maricopa County Sheriff's Office).

An 11:00 AM MST (1800 Zulu) special upper-air sounding conducted by the National Weather Service (NWS) Weather Forecast Office in Phoenix identified atmospheric parameters conducive for flash flooding (Fig. 7). Precipitable Water (PWAT), a measure of the amount of moisture in an atmospheric column, was measured at 2.10 inches, which was a record high value for mid-September based on Phoenix sounding climatology (Fig. 8). Mixed-Layer Convective Available Potential Energy (MLCAPE), a measure of how unstable the atmosphere is, was very high at nearly 2,000 J/kg. There was also a deep warm cloud layer, around 10,000-12,000 feet deep, which supported highly efficient warm rain processes. The combination of these elements facilitated intense rainfall rates exceeding 1 to 2 inches per hour and briefly higher rates over shorter timeframes. Synoptic-scale forcing was also present from a large upper-level low pressure positioned over the Pacific Northwest and a smaller low that moved up from west of Baja California. This forcing interacted with a remnant tropical low pressure system originating along the west coast of Mexico (Fig. 9). Furthermore, a unidirectional southerly wind profile throughout much of the tropospheric column promoted some linear storm structures and training of thunderstorms, which led to observed high rainfall totals over concentrated areas (Fig. 7).

Fig. 7: A sounding obtained from a special weather balloon released at 11 AM MST (1800Z) on September 16, 2026 from the NWS Phoenix Office. The data reflected a saturated environment and a highly unstable air mass, which supported the potential for heavy rain producing thunderstorms.

Fig. 8: Precipitable Water climatology based on observed soundings in Phoenix. The 2.10 inches measured by the 11 AM MST September 16, 2026 sounding was an all-time record for mid-September. Note, the NWS Phoenix Weather Forecast Office almost exclusively launches weather balloons during the monsoon (June 15-September 30), which explains the lack of climate data during the months outside the monsoon in this chart.

Fig. 9: GOES-19 Water Vapor Satellite Imagery between 9 AM MST September 16 and 3 AM MST September 17 showing the low pressure features responsible for the upper-level ascent and thus the widespread thunderstorm coverage.

Approximately 24 hours prior to the onset of the event, the NWS Phoenix office, in coordination with NWS Tucson and NWS Flagstaff offices, issued a Flood Watch across central and eastern Arizona, which was subsequently expanded to include portions of La Paz and Yuma counties (Fig. 10). Due to the high probability of flash flooding, the Weather Prediction Center upgraded the Excessive Rainfall Outlook to a Moderate Risk (at least 40% probability of excessive rainfall resulting in flash flooding) for much of the south-central and southern Arizona lower elevations as well as the foothills and higher terrain areas north and east of Phoenix (Fig. 11) . This marked the first Moderate Risk issued within the NWS Phoenix County Warning Area since 2023.

Fig. 10: Flood Watch graphic highlighting the areas under the threat.

Fig. 11: Day 1 Excessive Rainfall Outlook graphic highlighting the Moderate Risk area.