Deep monsoon moisture over northern Arizona brought scattered thunderstorms with heavy rain and flash flooding.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS FGZ wrote it up.
Deep monsoon moisture over northern Arizona brought scattered thunderstorms with heavy rain and flash flooding.
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 Aug 24, 10:01 AM MST, 3 hours 39 minutes earlier.
Area Forecast Discussion National Weather Service Flagstaff AZ 1001 AM MST Fri Aug 24 2018 .SYNOPSIS...Deep monsoon moisture will bring another round of afternoon and evening thunderstorms today. Drier air advecting into the region will lead to a downturn in shower and thunderstorm activity later in the weekend into early next week. && .DISCUSSION...Ample moisture remains in place over much of northern Arizona setting the stage for another active monsoon day. Surface dewpoints range from the mid 50s to around 60 degrees indicating significant low level moisture in the region. Additionally, mostly sunny skies are allowing for sufficient heating of the boundary layer this morning. Expect convection to develop by midday with the most widespread showers and thunderstorms along the Rim from Flagstaff southeast to the White Mountains. A complex over northern Mexico will likely provide additional moisture and lift later this evening to the southeast portion of the region. This could keep shower and thunderstorm activity going well into the evening hours in southern Navajo and Apache counties. We will continue to monitor this potential. A few minor changes were made to precipitation chances, but the overall forecast remains on track. && .PREV DISCUSSION /444 AM MST/...Flood water from storms yesterday is still flowing down the Agua Fria and Paria Rivers this morning. This water should recede by mid morning. The rain from yesterday also helped fog to form in a few areas. Most notable was along I-40 between Flagstaff and Williams. This fog should also burn off by mid morning. Moisture values (from satellite) have changed very little over this portion of northern Arizona since yesterday morning...still an inch or higher. Some drying has taken place over southern Nevada in the past 24 hours (1.4 inch yesterday vs 0.8 inch this morning). Cloud cover over northern Arizona is thinner than yesterday morning. This will allow an earlier start to storms. An active day is expected across northern Arizona today. Storm motion west and north of Flagstaff is expected to be southwest to northeast. Storms south and east of Flagstaff are expected to move more south to north. Many areas across Yavapai County have received heavy rain over the past few days. Additional heavy rain over these areas will generate more runoff rather quickly. Drier air will continue to slowly advect into the region over the weekend resulting in a continued decrease in convection. However, enough moisture will remain in the White Mountains for scattered to numerous storms each afternoon. A drier airmass will remain in place early next week so we are only expecting isolated monsoon activity each afternoon. Long range model guidance indicates these drier conditions could last well into next week. && .AVIATION...For the 18Z package...Scattered showers and thunderstorm are expected this afternoon with the most active areas along the Rim from KFLG southeast to the White Mountains. IFR possible in the stronger storm cells. Storm activity decreasing after 02Z. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Monsoon moisture will remain over the district with good chances for showers and thunderstorms again today and Saturday. Expect scattered storms to develop each afternoon with the greatest coverage along the Mogollon Rim southeast to the White Mountains. Sunday through Tuesday...Drier air will move into the region leading to decreased shower and thunderstorm activity. Scattered showers and thunderstorms are still expected in the White Mountains region each afternoon. && .FGZ WATCHES/WARNINGS/ADVISORIES...NONE. && PUBLIC...TPS/MAS AVIATION...MCT FIRE WEATHER...KD For Northern Arizona weather information visit weather.gov/flagstaff
The Flagstaff radar (KFSX), 97 km away, was scanning. The volume nearest the first event began at 20:39 UTC.
2 flash flood. Local standard time, as filed.
The stream gauge on Big Bug Creek recorded a 2.1 foot rise at 140 PM MST. The Agua Fria gauge showed a 2.7 foot rise (730 CFS increase) at 613 PM.
Tonto Creek near Rye measured a 1.5 foot rise (1000 CFS increase) at 725 PM.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Arizona in 2018Every state in 2018This day in weather historyWarnings in Arizona, August 2018
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: 786337, 786356.