October 20-22, 2026: Supercells with Large Hail and Damaging Winds

October 20-22, 2026: Supercells with Large Hail and Damaging Winds

Fig. 1: GOES-19 Water Vapor Satellite Imagery between 8 PM MST Oct 20th and 8 PM MST Oct 22nd, 2026 showing a potent low pressure system off the West Coast driving a moisture plume northward and instigating multiple rounds of thunderstorms across the Southwest.

Between October 20 and 22, 2026, a potent low-pressure system triggered several rounds of severe thunderstorms across much of central and eastern Arizona (Fig. 1). The primary hazards from this outbreak were large hail and damaging winds. There were also two confirmed EF-1 tornadoes that produced over 100 mph winds: one in the Heber-Overgaard area in Navajo County and another at Spruce Mountain, south of Prescott in Yavapai County. While very heavy rainfall accompanied the storms, rapid storm motion reduced residence times over any given location, which limited the overall coverage of flood impacts. Across south-central Arizona, the vast majority of the impacts were driven by two individual supercells.

The first supercell impacted the Chandler and Mesa areas on the evening of Tuesday, October 20, 2026. An isolated thunderstorm formed near the city of Maricopa in northwestern Pinal County before rapidly intensifying into a supercell as it traveled northeastward toward Chandler and Mesa (Fig. 2-3). The storm produced large hail up to 1 to 2 inches in diameter and wind damage. Numerous trees were uprooted across the area, including an estimated 25 to 30 trees at a park in Chandler. In Mesa, roughly 5 large trees were uprooted at a church, with additional downed trees/branches damaging carports and vehicles at a nearby apartment complex (Fig. 4-7).

Fig. 2: Base reflectivity (left) and velocity (right) between 9:12 PM MST and 10:58 PM MST on October 20, 2026 from the KIWA (Phoenix, AZ) radar showing the supercell thunderstorm moving through the Chandler and Mesa areas. 

Fig. 3: Base reflectivity (left) and correlation coefficient (right) at 10:11 PM MST on October 20, 2026 from the KFSX (Flagstaff, AZ) radar. The spike feature extending out of the storm to the southwest is an artifact called Three-Body Scatter Spike (TBSS). TBSS is a radar beam scattering artifact that extends down radial from the storm core, or away from the radar, and indicates the presence of large hail. The drop in the correlation coefficient value near the storm core indicates the presence of mixed hydrometers and thus in this case a mixture of heavy rain and hail.

Fig. 4-7: Images of hail and wind damage in the Chandler and Mesa areas. Image location and sources (Clockwise from top-left): 1-2 inch diameter hail in the area of Brown and Gilbert Roads in Mesa (Trained Storm Spotter Brandon Knapp); Large uprooted tree in a park in Chandler, located in the area of Rural Road and Chandler Boulevard (ABC 15); Carport and vehicle damage in an apartment complex located near the area of Gilbert Road and University Drive in Mesa (FOX 10); Large uprooted trees at Grace United Methodist Church in Mesa (FOX 10). 

A second, more destructive, supercell impacted Central Phoenix on the afternoon of Wednesday, October 21, 2026. After initiating south of Avondale, the storm quickly moved northward and intensified, reaching supercell intensity as it passed north of the Estrella Mountains (Fig. 8-9). Hail up to 2 inches in diameter, potentially locally larger, caused significant damage by denting vehicles, shattering auto glass, and breaking residential windows. The storm also generated severe winds in excess of 60 mph, which uprooted numerous large trees throughout the area (Fig. 10-13).

Fig. 8: Base reflectivity (left) and velocity (right) between 2:19 PM MST and 4:38 PM MST on October 21, 2026 from the KIWA (Phoenix, AZ) radar showing the supercell thunderstorm moving through the Glendale and Phoenix area. 

Fig. 9: Base reflectivity (left) and correlation coefficient (right) at 3:30 PM MST on October 21, 2026 from the KIWA (Phoenix, AZ) radar as the storm was over the Metrocenter area along Interstate 17. Similar to the supercell storm affecting Chandler and Mesa the previous day, a Three-Body Scatter Spike is present indicating the presence of large hail.

Fig. 10-13: Images of hail and wind damage in communities located near Interstate 17 in Central Phoenix. Image location and sources (Clockwise from top-left): Hailstones up to 2 inches in diameter in the area of 22nd Avenue and Cactus Road in Phoenix (Dee Dee Darby on Facebook); Broken windows at a residential complex located near the area of 15th and Peoria Avenues in Phoenix (JoAnne Beard on Facebook); Large uprooted tree on a vehicle inside an apartment complex located in the area of Peoria Avenue and Interstate 17 in Phoenix (Ashley Larson on Facebook); Back window of a car completely shattered by hail in the area of 19th and Sunnyside Avenues in Phoenix (Kim Purple on Facebook). 

Storm Environment

Atmospheric conditions were exceptionally favorable for a severe weather outbreak across much of central and eastern Arizona. Precipitable water values, a measure of moisture in an atmospheric column, surpassed 250% of normal, setting late-October records as values peaked at 1.0–1.4” in the higher elevations and 1.5–1.7” in the lower elevations (Fig. 14). Combined with cold 500 mb temperatures of around -10°C, this high moisture content generated moderate instability with MLCAPE values peaking at 1000–1500 J/kg (Fig. 15-16). Very strong 0-6 km wind shear in excess of 50 kts supported the development of organized severe thunderstorms, including supercells (Fig. 17). In addition to largely linear speed shear, there was also some directional shear present. Upper-air soundings from both Flagstaff and Phoenix confirmed notable directional shear alongside the strong speed shear, featuring southeasterly surface winds that veered south-to-southwesterly aloft (Fig. 18-19). The high shear and instability favored strong rotating storm updrafts, which supported the production of large hail and tornadogenesis. The NWS recognized this outbreak as the most significant severe weather event in Arizona since early October 2010. The early October 2010 event featured a nearly identical synoptic setup, where a powerful low-pressure system off the West Coast sparked widespread severe thunderstorms throughout central and northern Arizona (Fig. 20-21). Coincidently, the 2010 outbreak produced multiple tornadoes across the Arizona High Country and generated very large hail in the Phoenix metropolitan area. An event write-up on the hail event in Phoenix can be accessed here: October 5, 2010-Largest Hail Recorded in Phoenix

Fig. 14: Animated loop of precipitable water values between 5 PM MST Oct 20th and 5 PM MST Oct 21st, 2026.

Fig. 15: 500 mb heights, temperatures, and winds at 2 PM MST on Oct 21st, 2026.

Fig. 16: Animated loop of the Mixed Layer Convective Available Potential Energy (MLCAPE) and Mixed Layer Convective Inhibition (MLCIN) between 5 PM MST Oct 20th and 5 PM MST Oct 21st, 2026

Fig. 17: Animated loop of surface to 6 km wind shear between 5 PM MST Oct 20th and 5 PM MST Oct 21st, 2026.

Fig. 18-19: Sounding data from weather balloons released at 11 AM MST on October 21, 2026 from the NWS Phoenix (left image) and NWS Flagstaff (right image) offices. The data reflects a moist and moderately unstable air mass with very strong speed and directional shear, signaling an environment favorable for supercells with large hail and isolated tornadoes. (click on the images for higher resolution version)

Fig. 20-21: (Top Image) 500 mb upper air analysis across the United States from 5 PM MST on October 5th, 2010. (Bottom Image) Same map for 5 PM MST on October 21, 2026.

Messaging

Messaging for the impending storm threat began on Saturday, October 17th, when NWS Phoenix released a weather story graphic on its website (weather.gov/psr) (Fig. 22). The initial graphic highlighted a strong low-pressure system expected to bring elevated precipitation and thunderstorm chances to the region. As the event drew closer and confidence on the overall hazards increased, more detailed graphics regarding timing and impacts were disseminated (Fig. 23).

Fig. 22: Weather story graphic published on October 17, 2026 highlighting the potential for a significant weather event.

Fig. 23: Weather story graphic published on October 20, 2026 providing more detailed information on rain chances, amounts, and overall impacts.

In anticipation of the potential for flooding from heavy rainfall, flood watches were issued for much of central and northern Arizona (Fig. 24). On Tuesday, October 20th, during the 16 UTC (9 AM MST) convective outlook issuance, the Storm Prediction Center (SPC) placed portions of south-central and north-central Arizona under a slight to enhanced severe weather risk, marking Arizona's first enhanced risk since September 2019. The outlook identified areas under risks for isolated tornadoes, large hail, and damaging winds (Fig. 25). Furthermore, a Conditional Intensity Level 1 (CIG1) - a new metric from SPC to indicate the potential severity of severe weather - was introduced for hail to reflect the possibility of hail exceeding 2 inches in diameter.

Fig. 24: Flood Watch issued on October 20, 2026 for much of central and northern AZ.

Fig. 25: Severe Weather Outlook graphic posted on October 20, 2026.

By midday on October 21st, SPC, in coordination with the NWS offices in Phoenix, Flagstaff, and Tucson, issued a Severe Thunderstorm Watch for much of south-central and east-central Arizona, along with a Tornado Watch covering the northern half of the state (Fig. 26-27). This marked the state's first Tornado Watch since the severe weather outbreak of early October 2010. Severe thunderstorm and tornado watches continued into October 22nd as storms capable of producing hail, damaging winds, and isolated tornadoes persisted. In total, 158 Severe Thunderstorm Warnings, 16 Tornado Warnings, and 11 Flash Flood Warnings were issued across Arizona during the three-day period from October 20th-22nd (Fig. 28).  To ensure effective communication and public safety coordination throughout the event, NWS Phoenix, NWS Flagstaff, and NWS Tucson all hosted multiple live webinars for emergency management personnel. These interactive briefings provided detailed analysis of the severe thunderstorm and heavy rainfall threats, enabling emergency managers to make informed decisions and pre-position key resources.

Fig. 26: Severe Thunderstorm Watch issued on October 21, 2026.

Fig. 27: Tornado Watch issued on October 21, 2026

Fig. 28-29: (Top Image) Severe Thunderstorm, Tornado, and Flash Flood Warning polygons issued in Arizona during the three-day period from Oct 20-22, 2026. (Bottom Image) Local Storm Reports issued for Thunderstorm Wind Gusts and Damage, Tornadoes, and Flash Floods during the same three-day period.