Thursday, 6 November 2014
Capitol Ballroom AB (Madison Concourse Hotel)
A storm that produced many tornadoes over a 2-h period across portions of central South Dakota is examined using WSR-88D data, damage surveys, and visual observations. The tornadic storm evolved near the intersection of a warm front and inverted trough axis, along the southern end of a broken line of thunderstorms. The low-level reflectivity structure was ill-defined, in part due to subsequent upstream updraft development merging with the storm cluster, yet the storm generated six primary low-level mesocyclones during its lifespan, each persisting in excess of 30 min. Several significant tornadoes (rated EF2+), including an EF4 near the town of Alpena, SD, occurred with the storm, but only one Tornado Debris Signature (TDS) was briefly observed by surrounding radars. This case illustrates the sensitivity of the TDS, where precipitation entrainment around the tornado, radar range, and a general absence of damage targets in the form of vegetation and engineered structures prohibited a reduction in the ρHV fields during the damaging tornadoes. The research explores some of the new features of the WSR-88D RDA/RPG Build 14, including the increased temporal resolution of velocity data at the lowest elevation angle from the Supplemental Adaptive Intra-Volume Low-Level Scan (SAILS). Challenges with the Storm-Based Auto PRF (also a new feature in Build 14) unexpectedly changing the maximum unambiguous range, masking tornadic circulations, are also discussed.
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