An upper level low pressure area over southern Nevada acted upon moisture already in place over northern Arizona to produce showers and thunderstorms with heavy rain and strong winds.
Every event with a place to put it. Tap one for its entry below.
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.
An upper level low pressure area over southern Nevada acted upon moisture already in place over northern Arizona to produce showers and thunderstorms with heavy rain and strong winds.
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 22, 9:35 PM MST, 4 hours 4 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE FLAGSTAFF AZ 935 PM MST WED AUG 22 2012 .SYNOPSIS...A LOW PRESSURE SYSTEM WILL MOVE ACROSS NORTHERN ARIZONA THROUGH THURSDAY...BRINGING A PERIOD OF INCREASED SHOWER AND THUNDERSTORM ACTIVITY. THUNDERSTORM CHANCES WILL DECREASE BY THE WEEKEND AS DRIER AIR MOVES IN FROM THE WEST. && .DISCUSSION...THE CENTER OF THE LOW PRESSURE CAN BE SEEN ABOUT 150 MILES NORTHWEST OF LAS VEGAS THIS EVENING. THE MOSAIC RADAR IMAGE THIS EVENING IS SHOWING NEARLY A CONTINOUS BAND OF SHOWERS AND THUNDERSTORMS EXTENDING FORM NEAR YUMA TO THE NORTH RIM OF THE GRAND CANYON. THE INDIVIDUAL CELLS ARE MOVING NORTHEAST AS THE WHOLE BAND IS SLOWLY PUSHING EAST. WILL UPDATE FORECAST TO INCREASE POPS EASTWARD FOR THE 06-12Z TIME PERIOD AS THE BAND OF STORMS MOVES EASTWARD. && ...FLASH FLOOD WATCH IN EFFECT FOR ALL OF NORTHERN ARIZONA THROUGH THURSDAY MORNING... .PREV DISCUSSION /343 PM MST/...UPPER LEVEL TROUGH OF LOW PRESSURE OVER SOUTHERN CA/SOUTHWEST NV THIS AFTERNOON WILL GRADUALLY SHIFT EAST ACROSS AZ TONIGHT THROUGH THURSDAY. THE TROUGH HAS BEEN SUPPORTING ONGOING CONVECTION FROM LAST NIGHT THROUGH THIS AFTERNOON ACROSS SOUTHERN CA/SOUTHERN NV IN ASSOCIATION WITH SOME COOLER AIR AND DIFFLUENT FLOW ALOFT. THIS FAVORED AREA OF LIFT WILL MOVE ACROSS NORTHERN AZ TONIGHT AND INTERACT WITH MOISTURE AMOUNTS NEAR TO AND IN EXCESS OF 1.00 INCH PW. EXPECT WIDESPREAD SHOWERS AND STORMS TO DEVELOP WITH A FEW EMBEDDED STRONG STORMS POSSIBLE. WITH THE INCREASED THREAT OF HEAVY RAIN TONIGHT HAVE GONE AHEAD AND EXPANDED THE FLASH FLOOD WATCH TO INCLUDE ALL OF NORTHERN AZ THROUGH THURSDAY MORNING. BY THURSDAY AFTERNOON...SOME DRIER MID LEVEL AIR BEGINS WORKING IN FROM THE WEST AND THINS OUT MOISTURE PROFILES ACROSS WESTERN AZ SO BELIEVE FLOOD THREAT WILL BE LESSENED HERE. EASTERN AZ STILL REMAINS UNDER THE FAVORED AREA OF LIFT THURSDAY AND WILL CONTINUE THE WATCH FOR APACHE/NAVAJO COUNTIES TO THURSDAY EVENING. IN ADDITION...WIND SHEAR APPROACHES 30-40 KTS ACROSS MUCH OF THE AREA TONIGHT SO EXPECT SOME MORE ORGANIZED CONVECTION TO DEVELOP WITH A FEW EMBEDDED STRONG STORMS POSSIBLE. STRONG STORMS WILL BE CAPABLE OF PRODUCING SEVERE HAIL AND STRONG WINDS. THE BEST CHANCE FOR STRONG STORMS TONIGHT WILL BE NORTH AND WESTERN PORTIONS OF OUR AREA...SHIFTING TO EASTERN PORTIONS OF AZ ON THURSDAY. LINGERING MOISTURE WILL BRING SCATTERED SHOWERS AND THUNDERSTORM ON FRIDAY...THEN A SLIGHTLY DRIER WEST TO NORTHWEST FLOW BEHIND THE TROUGH ON SATURDAY AND SUNDAY WILL BRING A DECREASE IN THUNDERSTORM CHANCES. DUE TO THE DRIER CONDITIONS DAYTIME TEMPERATURES WILL RUN ABOVE NORMAL AND NIGHT TEMPERATURES WILL BE COOLER THAN EXPECTED FOR THIS TIME OF YEAR. MOISTURE WILL LIKELY INCREASE AGAIN EARLY NEXT WEEK AS THE RIDGE BUILDS BACK INTO THE SOUTHWEST. && .AVIATION...FOR THE 06Z PACKAGE...SHOWERS AND THUNDERSTORMS WILL CONTINUE ACROSS NORTHERN ARIZONA OVERNIGHT AS A LOW PRESSURE SYSTEM NORTH OF LAS VEGAS MOVES EASTWARD. THE BEST CHANCE OF STRONGER STORMS SHIFTS INTO EASTERN ARIZONA BY THURSDAY AFTERNOON. HEAVY RAIN...HAIL AND STRONG OUTFLOW WIND GUSTS WILL ACCOMPANY THE STRONGEST STORMS THROUGH THE PERIOD. EXPECT MVFR/IFR CONDITIONS NEAR SHOWERS AND STORMS. AVIATION DISCUSSION NOT UPDATED FOR AMENDMENTS. && .FGZ WATCHES/WARNINGS/ADVISORIES...FLASH FLOOD WATCH THROUGH THURSDAY MORNING FOR AZZ004>008-012-015-018-037-038. FLASH FLOOD WATCH THROUGH THURSDAY AFTERNOON FOR AZZ009>011-013- 014-016-017-039-040. && PUBLIC.........MAS/MCS AVIATION.......MAS/JJ FOR NORTHERN ARIZONA WEATHER INFORMATION VISIT WEATHER.GOV/FLAGSTAFF
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE FLAGSTAFF AZ 323 PM MST WED AUG 22 2012 AZZ004>018-037>040-231100- KAIBAB PLATEAU-MARBLE AND GLEN CANYONS-GRAND CANYON COUNTRY- COCONINO PLATEAU-YAVAPAI COUNTY MOUNTAINS- NORTHEAST PLATEAUS AND MESAS HWY 264 NORTHWARD-CHINLE VALLEY- CHUSKA MOUNTAINS AND DEFIANCE PLATEAU- LITTLE COLORADO RIVER VALLEY IN COCONINO COUNTY- LITTLE COLORADO RIVER VALLEY IN NAVAJO COUNTY- LITTLE COLORADO RIVER VALLEY IN APACHE COUNTY- WESTERN MOGOLLON RIM-EASTERN MOGOLLON RIM-WHITE MOUNTAINS- NORTHERN GILA COUNTY-YAVAPAI COUNTY VALLEYS AND BASINS- OAK CREEK AND SYCAMORE CANYONS-BLACK MESA AREA- NORTHEAST PLATEAUS AND MESAS SOUTH OF HWY 264- 323 PM MST WED AUG 22 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF EAST CENTRAL ARIZONA...NORTH CENTRAL ARIZONA...NORTHEAST ARIZONA AND WEST CENTRAL ARIZONA. .DAY ONE...THIS AFTERNOON AND TONIGHT DEEP MOISTURE ACROSS NORTHERN ARIZONA WILL INTERACT WITH A LOW PRESSURE SYSTEM MOVING ACROSS THE STATE TONIGHT TO PRODUCE WIDESPREAD SHOWERS AND THUNDERSTORMS TONIGHT THROUGH THURSDAY MORNING. THESE STORMS HAVE THE POTENTIAL TO BECOME STRONG AND PRODUCE FLASH FLOODING AND THE POSSIBILITY OF HAIL...UP TO AN INCH IN DIAMETER. SOME STORMS MAY ALSO PRODUCE STRONG WIND GUSTS. A FLASH FLOOD WATCH IS IN EFFECT FOR ALL OF NORTHERN ARIZONA THROUGH THURSDAY MORNING. .DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY THE UPPER LEVEL STORM SYSTEM WILL CONTINUE TO MOVE EAST ACROSS NORTHERN ARIZONA ON THURSDAY WITH WIDESPREAD CLOUDS...SHOWERS AND THUNDERSTORMS EXPECTED THROUGH THE DAY. THE THREAT FOR FLASH FLOODING WILL BE GREATEST OVER EASTERN ARIZONA WHERE A FLASH FLOOD WATCH CONTINUES THROUGH THURSDAY AFTERNOON. A MOIST AND UNSTABLE AIR MASS WILL REMAIN ACROSS NORTHERN ARIZONA ON FRIDAY...EXPECT A GOOD CHANCE OF SHOWERS AND THUNDERSTORMS. A SLIGHTLY DRIER WEST TO NORTHWEST FLOW BEHIND THE TROUGH ON SATURDAY AND SUNDAY WILL BRING THUNDERSTORM CHANCES DOWN TO 20 TO 40 PERCENT. .SPOTTER INFORMATION STATEMENT... WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.
AZZ008-037-038-230745- SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE FLAGSTAFF AZ 1044 PM MST WED AUG 22 2012 ...THUNDERSTORMS DEVELOPING OVER YAVAPAI COUNTY... AN AREA OF THUNDERSTORMS IS DEVELOPING OVER YAVAPAI COUNTY LATE THIS EVENING. THESE STORMS ARE MOVING NORTHEAST AT 15 TO 20 MPH. RAINFALL RATES WITH THE HEAVIER STORMS ARE 1 TO 2 INCHES AN HOUR. RAINFALL RATES THIS HIGH CAN CAUSE FLOODING IN LOW LYING AREAS AND NORMALLY DRY WASHES. USE CARE WHEN DRIVING AT NIGHT AS IT MIGHT BE HARD TO SEE WHERE WATER COVERS ROADWAYS. && MAS
The Flagstaff radar (KFSX), 64 km away, was scanning. The volume nearest the first event began at 08:40 UTC.
2 flash flood, 1 heavy rain, 1 thunderstorm wind. Local standard time, as filed.
Sunset Wash overflowed on to Sunset Drive. Police set up barricades to close the road.
Street flooding was reported in St Johns. Manhole covers were being displaced because there was so much water.
Heavy rain caused two feet of water to flow over Navajo Route 15 near the community of Cornfields.
A peak wind gust of 70 MPH was recorded at the St Johns Airport. Law enforcement also reported that trees were blown down in town.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Arizona in 2012Every state in 2012This day in weather historyWarnings in Arizona, August 2012
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: 411080, 411074, 411072, 411075.