Arizona Tornado Warnings: How Tornadoes Form and How to Stay Safe

Searches for tornado alerts spiked in Arizona this week after the National Weather Service (NWS) office in Flagstaff issued tornado warnings and watches across northern parts of the state. The event has put a spotlight on a type of severe weather that is uncommon in the desert Southwest but not impossible, and on the basic science of how tornadoes form.

What happened in Arizona this week

On the morning of Wednesday, October 21, 2026, the NWS issued multiple tornado warnings for portions of Yavapai and Coconino counties, including a warning for Flagstaff and surrounding areas issued shortly after 9 a.m. local time, according to azfamily.com. The station reported that these warnings were based on radar-indicated rotation rather than a confirmed tornado sighting on the ground. Tornado warnings were also issued for parts of the Prescott area.

Northeastern, north and northwestern Arizona were placed under a tornado watch until 6 p.m. local time. The NWS said, as quoted by azfamily.com, that "a few tornadoes, hail up to the size of a tennis ball and scattered gusts up to 70 miles per hour are all possible." Wednesday and Thursday were designated First Alert Weather Days by azfamily.com's station and ABC15 Weather Action Days by abc15.com, reflecting an elevated — described as "slight" — risk of severe storms for both the High Country and the Phoenix Valley.

According to abc15.com, the Phoenix Valley itself faced a lower tornado risk than the high country, with isolated tornadoes considered possible mainly in higher-elevation areas on Wednesday afternoon. The main hazards for the Valley were damaging winds of 60–70 mph, hail of 2 inches or larger, and heavy rain of up to 1 inch, with 1–3 inches and locally 3–5 inches possible in the high country. A flood watch was also in effect statewide because of the rain, separate from the tornado threat.

How tornadoes actually form

Most strong and long-lasting tornadoes develop from supercell thunderstorms — storms with a persistent rotating updraft — according to an explainer published by the National Oceanic and Atmospheric Administration (NOAA). The key ingredient is wind shear: a change in wind speed or direction with height.

NOAA's explainer describes two forms of shear that combine inside a storm. Speed shear occurs when wind speed increases with altitude, creating a rolling motion in the atmosphere; directional shear occurs when wind direction changes with height. When a storm's rotating updraft lifts this rolling air, the directional shear strengthens the storm's counterclockwise rotation while weakening the clockwise rotation, until a single rotating column called a mesocyclone remains. If that rotation tightens and extends down to the ground, the result is a tornado.

NOAA's figures put the average number of tornadoes recorded in the United States at roughly 1,200 per year, with peak activity from April through June concentrated in the central United States, an area often called "tornado alley." Arizona tornadoes are comparatively rare and tend to be weaker, but the same basic shear-and-updraft mechanism applies wherever a sufficiently organized thunderstorm forms.

Measuring tornado strength: the EF scale

Tornado intensity in the United States is rated on the Enhanced Fujita (EF) Scale, which NOAA says has been used operationally since 2007, replacing the original Fujita scale that had been in use since 1971. The EF scale assesses damage using 28 damage indicators — such as building type, structure and tree damage — each scored across eight degrees of severity, a refinement intended to correct inconsistent ratings and overestimated wind speeds under the older system.

Watch vs. warning: what each term means

The NWS draws a clear line between the two main alert types. A tornado watch means tornadoes are possible in and near the watch area; it covers a broad region and is meant to prompt preparation — reviewing emergency plans, checking supplies, and identifying a safe room — according to NWS guidance.

A tornado warning is more urgent: it means a tornado has been sighted or indicated by weather radar, and the NWS says this requires immediate protective action. Warnings cover smaller areas than watches and typically give only minutes of lead time.

What to do during a warning

• At home: go to a basement, safe room, or an interior room on the lowest floor, away from windows, and bring pets if possible, per NWS guidance.
• In a mobile home: these structures are considered unsafe in a tornado; evacuate immediately to a sturdy building.
• In a vehicle: driving to shelter is preferable; if trapped, either stay low and protect your head inside the vehicle, or exit for a low-lying area such as a ditch, away from the vehicle.
• Outdoors: find a sturdy building immediately; sheds, storage facilities, tents and mobile homes do not provide adequate protection.
• In all cases: monitor local news or NOAA Weather Radio for updates and act as soon as a warning is issued.

Arizona's elevated flood risk alongside the storms

Beyond tornadoes, abc15.com reported that flash flooding was a significant concern during the same storm system, with a flood watch covering much of Arizona through Thursday. Temperatures in the Valley were expected to run about 5 degrees below normal for late October, dropping into the 70s by Friday; abc15.com reported that highs would stay near 80 degrees over the weekend before climbing into the mid-80s early the following week, once dry conditions returned.