Introducing Synoptic Classifications Within Cold-Flavor Santa Ana Windstorms Using ERA5 and Local Mesonet Data
1. Introduction
Santa Ana wind events vary widely in duration and intensity, directly influencing destructive wildfire behavior across Southern California’s urban-wildland interface. This study analyzes four major cold-flavor Santa Ana events from 2020 to 2025 and proposes a synoptic classification framework to improve the prediction and public communication of windstorm duration and severity.
Using ERA5 reanalysis in conjunction with 10-minute sustained and peak gust observations from Eaton Canyon, we identify three potential synoptic variants based on 500 mb relative vorticity patterns that cause strong mountain wave windstorms in Southern California: single-trough, double-trough, and rotor-trough events. To test the hypothesis that major cold-flavor windstorms fall into one of these three categories, we retrospectively applied the framework to 50 years of historical windstorms, identifying candidates using ERA5 data. Historical windstorms were cross-verified using newspaper archives. Results indicate that 13 of the 17 flagged windstorm events between 1970 and 2019 were confirmed through media archives, with the four unverified cases all occurring prior to 1987.
Findings also reveal that double-trough events, while not always producing the highest gusts, sustain damaging winds for significantly longer durations than other variants and therefore pose the greatest risk for wildfire spread into urban areas, particularly when they occur before meaningful rainfall. In contrast, rotor-trough events are associated with the most powerful windstorms observed to date. Improved communication of the approaching synoptic variant may enhance resource allocation and preparedness among emergency personnel.
2. Methodology
We selected all four of the major Santa Ana windstorms to have struck Altadena, where the historic Eaton Fire occurred, from the past 5 years. These events occurred on:
- February 3-4, 2020
- January 21-22, 2022
- March 14, 2024
- January 7-8, 2025
Each event was identified via eyewitness recount and available weather station data at SCE Eaton Canyon (SE215) and SCE Loma Alta / McNally (SE609), both of which located in the Altadena area. These stations were selected due to their unobstructed exposure, with minimal nearby trees or structures that could otherwise affect wind speed or direction measurements. Wind events were included in this analysis only if peak gusts reached at least 50 MPH at either station. For each qualifying event, the corresponding upper-level trough was examined using ERA5 reanalysis data accessed through the PG&E 2-km WRF Model Visualization Project, maintained by the Wildfire Interdisciplinary Research Center at San José State University. Troughs were identified and tracked using 3-hour 500 mb relative vorticity fields.
Wind speed data from mesonet sensors built and operated by Southern California Edison (SCE) at Eaton Canyon near Altadena were compiled into histograms showing the frequency distributions of 10-minute sustained wind speeds and 10-minute maximum observed gusts at 5 MPH intervals.
Each event began when either wind gusts reached 30 MPH or 10-minute sustained winds reached 15 MPH, and ended when those thresholds were no longer met for at least 12 hours. These thresholds are general considered consequential for wildfire spread according to National Weather Service Red Flag Warning criteria. These specifications allow an event separated into two halves with a lull in-between to still be considered one single windstorm. Once the histograms were created, each event was then sorted by both strength and duration.
Events were classified based on surface observations to look for a correlation. Once it was established how many variants there could be, a full investigation into 50 years of ERA5 reanalysis was conducted to determine whether past wind events between 1970 and 2019 could also be place into one of the three variants. These additional wind events, which will not be examined further due to a lack of weather station data, were then searched in media databases for possible stories published about them.
3. Observations
The February 2020 event lasted a cumulative 17.0 hours during which wind gusts exceeded 30 MPH. Despite its duration, the storm produced only 1.5 hours of damaging gusts greater than 50 MPH. Two peaks in intensity were observed: one near the onset and a more significant one at the tail end. Of the 1.5 hours of damaging winds, 1.3 hours occurred during this final surge. The event featured three notable lulls—after sunrise, shortly after solar noon, and again following sunset. Between lulls, winds were persistent, with long stretches of gusts ranging from 25 to 40 MPH.
The January 2022 event persisted for 8.5 hours—half the duration of the 2020 storm—but generated 2.7 cumulative hours of damaging wind gusts ≥50 MPH. Wind activity was concentrated into three distinct peaks: a weak pre-sunset surge, a stronger episode just before midnight, and a slightly stronger peak before sunrise. Damaging gusts were evenly distributed across the two overnight surges. The peaks were separated by a prolonged lull after sunset and a brief lull just after midnight. Wind intensity fell sharply following each peak without forming plateaus.
The March 2024 windstorm lasted 8.5 hours and produced 3.0 hours of damaging wind gusts ≥50 MPH. The event developed gradually, initiating a few hours before sunrise and intensifying steadily through mid-morning. Damaging gusts began shortly after sunrise and peaked during mid-morning hours. Wind speeds gradually declined by solar noon and diminished further in the hour following. Unlike the 2020 and 2022 events, the wind speed profile for this storm resembled a semi-circular curve—characterized by a slow rise and fall with no sharp peaks or lulls.
The January 2025 windstorm was exceptionally long and intense, lasting 23.3 hours and producing 12.2 cumulative hours of damaging wind gusts ≥50 MPH—nearly double the total of the other three events combined or 5x any other event analyzed. The event began rapidly a few hours before sunrise on January 7, with damaging gusts recorded within the first hour. A plateau in wind intensity followed, extending through the mid-morning hours, with the strongest gusts occurring near sunrise. A sharp collapse in wind speed occurred mid-morning, leading to a lull that lasted until shortly after solar noon.
A second phase began abruptly in the early afternoon, returning to damaging gusts within one hour. A new plateau in intensity then extended through midnight and into the early morning of January 8. During this period, the Eaton Canyon SCE weather station was burned over as the Eaton Fire passed through, causing a slight bump in windspeed. The maximum gust of the event occurred a few hours after burn over. A brief lull followed midnight, succeeded by the most powerful surge of the event in the pre-dawn hours. This surge was the only occurrence in the dataset of tropical-storm-force sustained wind speeds. The event weakened rapidly before sunrise, followed by a final qualifying surge mid-morning, and ended two hours before solar noon on the second day.
4. Results
Analysis of the four recent wind events alongside their synoptic environments revealed that all could be categorized into one of three distinct upper-level patterns: single-trough, double-trough, or rotor-trough events. Each variant exhibited consistent characteristics in terms of vorticity structure, jet placement, windstorm duration, and intensity profile. A subsequent scan of ERA5 data from 1970 to 2019 identified 17 additional historical periods with similar synoptic setups. While surface wind observations were not available for these older events, their upper-level characteristics suggest they are strong candidates for past major cold-flavor Santa Ana windstorms. Of these 17 cases, 13 events produced articles in local media outlets. The four that did not all occurred before 1987.
Rotor Events: Rotor-trough events are characterized by large, cold-core upper-level troughs drifting south-southwest from the Great Basin. As these troughs intensify over the Mojave Desert, they often generate shortwave ejections to their south and west. These ejections result in pulses of cyclonic relative vorticity ranging from 25 × 10⁻⁵ s⁻¹ to 60 × 10⁻⁵ s⁻¹ crossing the Transverse Ranges. This dynamic interaction produces brief but exceptionally intense mountain wave windstorms. Wind speed profiles during these events are typically arc-shaped, lacking sharp peaks or distinct lulls, and often last fewer than 12 hours, with peak gusts ≥50 MPH occurring for under 6 hours.
Examples likely include events on January 20, 1980; January 27, 1987; November 30, 1991; November 14, 1993; October 27, 1996; December 1, 2011; February 17, 2012; and March 14, 2024. The March 2024 event, for which observational data are available, showed a clear arc-like wind profile consistent with a rotor-trough structure. A similar feature was observed in the first phase of the January 7, 2025 event, suggesting a hybrid structure with both rotor and double-trough dynamics. In both cases, observed surface wind gusts patterns coincided with the passage of a shortwave ejection aloft.
Single-Trough Events:
Single-trough events are the most common variant. They typically involve a single, moderately sized trough diving southeastward from the Great Basin into the lower Colorado River Valley. As the trough progresses, the western flank of its jet streak often becomes positioned over Los Angeles County, initiating mountain wave formation across the Transverse Ranges. These events are associated with continuous relative vorticity advection (as opposed to discrete pulses), resulting in prolonged but less intense wind episodes.
Duration often exceeds 12 hours, with damaging wind gusts ≥50 MPH sustained for at least 3 hours. Wind speed profiles tend to show a singular peak followed by gradual decline, although multiple surges may occur depending on smaller-scale embedded features. Historical examples include January 1, 1973; January 27, 1984; February 21, 1985; November 2, 1986; February 19, 1988; December 8, 1988; January 16, 1991; January 6, 1997; January 6, 2003; and January 22, 2022. The 2022 event, supported by observational data, exhibited multiple well-defined surges without extended plateaus, likely associated with localized vorticity maxima moving along the jet.
Double-Trough Events: Double-trough events are the rarest but most hazardous variant due to their prolonged duration and potential for multiple episodes of damaging winds. These setups begin similarly to single-trough events, with a trough diving through the Great Basin and initiating a mountain wave. However, a second shortwave trough, often originating in the Northern Rockies, rapidly follows the first and plunges toward Southern California along the same upper-level jet axis.
This two-stage process leads to distinct windstorm phases separated by a lull, often producing two overnight periods of damaging winds. Total event duration may exceed 24 hours, with damaging gusts ≥50 MPH sustained for 6 – 12 hours. Wind profiles often contain long plateaus and strong surge onsets. Confirmed double-trough cases include December 11, 1984, February 4, 2020, and January 7–8, 2025. Both the 2020 and 2025 events exhibited significantly longer windstorm durations than other variants. Notably, the first phase of the 2025 event likely included rotor dynamics, while the second phase maintained high vorticity and strong jet support for more than 9 hours. The inclusion of rotor motion, typically associated with stronger events, aided 2025 in becoming an unusually powerful and long-lasting variant.
5. Discussion and Conclusion
The classification framework developed in this study demonstrates that cold-flavor Santa Ana windstorms can be meaningfully grouped into three synoptic variants—single-trough, double-trough, and rotor-trough—based on 500 mb vorticity structure and jet streak alignment. These variants differ not only in meteorological setup but also in their surface expression, duration, and potential to exacerbate wildfire spread. Analysis of the four modern events using mesonet data shows clear alignment between observed wind profiles and the upper-level dynamics captured by ERA5 reanalysis.
Among these, double-trough events emerge as the most dangerous. Both the February 2020 and January 2025 windstorms were double-trough events, and both exhibited total durations at least twice that of other variants. These storms sustained sub-peak damaging winds for extended periods, with maximum gusts occurring intermittently rather than during a single burst. This behavior suggests a sustained synoptic support for mountain wave activity, which is consistent with the prolonged vorticity advection and dual shortwave trough interactions observed in ERA5. Critically, the January 2025 event occurred before meaningful rainfall. The rare combination of this vigorous and long-lasting Santa Ana windstorm enabled the Eaton and Palisades wildfires to simultaneously evolve into one of the largest urban conflagrations in modern history.
In contrast, single-trough events, such as the January 2022 storm, were shorter in duration and featured wind profiles with sharp, well-defined peaks followed by rapid declines. These were associated with transient jet streak alignments, where the most favorable dynamics for mountain wave generation passed quickly across Southern California. Despite producing locally intense gusts, the brief duration limits their capacity to drive world-record challenging conflagration expansion on their own.
Rotor-trough events, exemplified by March 2024, showed a gradual rise and fall in intensity, forming an arc-shaped wind profile. These storms appear to be triggered by the passage of a horizontal shortwave ejection, creating a wave-like response in mountain wind speeds. Though shorter in duration, rotor events can produce extreme instantaneous gusts and may represent the upper bound for windstorm intensity in the region. Their timing, waveform signature, and vorticity structure are distinct enough to merit their own category.
This study also underscores a critical gap in how fire weather threats are communicated. Current operational practices often emphasize peak wind gusts, yet our results show that event duration and sustained sub-peak winds may be more closely tied to structural fire risk and wildfire propagation. A wildfire occurring during a 20-hour windstorm with 40–50 MPH gusts may be more damaging than wildfire occurring during a a 3-hour storm peaking at 65 MPH. Incorporating variant-based synoptic classifications into fire weather forecasting and public messaging could enable better decision-making, including earlier evacuations and resource deployment.
Finally, the successful retrospective application of this classification framework to 50 years of historical ERA5 data—supported by independent media verification—suggests that this system has both predictive and diagnostic utility. It offers a foundation for more nuanced forecasting and post-event analysis, particularly in data-sparse regions or historical cases where surface observations are unavailable.
6. Future Work
Future work should apply this classification to additional cold-flavor Santa Ana events and evaluate whether these synoptic signatures can be reliably forecast several days in advance using ensemble models or high-resolution WRF simulations. Future work could also search for additional variants that could be derived from a windstorm not flagged in the ERA5, or one of the given variants in this study that should be separated further.
