Idaho Climate – Sep 2026September 2026 Update

The 1910 'Big Burn'

Northern Idaho took the worst of the Big Burn. Over two days, 20-21 August 1910, winds of up to 70 mph welded thousands of separate fires into one front that ran through the Coeur d'Alene and St. Joe country; Wallace was partly burned and evacuated by train. Most of the 87 dead - 78 of them firefighters - died here.

BAMS, 2013

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

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 1952 Northwest drought

1952 was Idaho's 2nd-driest year on record here - 467 mm, 25% below its 1961-90 average. This is a north-western drought, separate from the great 1950s drought of the Southern Plains that ran through the same decade.

NOAA NCEI

Idaho's multi-decade snowpack decline

April snowpack declined at 81% of measured western US sites from 1955-2023 (avg ~18% decline); Idaho's state climatologist has documented a clear multi-decade warming/drying trend feeding the Snake River system.

EPA Climate Indicators: Snowpack

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

US Clean Air Act

The 1970 Clean Air Act set strict national standards for sulphur dioxide and particulates, and American skies got measurably cleaner over the decades that followed. There is a climate sting in that success: sulphate aerosols reflect sunlight, so removing them removed a cooling effect that had been partly masking greenhouse warming. Measured here, Idaho averaged +0.94°C warmer across 2006-2025 than across the two decades before the Act, 1951-1970, against a median of +1.09°C across the 49 states whose records reach back that far. Every one of them warmed. The range runs from +0.66°C in Oklahoma to +1.67°C in Alaska, so the question a state's own chart answers is not whether it warmed, but by how much.

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

Pacific Decadal Oscillation flips to a warm phase

The Pacific Decadal Oscillation is this region's own ocean rhythm. Its warm phase gives the Pacific Northwest milder, drier winters and a thinner mountain snowpack, and the 1976-77 flip began about two decades of it - long enough to show in the region's salmon runs, which is how the pattern was first identified.

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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 has only a weak pull on Idaho's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Idaho's winter rainfall ran +10% against the 1961-90 average, with temperatures +1.6°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 aerosol veil left a clear mark on Idaho. 1993, two years after the June 1991 eruption, is Idaho's 5th-coldest year of the 76 on record and no year since has been colder. 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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The February 1996 Pacific Northwest floods

1996 is Idaho's wettest year in this site's 1950-2025 record - 815 mm, 30% above its 1961-90 average. Idaho lay at the eastern edge of the February 1996 rain-on-snow event, which did its worst damage west of the Cascades.

NOAA Climate Prediction Center, USGS Oregon remembers the 1996 flood, Oregon Encyclopedia

1997-98 El Niño

In 1997-98 Idaho's winter rainfall ran -10% against the 1961-90 average, with temperatures +1.3°C - though ENSO's pull on rainfall here is weak. It is often treated as the textbook strong El Niño.

ONI +2.4°C · RONI +2.3°C

NOAA CPC, NOAA NCEIENSO Tracker

Pacific Decadal Oscillation flips back to a cool phase

The Pacific Decadal Oscillation flipped back to a cool phase around 1998-99. For the Pacific Northwest a cool phase favours cooler, wetter winters and a deeper mountain snowpack - the opposite of the two decades before.

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The 2002 Western drought

2002 was Idaho's 6th-driest year on record here - 482 mm, 23% below its 1961-90 average.

NOAA Drought.gov, NOAA NCEI

Start of the sustained rise in US heatwave frequency

Idaho's own Heatwave Days (WSDI) count rose from a 1961-90 average of 8.8 days a year to 11.4 in 2016-25 - a modest rise, and one of the smallest in the country against a 50-state average of 2.4x.

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

Post-2013 acceleration in US temperatures

6 of Idaho's ten warmest years on record have occurred since 2013, and 9 of the ten since 2000, with 2015 the warmest of all.

NOAA Climate at a Glance

2015-16 El Niño

In 2015-16 Idaho's winter rainfall was close to its 1961-90 average (+5%), with temperatures +1.6°C - though ENSO's pull on rainfall here is weak. 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. Idaho's own record is a looser fit - six of its eleven warmest fall in that span, with 2014, 2012, 2007, 2003 and 1992 outside it. Its warmest year is 2015.

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The 2015 western snow drought

For Idaho, 2015 was its warmest year on record, its highest heatwave-day count (44.2 days) and the least snow of any year in this site's 44-year snow record. No other western state set all three records in the same year.

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

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

2021 Pacific Northwest heat dome

Lewiston reached 115°F on 29 June 2021, close to its all-time high, and Boise ran nine consecutive days at 100°F or above. Idaho recorded its 5th-highest heatwave-day count of the 1950-2025 record that year and its 3rd-warmest year overall - the dome sat far enough east to make this an Idaho event, not just a coastal one.

Philip et al. 2022, NOAA Climate.gov, USDA Climate Hubs

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