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Scan-by-Scan Storm-Motion Deviations for Concurrent Tornadic and Nontornadic Supercells

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  • 1 aNOAA/National Weather Service, Rapid City, South Dakota
  • | 2 bUniversity of Nebraska–Lincoln, Lincoln, Nebraska
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Abstract

In this exploratory study, storm-motion deviations are examined for concurrent tornadic and nontornadic supercells using 171 cases. This deviation, or “delta,” is defined as the shear-orthogonal distance between the observed supercell motion and a baseline supercell-motion prediction. Larger deltas—representing supercells moving farther right (in a shear-relative sense) compared to the baseline prediction—are hypothesized as more likely to be associated with tornadoes than nearby supercells with smaller deltas, consistent with recent research. Automated radar tracking is used to calculate supercell motion every scan, which then is compared to a model-derived hourly supercell-motion prediction to calculate the deltas. Tornadic supercells have larger average deltas (by 1.9–2.0 m s−1) than nearby nontornadic supercells when using 20- and 30-min storm-motion calculations, and the deltas are larger for the tornadic versus nontornadic supercells ∼80% of the time. Average delta trends also are positive 62%–70% of the time prior to tornadogenesis. The supercell-motion deltas show a modest positive correlation with EF-scale damage rating, indicating a possible relationship between tornado rating and storm deviation. The relative delta differences between tornadic and nontornadic supercells appear more meaningful than the absolute delta magnitudes (i.e., about 70% of tornadic cases with negative average deltas had deltas that were less negative compared to concurrent nontornadic supercells). This concept shows promise as a potential tool to assist operational forecasters in tornado warning decisions.

Significance Statement

Supercells are rotating thunderstorms, and these storms produce the most destructive tornadoes. However, it has been challenging to forecast which supercells will produce tornadoes. In this exploratory study to help better forecast supercell tornadoes, we looked at how the observed supercell motion compared to the predicted motion, based on a commonly used method. We found tornadic supercells tend to move somewhat differently from the predicted motion—compared to nearby nontornadic supercells. This unusual movement often starts prior to tornadogenesis, potentially providing lead time to tornado formation. Pending further validation, development, and testing of real-time analysis tools, this storm-motion behavior could be used by operational forecasters as a factor to help determine when (or when not) to issue a tornado warning for a supercell thunderstorm, thus providing better information to the public.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Matthew J. Bunkers, matthew.bunkers@noaa.gov

Abstract

In this exploratory study, storm-motion deviations are examined for concurrent tornadic and nontornadic supercells using 171 cases. This deviation, or “delta,” is defined as the shear-orthogonal distance between the observed supercell motion and a baseline supercell-motion prediction. Larger deltas—representing supercells moving farther right (in a shear-relative sense) compared to the baseline prediction—are hypothesized as more likely to be associated with tornadoes than nearby supercells with smaller deltas, consistent with recent research. Automated radar tracking is used to calculate supercell motion every scan, which then is compared to a model-derived hourly supercell-motion prediction to calculate the deltas. Tornadic supercells have larger average deltas (by 1.9–2.0 m s−1) than nearby nontornadic supercells when using 20- and 30-min storm-motion calculations, and the deltas are larger for the tornadic versus nontornadic supercells ∼80% of the time. Average delta trends also are positive 62%–70% of the time prior to tornadogenesis. The supercell-motion deltas show a modest positive correlation with EF-scale damage rating, indicating a possible relationship between tornado rating and storm deviation. The relative delta differences between tornadic and nontornadic supercells appear more meaningful than the absolute delta magnitudes (i.e., about 70% of tornadic cases with negative average deltas had deltas that were less negative compared to concurrent nontornadic supercells). This concept shows promise as a potential tool to assist operational forecasters in tornado warning decisions.

Significance Statement

Supercells are rotating thunderstorms, and these storms produce the most destructive tornadoes. However, it has been challenging to forecast which supercells will produce tornadoes. In this exploratory study to help better forecast supercell tornadoes, we looked at how the observed supercell motion compared to the predicted motion, based on a commonly used method. We found tornadic supercells tend to move somewhat differently from the predicted motion—compared to nearby nontornadic supercells. This unusual movement often starts prior to tornadogenesis, potentially providing lead time to tornado formation. Pending further validation, development, and testing of real-time analysis tools, this storm-motion behavior could be used by operational forecasters as a factor to help determine when (or when not) to issue a tornado warning for a supercell thunderstorm, thus providing better information to the public.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Matthew J. Bunkers, matthew.bunkers@noaa.gov
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