CW3E Event Summary: Heavy Rainfall in Arizona

September 19, 2023

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A Fall “Transition” Monsoon Event Brings Heavy Rain to Parts of Arizona

  • On September 12-13, 2023, a deep southwesterly flow of moist air was directed into Arizona, ahead of an advancing upper level trough.
  • This setup was a classic “Transition” monsoon event, signified by the transition from the hot, humid active summer into quieter fall months. Synoptic conditions for transition events are distinctly different from typical monsoon events when high pressure is overhead. While precipitation trends down in September, the combination of summer moisture and fall dynamics can result in significant rainfall events. The pattern resulted in persistent convection near and just east of the Phoenix area.
  • The Southwest Monsoon runs from June 15 through September 30.
  • Heavy precipitation fell over the Salt River watershed in central Arizona, with some locations receiving more than 20% of the normal total water year precipitation in a 24-hour period.
  • In excess of 6” of rain fell near Roosevelt, AZ. The Return Interval (Annual Probability) of 5.5”/6 hr is over 1000 years (<0.1%). For a nearby COOP station (Roosevelt 1 S, 1905+), the all-time 1-day record is 4.14" (1978-03-02). For comparison the average annual precipitation total is 15.91”.
  • The heavy rain resulted in flooding and rock slides, causing at least one highway to close.
  • IVT tools (based on the GEFS) indicated the potential for this notable moisture flux days in advance.

Headline Image

Main Points

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Satellite Loop

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Radar Loop

Monsoon Transition Pattern

Precipitation Map

Station Precipitation Map

Rainfall at Roosevelt Dam

Tweets from ADOT

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IVT Forecast

Summary provided by P. Iñiguez, C. Castellano, J. Cordeira, and M. Ralph; 19 September 2023

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