A strong ridge of high pressure over the Desert Southwest resulted in excessive heat conditions as temperatures climbed well above normal. The hottest day in Phoenix occurred on the 4th with a high of 115 degrees.
Every event with a place to put it; 31 filed by county or forecast zone are drawn at its center, in a lighter ring. 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 PSR wrote it up.
A strong ridge of high pressure over the Desert Southwest resulted in excessive heat conditions as temperatures climbed well above normal. The hottest day in Phoenix occurred on the 4th with a high of 115 degrees. According to the Maricopa County Department of Public Health, 25 heat-associated fatalities occurred during this event.
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 Phoenix, AZ wrote its last area forecast discussion before the first event at Jul 31, 9:19 PM MST, 2 hours 41 minutes earlier.
Area Forecast Discussion National Weather Service Phoenix AZ 919 PM MST Sat Jul 31 2021 .UPDATE...Updated Aviation Discussion. && .SYNOPSIS... High pressure and drier air will move into the region during the first half of next week limiting thunderstorm chances primarily to higher terrain areas of eastern Arizona. Temperatures will increase markedly with afternoon highs rebounding above normal. In fact, some areas of southeast California may flirt with 115 degrees and high heat risk. Thunderstorm chances may increase modestly towards the end of next week. && .DISCUSSION... Another complicated pattern evolution as several smaller scale MCV's have become entrained in the overall larger regional synoptic pattern. In general per satellite and objective analysis, the primary subtropical anti-cyclone was stationed over Colorado early this afternoon with a narrow, elongated trough lingering off the California coast. Concurrently, smaller MCV's were lifting through both central Arizona and southern Nevada. With the synoptic pattern undergoing an amplification and retrogressive phase, the location of all these features are becoming somewhat more difficult to ascertain. However, the trend among the ensemble mean suggests a jet streak aligned along the Baja peninsula will lift into Nevada while ridging builds and strengthens southwest into the forecast area over the next 24-48 hours. The position of the Baja jet streak will tend to place much of central Arizona under anti-cyclonic, subsident warming while the building heights under the midlevel ridge will only reinforce the trend. Other than the notable MCV's and residual cloud cover this morning, the other influence towards reduced thunderstorm coverage can be seen in the 12Z sounding data where moisture was substantially lower (~8 g/kg) in the 850-700mb layer. This drier layer will not immediately mix through the more shallow boundary layer (14-15 g/kg in the sfc-925mb layer), but lower level moisture profiles will gradually become influenced and depleted as mixing depths progressively deepen the next several days. Even this afternoon and evening, HREF output reflects the combination of subsidence and less favorable moisture profiles with just isolated storms around the region. Most likely, deep convection will materialize and become rooted at preferred terrain features (i.e. Kofa Wilderness, Table Top Reserve, Pinal Peak, etc) resulting from diurnal upslope flow, but have little sustenance removed from the forcing terrain. The only exception may be parts of SE California where sinking motion will not be pronounced, however current conditions exhibit a large amount of CinH that would need to be overcome. NBM 20-30% POPs over eastern Gila Co and Joshua Tree seem reasonable with generally 15% POP or less elsewhere (probably could be higher over some of the aforementioned preferred locations). HREF membership suggests little wind threat and mostly just a locally heavy rain potential. During the first half of next week, 500mb heights will increase as the main subtropical anti-cyclone retrogrades and consolidates over the lower Colorado River Valley. NAEFS means indicate very little spread with heights settling around 595dm in the Mon-Wed time frame (potentially maintaining strength later into the week). This progression will 1) create warming of the atmospheric profile - especially the midlevels, 2) increase boundary layer mixing depths, 3) scour low level moisture profiles, and 4) result in notable surface warming with temperatures approaching high heat risk thresholds in some locations. Forecast soundings suggest good warming the the 700-400mb layer while the boundary layer deepens up to 700 mb and sfc-700mb mean mixing ratios fall into a 7-8 g/kg range. This type of profile would be prohibitive of storms reaching into lower elevations, and even limit activity over higher terrain areas (even NBM POPs in eastern Gila County have trended towards 20% or lower). Perhaps the larger impact will be a return to above normal temperatures with much of the Phoenix metro possibly flirting with highs around 110F by the middle of the week. The greater threat and potential high heat risk will occur in the western CWA where readings may top over 115F at times (and may eventually require headlines). During the latter half of next week, there is fair ensemble agreement that the subtropical ridge weakens and becomes flattened across the SW Conus in response to a robust negative PV anomaly punching into the Pacific NW. Some model output keeps an E-W ridge axis splayed across northern Arizona allowing eastern waves to undercut the less favorable environment and advect better quality moisture back north. However, there's nearly equal support that the ridge will supplant closer to the international border bringing some measure of westerly winds and drying into the region. There is even potential that the Pacific wave dips even more south placing the forecast area under deep southerly, moist flow. This overall large uncertainty is probably best exemplified by NBM POPs late next week hardly differing from climatology. Greater confidence comes with respect to temperature forecasts where little ensemble uncertainty exists showing readings falling back near or below normal by the weekend. && .AVIATION...Updated at 0420Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Showers and thunderstorms that had formed over the distant higher terrain N-E and SE of Phoenix Saturday afternoon and early evening have dissipated. An isolated storm may pop up on Sunday afternoon around 20-23Z around the area but chances for storms over any given terminal is less than 10% and any storms that do form will tend to be fleeting and relatively modest in stature and the confidence of their occurrence was too low to include mention in the TAFs. Chances for strong outflows are also low, between 5-10% tonight and again for Sunday afternoon-evening. Typical diurnal wind patterns are otherwise expected with speeds generally in the 6-11 kt range. Otherwise skies will be mostly clear with mostly FEW to occasionally SCT mid-high cloud decks. Southeast California/Southwest Arizona including KIPL and KBLH: A few very isolated showers and thunderstorms could move through the terminal area Sunday afternoon, primarily between 20z and 23z closer to KBLH, but chances for direct impacts at the terminal remains low, less than 10% and the confidence of their occurrence was too low to include mention in the TAFs. Winds will otherwise favor a light diurnal or light and variable pattern through the period. Otherwise skies will be mostly clear to clear with FEW- SCT mid-high cloud decks. && .FIRE WEATHER... Tuesday through Saturday: Moisture levels will continue decline through at least the middle of the week with only isolated thunderstorm activity primarily relegated to higher terrain areas of eastern districts. There may be a slight increase in moisture and storm coverage by the end of the week. During the drying trend, temperatures will rebound into a slightly above normal category. Afternoon humidity levels will fall as low as 10-20% at lower elevations during the middle of the week, then increase closer to a 15-30% range into the weekend. Higher elevations will experience afternoon ranges closer to 20-40%. Overnight recovery will range widely from 30-80% mostly based on elevation. Aside from isolated thunderstorm gusts, winds will remain light and mostly follow typical upslope/drainage patterns. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && DISCUSSION...18 AVIATION...Sawtelle FIRE WEATHER...18
The national radar mosaic for the moment the first event began.
24 excessive heat, 7 heat. Local standard time, as filed.
Two fatalities (1 direct and 1 indirect) were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
Four fatalities (2 direct and 2 indirect) were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
Eight direct fatalities were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
Five direct fatalities were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
One direct fatality was reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
One direct fatality was reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
One direct fatality was reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
One direct fatality was reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
Two direct fatalities were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances, and exact location unavailable.
As the offices recorded each death: direct deaths were caused by the weather itself, indirect ones by something it set in motion.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Arizona in 2021Every state in 2021This day in weather historyWarnings in Arizona, August 2021
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: 1071560, 1071561, 1071562, 1071563, 1072038, 1072039, 1072040, 1072041, 1072042, 1072043, 1072044, 1072045, 1072046, 1072047, 1072049, 1072050, 1072051, 1072052, 1072054, 1072055, 1072056, 1072057, 1072058, 1072059, 1072060, 1072062, 1071564, 1071565, 1071566, 1071567, 1071568.