Daily 500-hPa heights (500H) from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis, version 5 (ERA5) (Hersbach et al., 2020) were used to analyze weather regimes (WRs) during April–May, the season of peak tornado activity (Graber et al., 2024), from 1981–2023. The seasonal cycle, defined as the long-term mean 500H for each calendar day, was removed from daily 500H. Unlike Graber et al. (2025), we did not remove the long-term linear trend in 500H of ERA5 to be consistent with the ECMWF seasonal forecast data. Rather than re-deriving WRs for this period, we used the WRs derived by Graber et al. (2025) using the K-means clustering method for 1960–2022 April–July as the reference WR patterns. WRs were assigned by finding the reference WR pattern with the smallest Euclidean distance from the daily 500H anomalies. We took this approach because K-means clustering yields slightly different WR patterns in different time periods and employing WRs during a longer time period as the reference patterns improves the robustness of the results.

Seasonal forecasts of daily 500H anomalies from the ECMWF were used to predict April-May WR frequency from 1981–2023 (Copernicus Climate Change Service, Climate Data Store, 2018; Vitart et al., 2017). The 25-ensemble forecasts are initialized once per month, are available on a 1° × 1° grid, are on 12 h temporal resolution, and go out 7 months. The forecasts initialized on 1 April were selected to mitigate the effect of the spring-predictability barrier (Duan and Wei, 2012). WRs were assigned for each ensemble forecast by projecting the forecast 500H anomalies to the reference WR patterns, and the resultant long-term mean WR frequencies are similar to those derived from ERA5. Persistent and nonpersistent WRs are defined as WRs lasting for ≥ 5 consecutive days and for < 5 consecutive days, respectively.

Tornado reports during April–May 1981–2023 were obtained from the NOAA Storm Prediction Center Severe Weather Database. Reports are georeferenced with time, date, and EF/F rating. To be consistent with previous work, tornado days (TDs) are defined as days with ≥ 1 tornadoes ranked EF/F-1 or greater, and tornado outbreaks (TOs) are defined as any day with > 10 tornadoes ranked EF/F-1 or greater (Graber et al., 2024, 2025). EF/F-0 reports were excluded due to reporting uncertainties (Brooks et al., 2014; Trapp, 2013). Known biases remain in this database, which a focus on TDs rather than raw tornado reports attempts to alleviate (Brooks et al., 2014; Graber et al., 2024; Trapp, 2014).

Following Graber et al. (2025), an empirical model was used to evaluate the seasonal prediction of tornado activity: 1TI(t)=∑i=15fi,tp×Pi,p+∑i=15fi,tnp×Pi,np where Pi denotes the TD probability for each WR, which is defined as the number of TDs with WR-i divided by the number of total WR-i days. The predictand, TI(t), which is the tornado index for year t, is computed using the seasonal count of WR-i days in year t, f(i,t), multiplied by Pi. TD probabilities and counts were calculated for persistent and nonpersistent WRs separately, denoted by subscripts p and np, respectively. The same procedure was applied to TOs.

To evaluate tornado prediction skill, a leave-one-year-out cross-validation method was employed. For example, 1981 was first held for testing, and Pi,np and Pi,p were determined using the WR information derived from the ERA5 reanalysis and tornado reports during 1982–2023. The seasonal WR counts from the ECMWF ensemble forecasts for 1981 were used in Eq. (1) to predict the tornado index value in 1981. This process was repeated for each year, yielding a predicted time series of the tornado index. The tornado index time series for all ensemble members were averaged to form the ensemble mean of tornado index, which was standardized using z-score normalization.

The prediction skill was quantified using the Pearson correlation between the predicted and observed tornado index time series. Additionally, a three-tier categorical verification was used by classifying both observed and predicted tornado index values into lower (0th–33rd percentile), middle (33rd–67th percentile), and upper (67th–100th percentile) terciles. A correct prediction was recorded when the observed and predicted tornado index values fell within the same tercile for a given year. The proportion correct (PC) is defined as the number of correct predictions divided by the number of total predictions.

To assess the potential impacts of climate modes on tornado predictability, PC was calculated separately for positive, negative, and neutral phases of various climate modes (see more information on climate modes in Sect. 2.4). Significance (p ≤ 0.05) between PC of each phase and random chance (33 % for three-tier verification) was determined using a Monte Carlo test with 10 000 resamples.

The impacts of some climate modes, ENSO (via Nino3.4), PNA, NAO, and AO, on tornado activity and WR counts were investigated to provide a physical basis for TD and TO predictability. We focus on their low-frequency variability on the seasonal and longer time scales and derived their April–May seasonal indices using the data from the NOAA Climate Prediction Center (2024a, b). Positive and negative phases of a climate mode are defined as the years when the standardized springtime (April–May) index exceeded ±0.9. 0.9 was used instead of 1.0 to slightly increase the sample size.

TD and TO probability anomalies were calculated for each climate mode phase as: 2Pa=Pr-PcPc×100% where the climatological mean TD (TO) probability (Pc) is defined as the total number of TDs (TOs) divided by the total number of days; Pr is TD (TO) probability for the given phase of the climate mode; and Pa is the percentage anomaly.

In addition, SSTs were investigated as a possible source of TO predictability. SST and 500H composite years were selected as years in which the April–May TO count exceeds +1 standard deviation (active) or falls below -1 standard deviation (inactive) relative to the 1960–2023 mean. Extended Range SST (ERSST) (Huang et al., 2017) of 2° × 2° resolution is used here, and we focus on SST anomalies in April because SST signals for TOs in April are more pronounced than in May based on a previous study (Chu et al., 2019) and our own analysis. In addition, the analysis is done for 1960–2023 to increase the sample size..

For completeness, the five WRs (Fig. 1a–e) are briefly described here. WR-A features anomalous highs over the Pacific Northwest and off the US east coast. WR-B is characterized by a prevailing anomalous low centered over central North America and an anomalous high over the Southeast. WR-C exhibits a three-cell wave pattern with anomalous lows over both coasts. WR-D and WR-E display west-east dipole patterns that nearly mirror each other. The WR spatial structures closely resemble WRs in Miller et al. (2020), Zhang et al. (2024), and Lee et al. (2023). Specifically, WR-B and WR-E resemble the Pacific Ridge and Alaskan Ridge regimes in Lee et al. (2023), respectively, but noticeable differences exist due to differences in data processing procedures and the time periods analyzed. The impacts of the WRs on CONUS tornado activity during April–May 1981–2023 (Fig. 1f and h) are similar to those during April–July 1960–2022 from Graber et al. (2025; their Fig. 4). WR-A is the least favorable WR for springtime tornado activity, and WR-B is the most favorable. Tornado activity in WR-C and WR-E are slightly unfavorable, and in WR-D is slightly favorable. The estimated TDs and TOs based on the empirical model (Eq. 1) and the ERA5 during April–May 1981–2023 are significantly correlated with the observed time series (Fig. 1g and i).

Before assessing the hybrid predictions of tornado activity, we first evaluate the prediction skill of WRs by the ECMWF springtime forecasts. The ECMWF ensemble mean prediction of springtime WR counts are significantly correlated with the ERA5 springtime WR counts (Fig. 2a–e). This indicates the seasonal predictability of WRs and provides the basis for tornado prediction using the hybrid framework (i.e., Eq. 1).

We assess the hybrid predictions of TDs and TOs using leave-one-year-out cross-validation method (Fig. 3). The hybrid prediction of TOs has a significant Pearson correlation of 0.38 with the observed TO time series, while TD prediction shows no skill, with a Pearson correlation close to zero. The skill contrast between the TO and TD predictions is probably because TOs usually occur under strong and persistent synoptic-scale patterns (Mercer et al., 2012; Cwik et al., 2022; Jiang et al., 2025; Graber et al., 2025), while transient (i.e., non-persistent) and weak patterns (e.g., patterns with weaker 500H anomalies than those associated with TOs), which are not well captured by the ECMWF springtime forecasts, may have strong impacts on TDs. In addition, it is worth noting that the percentage anomalies of TOs associated with various WRs are generally stronger than those of TDs (Fig. 1).

We also evaluated the tercile-based predictions using the TO and TD time series in Fig. 3 (see the tercile thresholds in Table S1 in the Supplement). The hybrid model correctly predicts TO terciles 53.5 % of years. Additionally, the PC is strongly modulated by some climate modes (Fig. 4). In particular, the hybrid model performs significantly better during the positive phases of the AO (75 % of correct prediction), NAO (71.4 %), PNA (71.4 %), and ENSO (70.0 %). PC also improves in the negative phases of NAO (62.5 %), AO (55.6 %), and ENSO (55.6 %), but the increases in PC are not significant (p > 0.05). In contrast, the PC in a neutral phase is below 53.5 %. Although the PC of the tercile-based TD prediction is close to random chance (34.9 %), the hybrid model performs better in +PNA years (57.1 %, p = 0.23) (figure not shown). This suggests that climate modes may be used to identify forecasts of opportunity for springtime tornado prediction.

Additional cross-validation tests by leaving 2-, 3-, 4-, and 6-year out yield similar results (Fig. S1 in the Supplement). TO predictions maintain skillful across all tests, while TD predictions remain unskillful. Given the low model skill (Fig. 3) and weaker connection to WRs for TDs (Fig. 1), the remainder of this paper focuses solely on TOs.

To investigate predictability sources for tornado activity, the relationship between tornado activity and low-frequency variability of climate modes is examined using springtime TO probability anomalies associated with the AO, NAO, PNA, and ENSO phases (Fig. 5a–d). In addition to CONUS, we examined different regions, Midwest, Southern Great Plains (SGP), Southeast, and Southern Great Plains (SGP) (Fig. S2). The region definitions follow Moore (2018), with exception of the SGP, which herein contains New Mexico because it has storm events similar to those in the Texas Panhandle.

Significantly enhanced TO probability in the CONUS and Midwest is associated with the -AO phase. TO probability is also enhanced in the Southeast, NGP and SGP, but the anomalies are not statistically significant. +AO years are characterized by an anomalous high over the southeastern CONUS and negative anomalies over the north-central CONUS (Fig. S3a), implying enhanced westerly flow aloft and promoting moisture and warm-air advection from the Gulf of Mexico. These circulation anomalies lead to positive MUCAPE and TO probability anomalies over the Southeast and SGP (Fig. S3a). In contrast, -AO years feature negative 500H anomalies over the central CONUS, accompanied by anomalously positive VWS (Fig. S3b). Increased VWS supports positive TO probability anomalies when it occurs on days where MUCAPE is high (Diffenbaugh et al., 2013; Sherburn et al., 2016). These positive TO probability anomalies during -AO are only statistically significant over the Midwest, consistent with the region of anomalously positive VWS anomalies.

The TO probability anomalies during the NAO phases are generally consistent in sign with those during AO phases when one phase is statistically significant, but quantitative differences exist. TO probability is reduced significantly over the NGP during the +NAO phase. +NAO years feature an anomalous 500H high over the west-central CONUS, which hinders moisture and heat transport from the Gulf of Mexico. Additionally, anomalously low VWS over the SGP further limits tornado potential during +NAO years. However, +NAO years also feature an anomalous 500H high over the western Atlantic and anomalous 500H low over the Southeast, which enhances southerly flow and moisture transport into the Southeast (Zhao et al., 2025), supporting positive MUCAPE anomalies (Fig. S3c) and corresponding positive TO probability anomalies in that region (Fig. 5b).

The positive PNA phase (+PNA) is unfavorable for TOs across the CONUS, Midwest, SGP, and Southeast (Fig. 5c). +PNA years feature an anomalous 500H high over the western CONUS and an anomalous 500H low over the eastern CONUS (Fig. S3e), implying reduced moisture transport from the Gulf of Mexico and anomalously low MUCAPE and VWS. These circulation anomalies hinder tornado activity across the Midwest, SGP, and Southeast. Positive TO probability anomalies are present across the CONUS in the negative phase of PNA (Fig. 5c), though there is no clear circulation pattern over the CONUS during these years (Fig. S3f). Springtime PNA has become more negative over time (Fig. S4), possibly explaining the increasing trend in TOs (Graber et al., 2024), though further work with other seasons needs to be done to confirm this, which goes beyond the scope of this study.

TO probability is enhanced in La Nina years in various regions and is accompanied by positive VWS anomalies, although the TO enhancement is significant only in the Southeast. Additionally, TO probability over the NGP are enhanced in El Niño years. The La Niña-tornado-activity link is largely consistent with previous work (e.g., Cook and Schaefer, 2008; Cook et al., 2017; Tippett et al., 2022; Allen et al., 2015; Moore, 2019), but it is worth noting that the ENSO state in the springtime is examined here, while some previous studies focus on the winter ENSO state. Previous studies have found that ENSO modulates the phase of PNA during boreal winter (Li et al., 2019; Soulard et al., 2019); however, these spring results are not consistent with those findings. Accordingly, the correlation between the ENSO and PNA indices are much stronger during the peak stage of ENSO in December than during the ENSO decay stage in April–May (Fig. S5).

The model appears to perform better (Fig. 4) in years when climate modes unfavorable for TOs, such as +NAO and +PNA, are present. To further evaluate skill, the Threat Score (Schaefer, 1990) was calculated for upper-tercile TO years. These years yield a Threat Score of 0.375 with 6 hits, 2 misses, and 8 false alarms, indicating a tendency for the model to overpredict TOs. For lower-tercile years, the Threat Score is slightly higher at 0.429, with 9 hits, 7 misses, and 5 false alarms. While lower-tercile years have greater overall skill, they also include more misses. Overall, the model demonstrates higher skill for lower-tercile years, and modest skill for upper-tercile years.

Overall, the link between climate modes and tornado activity suggests that some climate modes provide a source of predictability for springtime TO prediction, and that the predictability seems slightly higher during inactive years for tornado activity.

The connection between climate modes and WRs is examined next to investigate whether climate modes modulate tornado activity via WRs. Figure 6 shows the relative likelihood of each WR during different phases of climate modes, expressed as the ratio of its phase-specific frequency to its climatological frequency. WR-A is ∼ 36 % less likely to occur during the -AO phase, while WR-B is ∼ 35 % more likely to occur (Fig. 6a). The increased occurrence of WR-B and decreased occurrence of WR-A in -AO years compared to the long-term mean (Fig. 1) are consistent with the positive TO probability anomalies in the CONUS and Midwest during the -AO years (Fig. 5). WR-D is ∼ 27 % less likely to occur during -AO, so the above average tornado activity in -AO (Fig. 5a) is mainly due to above average frequency of WR-B. Additionally, WR-A is ∼ 21 % more likely to occur during +AO years.

During +NAO years, WR-A is ∼ 42 % more likely to occur, which is consistent with the reduced TO probability in the NGP and SGP (Fig. 6). Composite 500H anomalies for +NAO years resemble WR-A, while composite 500H anomalies for +AO show a mix of WR-A and WR-B (Fig. S3). WR-A's increased occurrence helps explain the high predictive skill of inactive TO years and indicates that +NAO offers a forecast of opportunity for inactive TO years (Fig. 6b). Additionally, WR-A is ∼ 30 % less likely to occur and WR-E is ∼ 30 % more likely to occur during -NAO years.

During +PNA years, WR-B is ∼ 35 % less likely to occur, whereas WR-E is ∼ 62 % more likely to occur (Fig. 6c). The reduced occurrence of WR-B and increased occurrence of WR-E are consistent with reduced TO activity over CONUS, Midwest, and SGP during +PNA (Fig. 5), and also help explain the high predictive skill of inactive TO years and indicate that +PNA offers a forecast of opportunity for inactive TO years (Fig. 6c). Composite 500H anomalies during +PNA years resembles WR-E, with an anomalous high over the western CONUS and an anomalous low over the eastern CONUS (Fig. S3c).

ENSO is not associated with significant changes in WR frequency, except for WR-E (Fig. 6d). WR-E is ∼ 32 % more likely to occur during El Niño, which is consistent with the reduced tornado activity during El Niño years reported in previous studies (Cook et al., 2017). Additionally, WR-E is ∼ 18 % less likely to occur during the neutral phase.

To summarize, WRs associated with increased springtime tornado activity occur more frequently during the climate mode phases that favor increased tornado activity, while WRs associated with suppressed tornado activity preferentially occur during climate mode phases that are unfavorable for tornado activity. These consistent relationships provide additional confidence in the robustness of the WR-based prediction framework and help to understand the predictability of tornado activity.

In this section, we explore the potential role of SSTs as a source of predictability, in some cases through air-sea coupled climate modes. Composite SST and 500H anomalies during active and inactive TO years are shown in Fig. 7. During inactive TO years, significant positive SST anomalies are found over equatorial eastern Pacific and significant negative SST anomalies over the North Pacific and subtropical western Atlantic. Inactive TO years feature an anomalous 500H high over the west-central CONUS and anomalous 500H lows over the East Pacific and Southeast CONUS, resembling WR-A. A positive phase of the North Atlantic tripole SST pattern is present, consistent with (Lee et al., 2016). In contrast, significant SST anomalies are nearly absent during active TO years except a limited region of cold SST anomalies over the East Pacific just off California. The 500H anomalies are characterized by an anomalous trough over CONUS, resembling WR-B, but the anomalies are much weaker than those during the inactive TO years. The lack of significant SST anomalies and weak 500H anomalies in the active TO years are consistent with the reduced model skill during active TO years and during climate mode phases that are favorable for tornado activity and suggest that inactive TO years are more predictable than active TO years. Statistical significance of results for inactive years should be interpreted with caution given the limited sample size available (N = 7).

To quantify the link between the SST anomalies and climate modes, we calculated the pattern correlations between SST composites in Fig. 7 and those associated with various climate modes (Table S2). Stronger correlations tend to be found in inactive TO years. The highest correlation occurs for inactive TOs and +PNA (cc = 0.71), thus demonstrating that +PNA can be used to identify a forecast of opportunity for inactive TO years.

We also examined composite SST and 500H anomalies for active and inactive WR years, defined as the years when the springtime WR count exceeds ±1 standard deviation from the mean (Fig. S6). WR-A shows opposite circulation patterns over the CONUS between its active and inactive years, with the inactive years resembling a -AO/-NAO pattern consistent with reduced WR-A occurrence during those phases. Active WR-B years exhibit a North Atlantic 500H pattern resembling a -AO pattern. In contrast, inactive WR-B years feature anomalously low SSTs over the North Pacific and an anomalous 500H high over Canada, resembling a +PNA pattern. These patterns are consistent with how PNA and AO modulate WR-B frequency (Fig. 6). The SST composite for active WR-B years has the highest pattern correlation with active TO years (cc = 0.45, Table S2). Active WR-C years feature enhanced southerly flow from the Gulf of Mexico and anomalously low SSTs in the Gulf of Mexico. Their offsetting effects help explain the near-climatology tornado activity. WR-D and WR-E show opposite and statistically significant SST anomalies in the subtropical Pacific, suggesting some potential predictability. Overall, the SSTs over the Pacific and Atlantic may help explain the predictability of WRs, though further investigation using longer observational records is needed to better understand the predictability of tornado activity.