On October 5–6, 2010, a destructive series of thunderstorms struck Arizona, resulting in the state's largest tornado outbreak and its costliest weather disaster on record.
Spawned by a nearby cold-core low, successive hailstorms in Phoenix and surrounding locations on October 5 caused damage to thousands of homes, businesses, and vehicles.
The approach of these mid- to upper-level lows, detached from the jet stream, allowed for southerly wind vectors to overspread the state.
Compared to previously analyzed tornado environments across the United States, researchers found that Northern Arizona events typically featured marginal instability, averaging around 450 J/kg.
This omega block was characterized by a high-pressure area over the Central United States and two separate cold-core lows, one over California and the other off the coast of New Jersey.
[4] As the western low remained nearly stationary, an unstable atmosphere began to develop across Arizona, prompting the Storm Prediction Center (SPC) to outline a Slight risk across the state.
[7] While this activity also produced sporadic straight-line wind damage,[8] a more robust event into the evening hours was hindered by the loss of daytime heating and the departure of favorable dynamics for severe storm development.
[12] However, as a well-defined piece of energy rotated around the base of the cold-core low,[13] a more significant tornado event than expected materialized.
One noteworthy aspect of the event is that every tornadic supercell began to show signs of rotation south of the Mogollon Rim, but none of them produced confirmed tornadoes until after they tracked over that geological feature.
One storm produced hail up to 1.5 in (38 mm) in diameter across Phoenix and into Carefree, damaging roofs and cars and breaking hundreds of skylights.
[35] The most substantial event of the day occurred with a storm that moved from Firebird Lake to south of Chandler into northern Glendale and finally across southern Peoria.
President Barack Obama declared the Havasupai Canyon a major disaster area and offered federal aid to assist in local recovery efforts.