Preliminary Storm Report for 9 August 2018

Preliminary Storm Report for 9-10 August 2018
By NWS Phoenix -- last updated at 7 AM on 10 August 2018

Flooding along Highway 60 in Tempe, AZ. Source: ADOT

Strong thunderstorms developed in two waves across the Southwest on Thursday 9 August 2018. The first developed over Mohave County in northwest Arizona during the evening hours and moved southwest into portions of La Paz County, AZ and eastern Riverside County, CA. Strong winds and heavy rain were observed with these storms along with dense blowing dust. A second wave developed much later, crossing Gila County and moving into eastern Maricopa and northwest Pinal Counties in Arizona. While the storms with this second wave were not particularly strong over the mountains, once the wave moved into the lower deserts where the air was more unstable, strong to severe storms blossomed quickly. Heavy rain, strong winds, and dense blowing dust became issues to contend with. Click here to view storm reports for this event.

Radar at 1144 PM on 9 August 2018. Click image for full event loop.

Lightning cloud-to-cloud flashes across central Arizona. Hover over image to show cloud-to-ground strikes.

Lightning cloud-to-cloud flashes across southwest Arizona and
southeast California. Hover over image to show cloud-to-ground strikes.

The storms over the East Valley of the Phoenix area produced very heavy rainfall. Two reports were received of approximately 3.5" of rain falling in under an hour and many more in the 1-3" range. Measured peak rainfall intensities of almost 7"/hr were observed with these storms. Rainfall amounts of 3.5" in one hour have a less than 0.2% chance of happening any given year in the Phoenix area, thus this intensity of rain is quite rare. The heaviest rain fell in central Tempe with other notable areas including Apache Junction and San Tan Valley.

Phoenix Rainfall Index for calendar day of 9 August 2018.

Peak 1-hr rainfall amounts across the Phoenix area (background image) and climatological probability of those
values being observed in a given year. The highest amounts of 3.5" in one hour have a less than 0.2% chance
of occurring across the Phoenix area at a single point in any given year.

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