An anomalously strong and expansive area of high pressure centered near the Four Corners allowed temperatures to climb up to 115 degrees, leading to excessive heat conditions. Record highs were broken in Phoenix on the 4th (114 degrees), 5th (115 degrees), and 6th (113 degrees).
Every event with a place to put it; 33 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.
An anomalously strong and expansive area of high pressure centered near the Four Corners allowed temperatures to climb up to 115 degrees, leading to excessive heat conditions. Record highs were broken in Phoenix on the 4th (114 degrees), 5th (115 degrees), and 6th (113 degrees). According to the Maricopa County Department of Public Health, 15 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 Sep 2, 4:20 PM MST, 40 minutes earlier.
Area Forecast Discussion
National Weather Service Phoenix AZ
420 PM MST Wed Sep 2 2020
.UPDATE...Updated Aviation Discussion.
&&
.SYNOPSIS...
Drier air has settled over the region with thunderstorm chances
essentially neglible through at least early next week. Temperatures
near the seasonal normal will warm significantly during the latter
half of the week with readings returning to above seasonal normal
levels, and closer to record territory over the weekend.
&&
.DISCUSSION...
Surface dew points are remarkably high across the forecast area
with readings in the low 70s around Yuma and El Centro, and in the
mid 60s around the Phoenix area after yesterday's shallow gulf surge.
While the surface feels very ripe, the 12Z KPSR and 15Z Yuma soundings
show the low level moisture extends only up to the H7 layer and is
drier compared to yesterday. Both soundings also reveal a strong
inversion below the H5 layer that appears more than adequate to
inhibit most convection. GOES-East derived stability indices does
indicate a stream of 200-800 j/kg of CAPE near the northern shore
of the Gulf of California protruding into southwestern AZ, but
considering the soundings, afternoon thunderstorms are only likely
to form over terrain features located close to the instability
with the aid of orographic enhancement. The latest HREF nearest
probability for storms depicts this outcome well, with the mountainous
areas of western Pima County and the Kofa Mountains area in Yuma
County as the only places in or near our CWA with much chance for
afternoon storms.
Most attention remains on the Excessive Heat Warning, replacing
the previous heat watch, for Friday through Monday. Ensemble
guidance remains consistent showing a very strong high pressure
ridge enveloping the West Coast and eventually centering near the
southern end of the Great Basin. This will support dramatic
increases in atmospheric thickness and height rises across the
region. Nearly all guidance indicates H5 heights will reach
595-598 dm over central Arizona by this weekend which would place
near the NAEFS maximum percentiles of climatology. Unfortunately,
this will translate to afternoon temperatures swelling into the
110-115 degree range Friday-Monday for numerous places with a high
or very high heat risk virtually everywhere in the CWA. The holiday
weekend may also mean that more people could be more exposed to heat
risk as they travel busy roads seeking reprieve in the cooler mountains
or recreating at nearby lakes, rivers, or pools.
Global ensemble guidance outputs diverge somewhat in forecast details
for next week but they generally show another trough digging into
the Intermountain West Tuesday/Wednesday. Height falls will surely
allow temperatures to cool much closer to seasonal normals and
hopefully ending the threat of anymore days of 110 degrees this
year. The latest 110 on record for Phoenix is September 19th,
although Yuma has seen 110 as late as October 2nd. There are some
signals showing a modest moisture increase by the middle of next
week that could bring shower and storm chances back into the
picture. Otherwise, conditions will continue be dry until then.
&&
.AVIATION...Updated at 2320Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation concerns expected through the TAF period. Breezy
westerly winds this afternoon with gusts upwards of 15-20 kts will
subside over the next couple of hours. The gradient will favor a
westerly wind component through most of tonight once again. However,
a period of light/variable winds can be expected during the
early/mid-morning hours on Thursday. Not expecting a typical wind
shift to the east overnight at KPHX but can't rule out a brief
easterly switch at times. Skies should remain mostly clear with no
rain/storm threat.
Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation concerns expected through the next 24 hours.
Winds at KIPL will continue to favor the southeast while winds at
KBLH favor the south. Winds are expected to mostly remain below 10
kts through the TAF period with typical afternoon breezes. Clear
skies will prevail through the next 24 hours.
&&
.FIRE WEATHER...
Saturday through Wednesday:
High pressure in the region will create very hot temperatures and
dry conditions most days. Daytime RH values will drop into the
10-20% range for most places while overnight recoveries will
struggle to surpass 30-40% for elevations below 3000 ft. Higher
elevations will see slightly higher RH values. Winds will
generally remain light with directions favoring diurnal/terrain
patterns. There are hints that moisture could return early next
week, perhaps as early as Tuesday/Wednesday, that would at least
improve the RH values across the board by 5-10%.
&&
.CLIMATE...
Record high temperatures:
Date Phoenix Yuma El Centro
---- ------- ---- ---------
Sep 4 112 in 2019 116 in 1945 115 in 1947
Sep 5 113 in 1945 115 in 1945 114 in 1945
Sep 6 111 in 2013 113 in 1986 114 in 1934
Sep 7 111 in 1979 114 in 1979 112 in 2019
&&
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Friday to 8 PM MST Monday for
AZZ530>556-559>562.
CA...Excessive Heat Warning from 10 AM Friday to 8 PM PDT Monday for
CAZ560>570.
&&
DISCUSSION...AD
AVIATION...Smith/Percha/Benedict
FIRE WEATHER...AD
CLIMATE...MOThe national radar mosaic for the moment the first event began.
31 excessive heat, 2 heat. Local standard time, as filed.
One heat-associated fatality was reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances and location unavailable.
Two heat-associated fatalities were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances and location unavailable.
Twelve heat-associated fatalities were reported by the Maricopa County Department of Public Health, with their details, such as age, gender, circumstances and location unavailable.
As the offices recorded each death: direct deaths were caused by the weather itself, indirect ones by something it set in motion.
| Event | Date | Age | Sex | Where | Kind |
|---|---|---|---|---|---|
| Heat, Central Phoenix | 2020-09-02 | Unknown | Direct | ||
| Heat, Central Phoenix | 2020-09-03 | Unknown | Direct | ||
| Heat, Central Phoenix | 2020-09-03 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-04 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-04 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-04 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-05 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-05 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-05 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-05 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-05 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-06 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-06 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-06 | Unknown | Direct | ||
| Excessive Heat, Central Phoenix | 2020-09-07 | Unknown | Direct |
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Arizona in 2020Every state in 2020This day in weather historyWarnings in Arizona, September 2020
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: 992715, 992716, 992717, 993391, 993392, 993393, 993394, 993395, 993396, 993397, 993398, 993399, 993400, 993401, 993402, 993403, 993404, 993405, 993406, 993407, 993408, 993409, 993410, 993411, 993412, 993413, 993414, 993415, 993416, 993417, 993418, 993419, 993420.