Deep monsoon moisture allowed for the development of a strong thunderstorm north of Supai. This caused a flash flood in Havasu Creek.
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The episode as NWS FGZ wrote it up.
Deep monsoon moisture allowed for the development of a strong thunderstorm north of Supai. This caused a flash flood in Havasu Creek.
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 29, 2:56 PM MST, 4 hours 34 minutes earlier.
Area Forecast Discussion National Weather Service Flagstaff AZ 256 PM MST Sat Jul 29 2023 .SYNOPSIS...The most active monsoon pattern of the year so far is settling into place across the region. Shower and thunderstorm activity will increase into Tuesday before a drying trend begins Wednesday. The coolest high temperatures in quite some time are expected over the next few days in concordance with the increase in moisture. && .DISCUSSION...The development of a robust monsoonal flow over the past week is dramatically increasing shower and thunderstorm activity over northern Arizona. A favorable placement of the subtropical high pressure ridge has induced persistent moist, southerly flow through the Southwest. This buildup of moisture both aloft and at the surface is evidenced by the increasing daily MCS activity to our south, and the northward surges of high dew points from the Gulf of California. The GFS and European ensembles both portend a drying trend beginning Wednesday, as a result of our subtropical high shifting eastward. Until then, however, the most active monsoon pattern of the year so far is in place. Mesoscale models are painting a wet picture for not only the high terrain, but the desert lowlands. It looks like convection will initiate each day in the late morning hours over the high terrain before spreading into the low elevations during the afternoon hours. This includes locations that have yet to see substantial rainfall this season, such as the canyon country near the Arizona-Utah border. Heavy rainfall (1" per hour), small hail, frequent lightning, and gusty winds (greater than 30 mph) are expected with some storms. It is possible that convection organizes over Yavapai County each evening, as storms slide into the lower elevations, tapping into deeper moisture. Relatively large dew point depressions at the surface in the deserts may encourage robust cold pool formation. Strengthening cold pools may help organize convection and create a scenario for damaging downbursts and severe winds. In spots where local terrain favors near-stationary or backbuilding storms, multiple inches of rain may fall within a few hours. This is something we have seen in some areas lately, especially in Yavapai County. We expect instances like this to increase in the next few days. As a result, the risk of flash flooding is growing, with areas in and around fire scars and rocky or saturated soils being the most vulnerable. Remember, NEVER drive through floodwater. Murky waters covering usually dry washes and low-water crossing conceal hidden threats, including dangerously deep waters and washed out surfaces. If planning on outdoor activities, understand the risk for thunderstorms before heading out. Never enter a slot canyon if you hear thunder or see a storm nearby. With precipitation coverage increasing in the coming days, high temperatures are expected to come down. Forecast highs Monday through Wednesday range from near normal for this time of year, to below normal. High temperatures will depend on your elevation and local convective activity. Winds away from showers, thunderstorms, and outflow boundaries will remain light. Keep your eyes to the sky and stay tuned to the forecast. && .AVIATION...For the 00Z TAF Package...Outside of showers and thunderstorms, VFR conditions are expected through the 24 hour TAF period. Scattered showers and thunderstorms this evening will wind down after sunset. Greatest chances for storms (over 50%) will remain along and south of a KCMR-KFLG-KINW-KRQE line. Activity will pick back up in the morning, with showers and storms becoming widespread (over 80%) after noon. Some storms may produce frequent lightning, heavy rain, and gusty, erratic winds. Away from shower and thunderstorm activity, light west-southwest winds of 5-15 kts are expected. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Sunday and Monday...Monsoon moisture deepens with the most activity so far this year expected. Widespread showers and thunderstorms are forecast each day, with greatest activity over the high terrain and in the afternoon and evening. Some storms may produce frequent lightning, heavy rain (up to 1 inch per hour), and gusty, erratic outflow winds (in excess of 30 mph). Temperatures finally cool to near average while southwest winds outside of showers, thunderstorms, and outflows remain light at 5 to 15 mph. Tuesday through Thursday...Temperatures remain near normal for this time of year. Widespread showers and thunderstorms on Tuesday become more scattered on Wednesday, before turning isolated on Thursday. Southwest winds remain light away from storms and outflows. && .FGZ WATCHES/WARNINGS/ADVISORIES... Excessive Heat Warning until 8 PM MST Sunday for AZZ006. && PUBLIC...Benji AVIATION...Benji FIRE WEATHER...Benji For Northern Arizona weather information visit weather.gov/flagstaff
Preliminary Local Storm Report
National Weather Service Flagstaff AZ
341 PM MST Sat Jul 29 2023
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0339 PM Hail 7 WNW Cherry 34.64N 112.16W
07/29/2023 E0.70 inch Yavapai AZ NWS Employee
Dime size hail reported by NWS Employee.
&&
MSDThe Cedar City radar (KICX), 143 km away, was scanning. The volume nearest the first event began at 02:30 UTC.
1 flash flood. Local standard time, as filed.
The stream gauge on Havasu Creek near the Colorado River measured a 1.5 foot rise.
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 2026-09-24. Each event’s entry at NCEI: 1127231.