The June 2026 heatwave established a new benchmark for early-summer heat in German

Ionita, Monica; Nagavciuc, Viorica

doi:10.5194/egusphere-2026-4179

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https://doi.org/10.5194/egusphere-2026-4179

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16 Jul 2026

| 16 Jul 2026

Status: this preprint is open for discussion and under review for Natural Hazards and Earth System Sciences (NHESS).

The June 2026 heatwave established a new benchmark for early-summer heat in German

Monica Ionita and Viorica Nagavciuc

Abstract. The June 2026 heatwave established a new benchmark for early-summer heat in Germany, producing the highest temperatures observed since instrumental records began and culminating in a new provisional national temperature record of 41.8 °C. Here, we quantify the extremeness of this event using high-resolution gridded observations, long-term station records extending back nearly two centuries, and ERA5 reanalysis, and investigate the relative contributions of atmospheric circulation and background climate warming. Across the 1951–2026 gridded record, the event ranks first in peak daily maximum temperature, cumulative heatwave intensity, and the number of days with daily maximum temperature exceeding 30 °C. Daily maximum temperature anomalies reached 12–16 °C locally, while regional peak temperatures exceeded the 1971–2000 climatology by up to 4.9 standard deviations and long-term station records by up to 3.7 standard deviations. At the national scale, heatwave duration and frequency have increased significantly since 1951, providing the long-term climatic context in which this event occurred. Flow-analogue analysis shows that the heatwave was generated by a persistent omega-type blocking anticyclone, but that comparable circulation patterns from the recent past would have produced 3–4 °C lower temperatures than those observed in 2026, indicating substantial thermodynamic amplification associated with background warming. Our results show that the June 2026 event represents a new class of intensity-dominated, record-shattering early-summer heatwaves in Germany, in which persistent atmospheric blocking acts upon a substantially warmer climate to produce unprecedented temperatures.

Received: 13 Jul 2026 – Discussion started: 16 Jul 2026

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Monica Ionita and Viorica Nagavciuc

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RC1:
'Comment on egusphere-2026-4179', Milad Basirifard, 16 Jul 2026 reply
The dynamic component is estimated from the mean T850 anomaly of the 25 closest Z500-pattern analogues, while the difference between that composite and the observed T850 anomaly is treated as the thermodynamic component. However, matching the pattern of Z500 anomalies does not guarantee that the circulation amplitude, pressure gradients, horizontal temperature advection, subsidence strength, or persistence are comparable. Indeed, the observed Z500 anomaly in Figure 11 appears substantially stronger than the analogue composite. Consequently, the reported 3 to 4 C residual may contain the effect of unmatched circulation intensity, antecedent soil moisture, advection, and other processes; not simply background warming. Could the authors repeat the analysis using analogues constrained by both Z500 pattern correlation and anomaly amplitude, or statistically adjust T850 for differences in Z500 amplitude? At minimum, the contribution associated with circulation mismatch should be quantified before the residual is described as evidence of thermodynamic amplification.

The manuscript states that the 2001–2025 analogue composite is systematically warmer than the 1970–2000 composite on all five event days. However, Figure 12 appears to show the early-period composite warmer than the late-period composite on approximately 26–28 June, with only limited or reversed separation on the remaining days. This visual result does not support the stated systematic warming across all days. The authors should provide the exact daily values, uncertainty intervals, and statistical significance of the early minus late differences. They should also verify whether the plotted curves, legend, or textual interpretation are incorrect. This issue directly affects one of the paper’s central conclusions.

The authors should define what constitutes a qualifying analogue, report similarity-distance distributions, account for unequal epoch lengths and repeated dates, and test whether the late-period excess differs from what would be expected by chance. Independent equal-sized analogue sets should also be selected within each epoch. In addition, Figure 13 visually labels the samples as n = 40 and n = 85, whereas its caption and main text report n = 38 and n = 87; this inconsistency must be corrected.

Bootstrapping the 25 selected analogue days quantifies variability within that particular set, but it does not account for uncertainty arising from the choice of analogue number, European matching domain, distance metric, calendar window, epoch boundaries, or temporal dependence among analogue days. These choices are especially important because the authors acknowledge that the estimated dynamic–thermodynamic partition depends on them. A comprehensive sensitivity analysis should vary at least the number of analogues, matching domain, ±calendar window, similarity metric, and epoch boundary. A year block bootstrap would also be more appropriate than resampling individual days where analogues from adjacent dates or the same years may not be independent. Confidence intervals should be reported for the thermodynamic residual and early-versus-late difference, not only for the dynamic composite.

The Methods define the standardized exceedance using the standard deviation of the daily temperature maximum series, whereas the Figure 5 caption states that the standard deviation is calculated from the regional annual-maximum series. These are fundamentally different distributions and could produce very different values, including the reported 4.9σ exceedance.

Cumulative intensity is calculated as exceedance above the climatological mean, although heatwave days are selected using the 90th-percentile threshold. This formulation can mix event duration, seasonal climatology, and threshold exceedance in a way that may influence the rank-1 result. The authors should clarify the exact distributions used for P90 and σ, rename warming ratio as a standardized exceedance or z-score, and demonstrate robustness using alternative accepted heatwave metrics and thresholds, such as exceedance above P90, 95th-percentile definitions, and different minimum-duration criteria. The conclusion that the event was intensity-dominated should be shown to be insensitive to these methodological decisions.

Trends in heatwave duration and frequency do not by themselves establish that an event of this magnitude must be expected to recur with increasing frequency and intensity. Such a statement requires an extreme-value or formal event-attribution analysis with estimated return periods and probability changes. The record claims also need more careful qualification because most heatwave metrics are restricted to May--June, whereas phrases such as the most intense heat event on record imply comparison with the complete warm season. In addition, the long-term station records require explicit information on homogenization, relocations, instrumentation changes, missing data, and urbanization effects. The station and HYRAS-DE analyses should not be presented as fully independent confirmation because HYRAS-DE itself is derived from station observations.

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Citation: https://doi.org/10.5194/egusphere-2026-4179-RC1

Monica Ionita and Viorica Nagavciuc

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https://doi.org/10.5194/egusphere-2026-4179-supplement

Monica Ionita and Viorica Nagavciuc

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Short summary

The June 2026 heatwave set a new benchmark for early-summer heat extremes in Germany, culminating in a provisional national record of 41.8 °C and ranking first in the 1951–2026 gridded record for peak daily maximum temperature, cumulative heatwave intensity, and number of days exceeding 30 °C. Regional temperature anomalies exceeded up to 4.9 standard deviations at the federal-state level and 3.7 standard deviations at long-term stations.