From 30 June 2026 onwards severe high temperature developed east of the Mississippi River and started to further intensify along the Appalachian Highlands culminating on the 4th of July in New Jersey. Such widespread extreme heat combined with high humidity fuelled numerous severe storms, impacting a large swath of the country, and left almost 1 million households without power after storms and weekend high temperature disrupted electric grid.

More specifically, this meteorological event was driven at synoptic timescales by a persistent upper-level anticyclone, commonly referred to as a heat dome, centered over northeastern Canada and northwest US remaining nearly stationary for about a week. The associated high-pressure system modulated the mid- to upper-level southerly flow on its western flank, advecting anomalously warm air northward into their individual heat dome centres. At the same time, large-scale subsidence beneath the anticyclone suppressed convection and cloud formation, allowing strong solar heating combined with greenhouse effect from high humidity, to accumulate heat within the lower troposphere and produce exceptionally high surface temperatures. Meanwhile, along the peripheries of the heat domes, the circulation transported warm, moisture-rich air toward the U.S.–Canada border. This moist inflow increased atmospheric instability and moisture availability, favouring the development of severe thunderstorms and locally intense rainfall. As the blocking circulation gradually weakened and shifted, these storms produced widespread heavy rainfall and flooding across parts of Canada.

The surface-pressure anomalies (relative to the 1950–present climatology), shown in the first figure, reveal an anticyclonic pattern of up to approximately +2 hPa, centred east of the Mississippi River. ``This high-pressure system was associated with the persistence of anomalously high temperatures across the northeastern United States, the central Appalachian Mountains, the Great Lakes region and parts of Canada south of the Hudson Bay. We see widespread warm anomalies reaching up to approximately +5 °C (+9°F) over larger areas. 

The precipitation filed during the event shows mostly low or modest  rainfall across most of this high-temperature region, consistent with the stabilising influence of the high-pressure system, i.e., the anticyclonic (clockwise) circulation, although the southerly warm wind anomly also played a role. The wind speed during the event shows generally light to moderate winds over eastern North America, while the strongest winds were confined to the North Atlantic and areas west of the Mississippi River. This pattern potentially suggests limited atmospheric ventilation over the region experiencing the most intense heat, favouring the persistence of the heatwave.

Climate and Data Background for the Analysis

The modern long-term global climate change is primarily driven by anthropogenic emissions of greenhouse gases and aerosols, and land use (The Royal Society). The intensity and frequency of heatwaves have increased globally and, in most cases, at the regional scale as well (Chapter 11, WGI, IPCC AR6). The United States and Canada on average are warming faster than the global mean, and these trends in North America that is expected to continue throughout the 21st century. Under nearly all future emissions scenarios, the frequency and/or intensity of heat extremes are expected to increase across North America, posing growing risks to human health and well-being, as well as to natural, managed and human systems (Chapter 14: North America, WGII, IPCC AR6). While heatwaves are among the deadliest weather disasters in the United States, further global warming will likely push extreme heat beyond critical thresholds for public health, food production and other sectors with increasing frequency (Chapter 12, WGI, IPCC AR6).

The applied approach uses observationally-constrained historical data - ERA5 atmospheric reanalysis, complemented with GFS forecasts for up-to-date coverage - and does not rely on un-constrained (free running) numerical model simulations. The ClimaMeter framework compares how the selected meteorological conditions captured by surface pressure over the relevant spatial domain have changed between the first half of the historical period (1950-1987) as "past" and the second half (1988-2024) as "present", and whether such changes are likely due to natural climate variability or human-induced climate change. For additional time series analysis in the previous section, we utilised MSWX atmospheric reanalysis (that is based on ERA5 and GFS outputs).   

ClimaMeter Analysis 

In our analysis of the 30 June - 4 July 2026 high-temperature event (red dots on the time series figure above) over the eastern North America – based on 5-day mean conditions - we focus on a broader domain encompassing large-scale atmospheric circulation patterns crucial for understanding weather systems influencing these surface conditions. The selected region for our analysis of surface pressure and related variables is [105°W–50°W, 30°N–55°N].     

Surface pressure changes between the two periods show only small, localized increases (up to approximately +0.5 hPa), primarily off the northeastern coast of the United States and, to a lesser extent, over the Great Lakes. In contrast, the associated temperature changes reveal substantial warming across the Midwest and parts of Canada north of the Great Lakes, where present-day conditions are up to approximately +2.5°C (+4.5°F) warmer than their historical counterparts. Wind speed changes indicate a modest reduction (down to approximately −3 km/h or -0.8 m/s) over the region of elevated near-surface temperatures, particularly around the Great Lakes and extending southward. Precipitation changes suggest generally drier conditions across much of the study region, with the exception of wetter conditions over the northern Midwest.

Similar past events indicate a seasonal shift from the “past” to the “present” conditions, with a higher fraction of cases now occurring earlier in summer (higher in June and July while lower in August). This suggests that such extreme heat events are occurring earlier in the boreal summer under present-day conditions. Changes at the selected urban areas show that they experienced significantly warmer conditions (up to approximately +0.8 °C (+1.4°F)) during this 5-day event compared to similar “past” events, while precipitation generally decreased slightly. The associated wind speed changes at these urban locations were mostly negative (down to approximately -2  km/h or -0.6 m/s).

Overall, our results suggest that meteorological conditions like those of the 30 June - 4 July 2026 heatwave over eastern North America are now associated with warmer surface conditions – most pronounced west of the Great Lakes - than in the “past”, consistent with the influence of long-term climate change since 1950. The large-scale atmospheric pattern closely resembles past events, yet, just as in the case of the 2021 Pacific Northwest heatwave, climate change leads to amplified effects. Natural variability represented by El Niño-Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), and the Pacific Decadal Oscillation (PDO) in the ClimaMeter framework appears to have played only a small role in shaping the event.

Exposure 

We quantify population and socioeconomic exposure to these recent high-temperature conditions in North America enhanced by climate change by combining event-day hazard masks with spatially gridded population and economic datasets over the study domain used in our meteorological analysis. Population data is obtained from the Global Human Settlement Layer (GHSL, 2025) and regridded to a 0.5° horizontal resolution (Schiavina et al., 2022). Economic exposure is estimated using gridded gross domestic product (GDP) per capita data at the same spatial resolution (Kummu et al., 2018). This framework is designed to quantify the spatial coincidence of extreme heat events with population and socioeconomic assets, rather than to assess vulnerability, adaptive capacity, or observed impacts.

The heat hazard is defined using detrended and deseasonalised temperature anomalies during the event and is restricted to areas where the ClimaMeter analysis detects a statistically significant positive warming signal under present-day conditions relative to the past. In this way, the exposure estimates approximate the regions where climate change has increased the intensity of the event. We define three mutually exclusive hazard classes - moderate, severe, and extreme - based on the local percentile of the event-day temperature anomaly. The moderate class includes grid cells with values between the 98th and 99th percentiles, the severe class those between the 99th and 99.5th percentiles, and the extreme class those exceeding the 99.5th percentile of the local reference distribution. Socioeconomic exposure is then quantified by summing the population and economic activity located within each hazard class under present-day conditions.

For the 30 June–4 July 2026 North American heatwave, our analysis (see the following figure) indicates that approximately 50.4 million people were exposed to these high-temperature conditions intensified by climate change, across areas representing around 3.3 trillion USD in economic activity.  Of these totals, approximately 24.8 million people (49.2%) and 1.4 trillion USD (42 .4%) were in the moderate hazard class, approximately 23.2 million people (46.0%) and 1.6 trillion USD (48.5%) in the severe hazard class, and approximately 2.4 million people (4.8%) and 0.3 trillion USD (9.1%) in the extreme hazard class. We emphasise that these figures represent exposure - that is, the population and economic activity located within areas affected by the heatwave - and are not estimates of actual impacts or losses. For further details and a discussion of the methodology and its limitations, the interested reader is referred to may consult Faranda et al. (2026).

Conclusion

Based on the above analysis, we conclude that meteorological conditions similar to this early summer 2026 North America heatwave have become up to 2.5°C (4.5°F) warmer in the present-day climate than in the past. The exposure analysis shows that about 50.36 million people and 3.30 trillion USD of economic activity were in the region that experienced these extreme heat conditions intensified by climate change. We interpret this heatwave as an extreme weather event driven by very rare meteorological conditions, a stalled blocking high, whose intensity has been amplified by long-term climate change.