Unfortunately, Saturday's first monsoon rainfall wasn't spread equally around the Tucson metro area, according to preliminary data from the Pima County Regional Flood Control District.

Tucson International Airport reported 0.29" of rain — marking a new record rainfall for the day. (The previous record of 0.20" was set in 1918.) The highest precipitation was recorded at a gauge on the Santa Cruz River and Ina Road — 0.39". There were many spots on the east side that managed to avoid the rain — including Mount Lemmon.

Here are some additional rainfall totals for Southern Arizona, provided by the National Weather Service, Tucson Weather Forecast Office:

  • Bisbee-Douglas Airport: Trace
  • Davis-Monthan Air Force Base: 0.03"
  • Nogales International Airport: 0.26"
  • Safford Airport: Trace
  • Sierra Vista Municipal Airport: 0.08"

Defining monsoon's start

As you might know, monsoon's official start — since 2008 — is June 15, rain or shine. Safety experts in Arizona and New Mexico agreed to this date, though almost three weeks before rain usually appears, for two reasons: One, it gives them time to spread around safety information about the season's potentially dangerous weather hazards (extreme heat, flash floods, lightning, etc.). Two, researchers can compare data for the same dates from year-to-year.

Previously, monsoon was determined by daily average dewpoint. It required three consecutive days at or above 54 degrees, with the first of those days being monsoon's start.

Under the old definition, Saturday would not have been the start of monsoon. Why? The daily average dewpoint was only 45 degrees. (Check out the National Weather Service chart in the photo area above.) Despite the rain, we'd still be waiting for monsoon to begin.