Investigation of a Negative Lead Time Tornado using Close-Range, X-Band Radar Observations

Using close-range X-band radar observations, we examine the case of a supercell tornado with a negative lead time. In this context, “negative lead time” means that a tornado was not warned by the local National Weather Service office until the tornado was ongoing. Multiple factors can impact if a timely warning is issued for a tornado, including tornado intensity on the Enhanced Fujita scale, distance from the nearest NWS radar, and sequencing of the tornadoes from the same storm (e.g., first versus second).

This tornado occurred on the Tippecanoe-Carroll County line in Indiana on 26 June, 2021, just after 0000 UTC. We compared data from the X-band Teaching and Research Radar (XTRRA), located near Purdue’s campus in West Lafayette, Indiana, to that of the NWS Indianapolis WSR-88D (KIND) which is used for warning issuance across central Indiana. The Warning Decision Support System Integrated Information (WDSS-II) system from the National Severe Storms Lab (NSSL) was used to derive azimuthal shear fields from both radars. In the supercell’s hook echo, KIND detected lower peak azimuthal shear (0.019 s^-1) than XTRRA (0.028 s^-1) did. This difference is due in large part to the relative beam heights of the two radars in the storm. The 0.5° beam of KIND, which was 88 km away, was sensing the storm approximately 1.3 km above ground level. In contrast, XTRRA’s 0.5° beam height at the storm range (16 km) was 170 m AGL. Therefore, KIND largely missed the near-surface rotation, where XTRRA, located much closer to the storm, picked up the azimuthal shear gradient very well. This case of a negative lead time tornado warning demonstrates the usefulness of supplemental radar coverage to help provide additional, high quality data that could potentially enhance the warning decision process.