A high pressure center located over New Mexico brought deep moisture over northern Arizona. In addition, an inverted trough moving northward in this flow brought additional instability which triggered stronger storms.
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The episode as NWS FGZ wrote it up.
A high pressure center located over New Mexico brought deep moisture over northern Arizona. In addition, an inverted trough moving northward in this flow brought additional instability which triggered stronger storms.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Flagstaff, AZ wrote its last area forecast discussion before the first event at Jul 28, 9:04 AM MST, 2 hours 29 minutes earlier.
Area Forecast Discussion National Weather Service Flagstaff AZ 904 AM MST Fri Jul 28 2023 .SYNOPSIS...The monsoon looks to ramp up over the coming days with increasing chances for afternoon and evening showers and thunderstorms. Hot conditions will continue across the region through the first half of the weekend before finally starting to cool down as monsoon activity increases. && .UPDATE...As expected, with a favorable pattern in place, a large thunderstorm complex developed over northwest Mexico. The result is an ongoing surge of moisture into southern Arizona from the Gulf of California. Yuma currently was carrying a 72 degree dew point with southeast winds at 20 mph gusting to 30 mph during the morning hours. Some of this moisture is sure to make it into portions of southern Yavapai County by late this afternoon. The flow today is more favorable for storms that develop over the eastern Mogollon Rim and White Mountains to move west to southwestward into the lower elevations. Gusty outflow from this activity could collide with the Yavapai County moisture for a better chance of severe thunderstorms and flash flooding late this afternoon and into the evening. There is always a bit of experiential guess work with these patterns but that is how we see it this morning. && .PREV DISCUSSION /252 AM MST/...Not much going on tonight in terms of the weather across northern Arizona, besides for some light and variable winds, near average temperatures, and some mid-level clouds with more coverage towards the south. An impressive mesoscale convective system (MCS) is currently impacting southern portions of the Mexican state, Sonora, as another, much weaker system continues to decay along the Arizona and Mexico border. Another day of isolated to scattered shower and thunderstorm activity is expected for this afternoon and evening with greatest chances (40-50%) along and south of the Mogollon Rim and into the White Mountains (60-70%). Hi-res guidance has storms beginning to develop along the higher terrain by noon, increasing in coverage thereafter. Leftover showers and storms persist through this evening, with activity ending by 11 pm MST. Temperatures remain hot across the region, peaking by noon just before clouds and thunderstorms increase in coverage. As we head into the weekend, the center of the high pressure system (currently over the AZ/NM border) begins to shift eastward, enhancing southerly flow into Arizona. This transition will allow for an increase in moisture advection as dew points and PWATs begin an upward trend across Arizona. As a result, POPs and shower and thunderstorm coverage have increased for Saturday, with widespread coverage forecast by Sunday. Temperatures look to remain roughly 5-10 degrees above average for Saturday afternoon, before beginning a cooling trend from Sunday onward. Both the GEFS and the ECMWF Ens are in agreement with the high shifting further to the east over Texas and Oklahoma for the first half of next week. Scattered to widespread thunderstorm activity is forecast for Monday and Tuesday with chances decreasing some on Wednesday as PWATs begin a decreasing trend. As we wrap up next week, ensemble guidance suggests a downturn in monsoonal activity with a return of warmer temperatures. Confidence remains more low to moderate at this time in regards to drier conditions for the end of next week. The high does look to shift back westward and position itself more directly over the Southwest, however, depending on how much it does rain in the beginning of the week and with no indications of any strong winds to scour out the surface moisture, chances for precipitation are still on the table. && .AVIATION...For the 18Z TAF Package...Mainly VFR conditions expected over the next 24 hours. Look for scattered showers and thunderstorms through 03Z, mainly along/south of a KPGA-KINW-KRQE line with isolated activity further north. Be on the lookout for wind gusts of 35 to 45 knots in the vicinity of storms. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Today and Saturday...Scattered showers and thunderstorms expected across northern Arizona each afternoon with the best chances along and south of the Mogollon Rim through the White Mountains. The primary threats will be lightning, erratic outflow winds of 30 to 40 mph, and rainfall rates of 0.50-1.00" per hour in any storms. Activity will gradually subside each evening. Sunday through Tuesday...Scattered to widespread shower and thunderstorm activity is expected each afternoon. Outside of any outflow winds, expect west-southwest winds 10 to 20 mph each afternoon. Daytime temperatures cooling through the period. && .FGZ WATCHES/WARNINGS/ADVISORIES... Excessive Heat Warning until 8 PM MST Sunday for AZZ006. && PUBLIC...McCollum/LaGuardia AVIATION...McCollum/Meola FIRE WEATHER...McCollum/Meola For Northern Arizona weather information visit weather.gov/flagstaff
The Flagstaff radar (KFSX), 109 km away, was scanning. The volume nearest the first event began at 18:32 UTC.
1 hail, 1 flash flood. Local standard time, as filed.
Quarter sized hail was reported in the Yavapai Hills area. One inch diameter hail was reported near Costco about 30 minutes later.
A video on social media showed a flash flood in Big Bug Creek in Mayer.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Arizona in 2023Every state in 2023This day in weather historyWarnings in Arizona, July 2023
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-30. Each event’s entry at NCEI: 1127221, 1127223.