US Southwest Climate – Sep 2026September 2026 Update

Novarupta (Katmai) eruption

The largest volcanic eruption anywhere in the world in the 20th century - 13 km3 of magma, about thirty times the 1980 Mount St. Helens eruption - at Novarupta in Alaska on 6-8 June 1912. Ash reached 100,000 ft and cooled the Northern Hemisphere by roughly 0.3°C over 1912-13. Unlike a tropical eruption such as Pinatubo, a high-latitude eruption's haze stays in its own hemisphere, so it did little to the Southern Hemisphere.

Smithsonian Global Volcanism Program, Alaska Volcano Observatory, National Park Service, Oman et al. 2005

The Dust Bowl

The defining environmental disaster of 1930s America. In the Southwest it reached Colorado and New Mexico.

Schubert et al. 2004, NCAR/UCAR

Black Sunday, the worst Dust Bowl storm

The worst single dust storm of the Dust Bowl years, and the day the phrase was coined. In the Southwest it swept across Colorado and New Mexico.

NWS Norman

Post-WWII industrial expansion begins

After the Second World War, coal and oil use grew rapidly across Europe, North America and the Soviet Union. Global sulphur dioxide emissions rose from about 30 to about 130 million tonnes a year by 1980, and the sulphate haze they formed masked part of the greenhouse warming underneath - the effect known as 'global dimming'.

Klimont et al. 2013

The 1950s drought

Among the most widespread and sustained droughts ever recorded in the US - worse than the Dust Bowl in several states. It reached every state in the Southwest.

NOAA Drought.gov, NOAA Climate Program Office, Texas Water Development Board

How much ENSO really controls the Southwest's winter rain

Of the fifty states, only seven have a December-February rainfall link to El Niño and La Niña strong enough to be worth reporting. In the Southwest only New Mexico clears it, with ENSO explaining 22% of the year-to-year swing in its winter rainfall. Arizona, Colorado and Utah fall below that line, so weaker ENSO events are left off their pages.

NOAA CPC Oceanic Nino IndexENSO Tracker

Agung eruption

Agung's eruption in Bali put a veil of sulphate haze into the stratosphere that cooled global temperature by about 0.3°C over 1963-66.

Smithsonian Global Volcanism Program, NOAA Global Monitoring Laboratory

The South Platte flood of 1965

The most expensive flood in Colorado's history, with a peak flow 1.8 times anything measured since 1889.

USGS Water Supply Paper 1850-B, Colorado Encyclopedia

US Clean Air Act

The 1970 Clean Air Act cut sulphur dioxide and particulate pollution across the country, and removing those sunlight-reflecting aerosols also removed a cooling effect that had been partly masking greenhouse warming. Across the Southwest, 2006-2025 averaged +1.15°C warmer than the two decades before the Act, 1951-1970.

US EPA

The 1973-74 La Niña

The strongest La Niña of the 1950-present record, part of a multi-year cool phase that also included a second, separate La Niña in 1975-76.

ONI -2.0°C · RONI -1.9°C

NOAA CPCENSO Tracker

The 1976-77 drought

A short, intense drought that ran from northern Minnesota to southern California. In the Southwest it reached Utah.

USGS, US GAO, USGS Water Data Labs

The cold winters of 1976-79

Three severe winters in a row. January 1977 was probably the coldest month the eastern United States had seen in two centuries - Atlanta's coldest month ever measured - and the winter of 1978-79 was the coldest on record for the contiguous United States. Missouri had its coldest winter on record; Illinois had its coldest January-February, 14°F below normal, with seventeen major winter storms.

Illinois State Water Survey, Missouri Climate Center, Monthly Weather Review (1978), Wikipedia

Pacific Decadal Oscillation flips to a warm phase

A warm Pacific Decadal Oscillation tends to bring the Southwest wetter winters, and the two decades after the 1976-77 flip were among the region's wetter spells of the century - which is part of why the drought that followed the flip back in 1998-99 came as such a contrast.

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Global SO2 emissions peak

Global sulphur dioxide emissions peak at about 130 million tonnes a year, then begin a long decline as Europe, North America and Japan clean up their air. As the aerosol mask thins, the rate of global warming picks up through the 1980s.

Klimont et al. 2013

El Chichón eruption

A sulphur-rich eruption in Mexico whose aerosol cloud stayed in the stratosphere for more than two years, cooling the Northern Hemisphere by about 0.2°C.

Smithsonian Global Volcanism Program

1982-83 El Niño

El Niño usually brings the Southwest a wet winter. In 1982-83 it was: winter rainfall ran +34% against the 1961-90 average, with temperatures +0.8°C. On NOAA's RONI index, which strips out background ocean warming, this is the strongest El Niño since 1950.

ONI +2.1°C · RONI +2.4°C

NOAA CPC, NOAA CPC (2)ENSO Tracker

Daytime warming catches up with the night-led pattern

Across 1961-90, 81% of the world's land saw nights warming faster than days. Across 1991-2020 the pattern reversed: 70% saw days warming faster, as the industrial haze that had been dimming daytime sunshine was cleaned up. The study compares two fixed thirty-year windows; measured as a turning point, this site's own global series turns around 1982.

Zhong et al. 2023

Mount Pinatubo erupts

Pinatubo's cooling is only faintly visible in the Southwest's own record. 1993, two years after the June 1991 eruption, ranks 18th-coldest of the 76 years there and no year since has been colder there. A single large eruption can hold temperatures down for two or three years; it cannot reverse the trend.

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Atlantic Multidecadal Oscillation flips to a warm phase

The Atlantic Multidecadal Oscillation flipped to a warm phase in 1995. It is an Atlantic cycle, but its reach is not confined to the coast: a warm AMO alongside a cool Pacific has been tied to more frequent drought across the western and central United States, including the widespread droughts of 1996 and 1999-2002. It is a natural cycle of roughly 60-70 years, so it shifts the baseline rather than driving the long-term trend.

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1997-98 El Niño

El Niño usually brings the Southwest a wet winter. In 1997-98 it was: winter rainfall ran +31% against the 1961-90 average, with temperatures +0.5°C. It is often treated as the textbook strong El Niño.

ONI +2.4°C · RONI +2.3°C

NOAA CPC, NOAA NCEIENSO Tracker

The 1999-2001 La Niña

A strong La Niña that ran for three winters after the record 1997-98 El Niño, peaking at an ONI of -1.53. La Niña usually leaves the Southwest dry in winter, and that first winter did so in the south of the region: Arizona ran 73% below its 1961-90 average and New Mexico 63% below, while Colorado was about normal and Utah 18% wetter.

ONI -1.5°C · RONI -1.6°C

NOAA CPC ONI

Pacific Decadal Oscillation flips back to a cool phase

The Pacific Decadal Oscillation flipped back to a cool phase around 1998-99. Cool phases favour dry winters across the Southwest, and the flip coincides with the start of the megadrought in 2000 - natural variability setting the stage, with warming then deepening it.

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Southwest megadrought (2000-2021+)

The driest 22-year period in the Southwest for at least 1,200 years, reconstructed from tree rings. It reached every state in the Southwest.

Williams et al. 2022

Colorado River flow decline / Lake Mead crisis

A decline in the river's flow of which roughly a third to a half is attributable to warming alone, not to reduced precipitation. The river's basin takes in every state in the Southwest.

Udall & Overpeck 2017

The 2002 Western drought

The sharpest single dry year of the early Southwest megadrought, and Colorado's driest year on record. In the Southwest it reached Arizona, Colorado and Utah.

NOAA Drought.gov, NOAA NCEI

The 2007-08 La Niña

A strong La Niña linked to reduced Atlantic hurricane wind shear and drought conditions across the southern US and East Africa.

ONI -1.8°C · RONI -1.7°C

NOAA CPCENSO Tracker

2009-10 El Niño

A moderate-to-strong El Niño that peaked around the turn of 2009-10. It helped make 2010 one of the warmest years on record at the time, and was followed straight away by the strong La Niña of 2010-12.

ONI +1.5°C · RONI +1.5°C

NOAA/IRI ENSO archiveENSO Tracker

The 2010-2012 La Niña

A strong double-winter La Niña peaking at an ONI of -1.57. In the Southwest that first winter split the region: New Mexico ran 46% below its 1961-90 average and Arizona 16% below, but Colorado was 12% wetter and Utah 51% wetter - the south of the region follows La Niña far more closely than the north.

ONI -1.6°C · RONI -1.6°C

NOAA CPC ONI

Start of the sustained rise in US heatwave frequency

Across the four states of the Southwest, Heatwave Days (WSDI) rose from a 1961-90 average of 4.8 days a year to 22.9 in 2016-25 - more than a fourfold increase, with every one of them recording more heatwave days than in 1961-90. It is not uniform: New Mexico is at 7.8x, Utah at 3.4x.

World Bank CCKP (CCKP), NOAA Climate at a Glance

The Wallow Fire

The largest wildfire in Arizona's modern record, about 538,000 acres. It burned across eastern Arizona and into New Mexico.

NASA Earth Observatory, CIRES

The 2012 drought and heat

A drought that covered about 70% of the country at its July peak. It reached every state in the Southwest.

NOAA NCEI, Summer 2012 Drought Update, NOAA NCEI, Drought

Post-2013 acceleration in US temperatures

Across the four states of the Southwest, an average of 8.0 of each state's ten warmest years have come since 2013 - every one of them has exactly eight of its ten since 2013. Every state in the region has at least 9 of its ten warmest since 2000.

NOAA Climate at a Glance

2015-16 El Niño

El Niño usually brings the Southwest a wet winter. In 2015-16 it did not: winter rainfall ran -7% against the 1961-90 average, with temperatures +1.1°C. By the raw ONI index it was the strongest El Niño ever measured - a reminder that a strong index does not guarantee a strong local outcome.

ONI +2.6°C · RONI +2.3°C

NOAA CPCENSO Tracker

2015-2025: the eleven warmest years on record

Globally, the eleven warmest years on record have all happened since 2015. The Southwest follows it closely: eight of its own eleven warmest fall in that span, with 2012, 2003 and 2000 outside it. Its warmest year is 2025.

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The West's air gets drier

The largest falls in relative humidity anywhere in the country, while the south-east barely changed. The drying shows in every state in the Southwest.

World Bank CCKP (CCKP), NOAA Climate at a Glance

The 2015 western snow drought

The year 81% of the West's monitored snow sites hit record-low snowpack - driven by warmth, not a shortage of precipitation. In the Southwest it reached Utah.

Mote et al. 2016, NOAA Drought.gov, USDA

The Hildale and Zion flash floods

On 14 September 2015 slow-moving thunderstorms over south-west Utah sent flash floods down Short Creek in Hildale and through Keyhole Canyon in Zion National Park. Twenty-one people died - 14 from Hildale, most of them in two vans swept away by the flood, and 7 canyoneers in Zion - the deadliest natural disaster in Utah's history.

NOAA NCEI Storm Events, FOX Weather

Low-sulphur shipping fuel rule (IMO 2020)

New international shipping rules cut the sulphur allowed in ship fuel from 3.5% to 0.5% almost overnight, removing about 8 million tonnes of sulphur dioxide a year. With less sulphate haze over the oceans, it may have added around 0.1°C of warming by 2023-24.

Gettelman et al. 2024

The 2020-2023 'triple-dip' La Niña

Three consecutive La Niña winters - a 'triple dip', which has happened only a handful of times since 1950 - peaking at an ONI of -1.11. The Southwest got the dry winter La Niña usually brings: every state in the region was below its 1961-90 average that first winter, Arizona by 45% and New Mexico by 37%, deep inside the region's long megadrought.

ONI -1.1°C · RONI -1.5°C

WMO, WMO (2)ENSO Tracker

The Marshall Fire, December 2021

Colorado's most destructive wildfire (1,000+ homes) - notable for occurring in WINTER, following a whiplash from a wet spring (fuel growth) to a record-dry, 10°F+-above-average fall/winter plus 100+mph winds.

NOAA Climate.gov

Great Salt Lake's record-low water level, 2022

Reached its lowest recorded level in November 2022. A 2025 peer-reviewed study (Bigalke et al., Geophysical Research Letters) found reduced streamflow explains about two-thirds of the decline, but warming-driven evaporation accounts for the remaining third - without the warming trend, 2022 would NOT have been a record low.

Bigalke et al. 2025

The Rio Grande runs dry through Albuquerque, 2022

A 5-mile stretch went dry for the first time in 40 years, driven by low snowpack and reduced stored water; drying events now occur earlier and farther upstream than in the past.

Washington Post

2023-24 El Niño

A strong El Niño arriving on top of long-term greenhouse warming, with the cut in shipping sulphur from the 2020 fuel rule adding a little more. Together they produced the record global temperatures of 2023 and 2024.

ONI +2.0°C · RONI +1.4°C

NOAA CPCENSO Tracker

Phoenix's 31-day 110°F+ streak, 2023

31 consecutive days at or above 110°F (June 30-July 31), shattering the prior record of 18 days (1974); the hottest calendar month on record for any major US city.

NWS Phoenix / NPR