And why the same mechanism that built the 2021 Pacific Northwest heat dome is doing it again, in early spring
As I write this in mid-March 2026, the western United States is in the grip of a heat wave that has no precedent in the cool-season observational record. Downtown Los Angeles hit 92°F last Thursday. Phoenix is on track for its earliest 100°F day ever, beating the previous record by more than a week. Las Vegas is flirting with triple digits in a month when average highs are in the low 70s. Temperatures across a vast swath of the West are running 20 to 30 degrees above normal, and the National Weather Service has issued heat alerts covering more than 25 million people.
The numbers are extraordinary. But what’s happening in the upper atmosphere is arguably more remarkable, and it connects this event, mechanistically, to one of the most studied extreme weather events in recent history.
The strongest cool-season ridge on record
The proximate cause of this heat wave is a 500-millibar ridge of high pressure centered over the Great Basin and Four Corners region that is, by several measures, unprecedented. Climate scientist Daniel Swain, writing at Weather West, described it as the strongest mid-tropospheric ridge ever observed in the southwestern United States in March, and then went further, calling it the strongest cool-season (November through March) ridge in the historical record. The ECMWF model’s three-week percentile rank analysis shows a vast region from offshore California to Colorado and Montana sitting at or beyond the record maximum in the ERA5 reanalysis, which extends back to 1979.
Swain used the term “record-shattering” deliberately: these are not records being broken by a tenth of a degree. They are long-standing records being exceeded by enormous margins, the kind of margins that, in a stationary climate, would be extraordinarily unlikely.
The ridge is producing two waves of intense heat. The first swept through Southern California and the Desert Southwest around March 13–14. The second, expected to be even stronger and more expansive, is building now across the Mountain West (Utah, Colorado, Oregon, Idaho, Wyoming) and will likely persist through at least March 22. The 250-hPa jet stream effectively vanishes over the ridge core, forced into a dramatic split with one branch shunted into Alaska and the other pushed south toward Mexico. Beneath the ridge, subsiding air warms adiabatically, skies clear, and the surface bakes.
A familiar mechanism
If this sounds familiar, it should. The dynamical fingerprints of this event bear a striking resemblance to the June 2021 Pacific Northwest heat dome, the event that sent Portland to 116°F, killed over a thousand people across the Pacific Northwest and British Columbia, and has since generated more than 70 peer-reviewed papers attempting to explain how something so far outside the historical envelope could occur.
The March 2026 event appears to share a key physical mechanism with the 2021 event: downstream ridge amplification driven by upstream latent heat release.
Here is the chain, described first for 2021 and then for what we’re observing now.
June 2021: An extratropical cyclone developed in the North Pacific. Warm, moist air ascended rapidly within the cyclone’s warm conveyor belt, the river of air that rises from the boundary layer ahead of a cold front. As the air ascended, water vapor condensed, releasing enormous quantities of latent heat. That heating reorganized the potential vorticity (PV) structure of the atmosphere: below the heating maximum, PV was enhanced (strengthening the cyclone), while above it, anomalously low-PV air was produced and transported into the upper troposphere. This low-PV outflow spread poleward, displacing the jet and amplifying a downstream ridge over western North America. The ridge then drove intense subsidence and adiabatic warming at the surface.
Neal, Huang, and Nakamura (2022), in the seminal paper on this mechanism, used a local wave activity budget to show that the upstream cyclone’s diabatic heating was a critical source of wave activity whose convergence over western Canada built the blocking ridge. When they reduced the upstream diabatic source by just 30% in their analysis, the resulting block was 41% weaker and displaced 10 degrees eastward. The upstream storm wasn’t incidental to the heat dome. It was the engine.
Subsequent studies deepened the picture. Schumacher, Hauser, and Seneviratne (2022) traced air parcels backward from the heat dome and found they had been heated by condensation in two distinct warm conveyor belts, one southeast of Japan and another south of Alaska, before descending and warming adiabatically under the ridge. White et al. (2023) quantified the contributions: roughly 78% of the temperature change along backward trajectories was diabatic (mostly from latent heating), while only about 22% was from adiabatic subsidence. And Oertel et al. (2023), in a paper aptly titled “Everything Hits at Once,” showed through ensemble sensitivity experiments that anomalous rainfall in the western Pacific triggered a cascade of weather events across the basin that built the extreme ridge over Canada.
March 2026: The synoptic setup is strikingly parallel. A Kona low, a deep subtropical cyclone, developed near Hawaii, producing catastrophic rainfall and flooding across the Hawaiian islands (Maui received over 40 inches of rain in some locations). That storm pumped massive amounts of moisture and moist static energy into the atmosphere. An atmospheric river then transported that moisture northeastward into the Pacific Northwest, where it generated heavy precipitation along the coast of British Columbia and the Cascades. The precipitation field on March 16 shows the signature clearly: a thick ribbon of rain and snow slamming into the Pacific Northwest, and absolute bone-dry conditions across the interior West under the ridge core.
Swain explicitly identified the parallel, writing that “the abundant upstream condensation over the Central Pacific into the PacNW will contribute to a classic diabatic ‘ridge-building’ event across the West, wherein warm moist air aloft helps to strengthen the magnitude of mid-tropospheric geopotential height anomalies and subsequently allow for a ‘heat dome’-like effect to develop as subsidence within the strengthening ridge causes adiabatic warming and drying.”
He called the process “fairly reminiscent of what transpired during the record-shattering June 2021 heat event in the Pacific Northwest and British Columbia.”
What we know, what we infer, and what remains to be quantified
It is important to be precise about the different levels of evidence here. For the 2021 event, the diabatic ridge-building mechanism has been demonstrated through rigorous diagnostic methods: local wave activity budgets, Lagrangian trajectory analysis, PV reconstruction, and ensemble sensitivity experiments. Multiple independent research groups reached convergent conclusions.
For March 2026, the evidence is necessarily different in character. The existence of the extraordinary ridge is observed. The upstream moisture transport, atmospheric river, and heavy Pacific Northwest precipitation are directly observable in real-time analysis fields. The spatial relationship between upstream precipitation and the downstream ridge matches the pattern predicted by the diabatic amplification mechanism. And the event has been interpreted in those terms by the researchers most familiar with the 2021 literature.
But the specific quantitative contribution of diabatic ridge amplification to this event’s extraordinary magnitude has not yet been formally diagnosed. That will require the kind of trajectory analysis, PV budgets, and sensitivity experiments that took months to complete for the 2021 case. What we have now is a strongly supported physical inference grounded in analogous prior work, not yet a quantitative demonstration.
This distinction matters because the mechanism is one of several factors contributing to the event’s extremity, and the relative importance of each factor is what formal diagnostics would help establish.
The broader science: why upstream storms build downstream heat domes
The idea that upstream precipitation can amplify downstream ridges is not specific to these two events. It rests on a substantial body of research into how moist processes interact with the large-scale atmospheric circulation.
Pfahl, Schwierz, Croci-Maspoli, Grams, and Wernli (2015) provided the first climatological demonstration that diabatic processes are of leading importance for atmospheric blocking, the persistent, stationary high-pressure systems that drive many extreme heat events. Using 21 years of trajectory data, they found that roughly 69% of air parcels entering blocking regions had experienced significant latent heating (potential temperature increases exceeding 2 K) over the preceding week. This was a paradigm-shifting finding: prior theories of blocking had been built almost entirely on dry dynamics.
Steinfeld and Pfahl (2019) extended this to a 38-year global climatology, showing that latent heating, predominantly in warm conveyor belts, is most important during blocking onset and in the most intense blocks. Crucially, they found that repeated injection of diabatically heated low-PV air by a series of upstream cyclones can sustain blocks against dissipation, explaining how some ridges persist for weeks.
Steinfeld, Boettcher, Forbes, and Pfahl (2020) then provided direct causal evidence through numerical experiments: when upstream latent heating was suppressed in model simulations, some blocking events failed to develop entirely, while others were substantially weakened. The relationship is not merely correlational. In many cases, upstream latent heating is necessary to produce or substantially strengthen the downstream anticyclone.
And Teubler and Riemer (2021), analyzing more than 6,000 Rossby wave packets over 40 years, found that the divergent flow associated with latent heating amplifies ridges at leading order, an effect larger than classical baroclinic growth. They proposed that the lifecycle of midlatitude ridges is best described as “downstream moist-baroclinic development.”
This body of work provides the scaffolding for interpreting both the 2021 and 2026 events. The mechanism is well established in the literature. What makes individual events extraordinary is the degree to which the ingredients align.
Why this is happening on a warmer planet
The March 2026 heat dome is occurring in a climate context that amplifies its impacts at multiple levels.
Winter 2025–2026 was the warmest on record across the majority of the American West. Seven states (Arizona, Colorado, Nevada, New Mexico, Oregon, Utah, and Wyoming) set all-time warmest winter records, some by margins exceeding 2°F. The La Niña base state through winter 2025–26 likely helped condition the large-scale jet pattern and the seasonal warm, dry background in the Southwest, though it does not by itself explain the extraordinary ridge amplitude. La Niña winters tend to favor above-normal temperatures and below-normal precipitation in the Desert Southwest, the canonical response, but the magnitude of this event goes far beyond the typical La Niña signature.
The climate change connection operates through several pathways that compound rather than simply add.
First, background warming shifts the entire temperature distribution. Even a modest shift in the mean produces a highly nonlinear increase in the frequency of extreme tail events. Rahmstorf and Coumou (2011) showed this theoretically, and Robinson et al. (2021) documented it empirically: heat extremes exceeding four standard deviations, nearly absent in the early 2000s, affected roughly 3% of global land area during 2011–2020, an approximately thousandfold increase compared to 1951–1980.
Second, once a strong ridge is in place, warm antecedent conditions and dry soils reduce evaporative cooling and amplify surface temperatures. Bartusek, Kornhuber, and Ting (2022) showed that for the 2021 event, nonlinear interactions between circulation, soil moisture, and warming amplified severity by approximately 40%. Zhang et al. (2023) found that heat extremes under similar dome circulations intensify faster than background warming: when global temperature rises from 1°C to 3°C, associated maximum temperatures increase by 3.5°C, not 2°C, due to soil moisture feedbacks.
Third, a warmer atmosphere holds more moisture. This means more water vapor is available for latent heat release in the warm conveyor belts and cyclones that drive diabatic ridge amplification. Several of the 2021 studies noted this explicitly. Neal et al. (2022) observed that increased atmospheric water vapor from warming likely enhances the diabatic processes that build ridges. White et al. (2023) similarly noted that increased moisture content “may influence the strength and development of blocking highs.” If confirmed for the 2026 case, this would mean that climate change is not just raising the baseline temperature under the ridge; it may be helping to build the ridge itself.
Fischer, Sippel, and Knutti (2021) provided the theoretical framework that ties this together: in a warming climate, we should expect not just more frequent extremes but record-shattering extremes, events that break previous records by much larger margins than historical experience would suggest. The probability of such events depends on the rate of warming, and in high-emission scenarios, week-long heat extremes that exceed records by three or more standard deviations become two to seven times more probable in the coming decades.
This is the key insight: warming does not simply add a degree to otherwise-normal events. An extreme circulation pattern, potentially strengthened by enhanced moist diabatic processes, acts on a warmer and in many places drier background state, producing disproportionately large surface impacts. As in 2021, the upstream diabatic contribution is most plausibly interpreted as helping build the anomalous ridge aloft, while the extreme surface temperatures arise from the combined effects of that ridge, subsidence, clear-sky radiative heating, and land-surface feedbacks operating on an anomalously warm baseline.
The snowpack crisis
The impacts of this heat dome extend well beyond uncomfortable temperatures. Western snowpack, the natural reservoir that provides up to 75% of the region’s freshwater supply as it melts through spring and summer, was already at or near record-low levels before this event began.
The NOAA National Integrated Drought Information System reported on March 12 that every major river basin and state in the West is experiencing a snow drought. Colorado reported record-low statewide snowpack as of March 8. The Colorado River Basin, the water supply for 40 million people, has record-low snow water equivalent. Some California SNOTEL stations had already lost all their snow by early March, with Tahoe City Cross melting out 40 days earlier than the long-term median. Snow cover across the West on January 4 was the lowest in NASA’s satellite record, which dates to 2001.
The heat dome will accelerate snowmelt dramatically and cause significant sublimation and evaporation of what remains. Swain warned that April 1 snowpack, the traditional benchmark for water supply forecasting, may be the worst on record across many, if not most, western watersheds. The Bureau of Reclamation’s forecast for inflow to Lake Powell, already grim, projects April-through-July runoff at just 36% of average, the fifth-lowest in the 63-year forecast history. Denver Water reported South Platte Basin snowpack at 54% of normal on March 16, the worst on record for that date, noting it would need seven to seven and a half additional feet of snow to reach a normal peak.
The heat that makes headlines in the valley is destroying the water supply in the mountains. For a region already deep into a multi-decade aridification trend, the timing could hardly be worse.
What this means
The March 2026 western heat dome is not an isolated anomaly. It belongs to a class of events, alongside the 2021 Pacific Northwest heat dome, the 2019 and 2022 European mega-heatwaves, and the 2020 Siberian heat wave, in which extreme dynamical forcing combines with enhanced diabatic processes and a warming baseline to produce temperatures that shatter the prior record by margins that would have been considered virtually impossible a generation ago.
The science connecting these events is increasingly clear. Upstream storms build downstream heat domes through diabatic ridge amplification. Climate change intensifies the impacts of this kind of event by raising the background temperature and drying soils that would otherwise buffer surface warming; it may also strengthen the moist diabatic processes that help build the ridge in the first place by increasing atmospheric moisture availability. The result is a world where record-shattering events become more frequent, not because the atmosphere has fundamentally changed how it works, but because the same dynamical engines now operate in a thermodynamic environment that lets them produce more extreme outcomes.
Colin McCarthy of UC Davis called the March 2026 event “the most extraordinary US heatwave since the 2021 Pacific Northwest Heat Dome.” Given the cool-season context, the relative anomalies, and the ridge amplitudes involved, it may deserve an even starker distinction: the most anomalous heat event in the recorded cool-season history of the American West.
The atmosphere, as always, is doing what physics dictates. The physics just hits different on a warmer planet.
Claude Opus 4.6 (Anthropic) contributed to this analysis through iterative analysis of synoptic patterns and literature review.
Further reading:
Bartusek, S., Kornhuber, K., & Ting, M. (2022). 2021 North American heatwave amplified by climate change-driven nonlinear interactions. Nature Climate Change, 12, 1143–1150. doi:10.1038/s41558-022-01520-4
Fischer, E. M., Sippel, S., & Knutti, R. (2021). Increasing probability of record-shattering climate extremes. Nature Climate Change, 11, 689–695. doi:10.1038/s41558-021-01092-9
Neal, E., Huang, C. S. Y., & Nakamura, N. (2022). The 2021 Pacific Northwest Heat Wave and Associated Blocking: Meteorology and the Role of an Upstream Cyclone as a Diabatic Source of Wave Activity. Geophysical Research Letters, 49(8), e2021GL097699. doi:10.1029/2021GL097699
Oertel, A., Pickl, M., Quinting, J. F., Hauser, S., Wandel, J., Magnusson, L., Balmaseda, M., Vitart, F., & Grams, C. M. (2023). Everything Hits at Once: How Remote Rainfall Matters for the Prediction of the 2021 North American Heat Wave. Geophysical Research Letters, 50(3), e2022GL100958. doi:10.1029/2022GL100958
Pfahl, S., Schwierz, C., Croci-Maspoli, M., Grams, C. M., & Wernli, H. (2015). Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geoscience, 8, 610–614. doi:10.1038/ngeo2487
Rahmstorf, S., & Coumou, D. (2011). Increase of extreme events in a warming world. Proceedings of the National Academy of Sciences, 108(44), 17905–17909. doi:10.1073/pnas.1101766108
Robinson, A., Lehmann, J., Barriopedro, D., et al. (2021). Increasing heat and rainfall extremes now far outside the historical climate. npj Climate and Atmospheric Science, 4, 45. doi:10.1038/s41612-021-00202-w
Schumacher, D. L., Hauser, M., & Seneviratne, S. I. (2022). Drivers and Mechanisms of the 2021 Pacific Northwest Heatwave. Earth’s Future, 10(12), e2022EF002967. doi:10.1029/2022EF002967
Steinfeld, D., Boettcher, M., Forbes, R., & Pfahl, S. (2020). The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments. Weather and Climate Dynamics, 1, 405–426. doi:10.5194/wcd-1-405-2020
Steinfeld, D., & Pfahl, S. (2019). The role of latent heating in atmospheric blocking dynamics: a global climatology. Climate Dynamics, 53, 6159–6180. doi:10.1007/s00382-019-04919-6
Teubler, F., & Riemer, M. (2021). Potential-vorticity dynamics of troughs and ridges within Rossby wave packets during a 40-year reanalysis period. Weather and Climate Dynamics, 2, 535–559. doi:10.5194/wcd-2-535-2021
White, R. H., et al. (2023). The unprecedented Pacific Northwest heatwave of June 2021. Nature Communications, 14, 727. doi:10.1038/s41467-023-36289-3
Zhang, Y., Zhou, W., et al. (2023). Increased impact of heat domes on 2021-like heat extremes in North America under global warming. Nature Communications, 14, 1690. doi:10.1038/s41467-023-37309-y
