Thunderstorm types associated with the “Broken-S” Radar Signature

The broken-S radar signature, often marked by a weakness in reflectivity data along a line of thunderstorms, has recently received attention as a signature associated with weak (F0-F1) tornadoes. The significance of these radar signatures and the associated large scale weather patterns are examined in this study using archived data from the State College (KCCX) WSR-88D, the National Weather Service (NWS) Storm Prediction Center (SPC) severe weather database, and the NOAA’s National Centers for Environmental Predictions (NCEP) reanalysis data.

Using the date, time, and latitude and longitude of weak tornadoes from the SPC data set, the location of tornadoes was plotted on radar reflectivity imagery using the NWS Weather Event Simulator (WES). A preliminary study of six severe weather events associated with weak tornadoes and multi-cellular lines revealed that several weak tornadoes were collocated with the weak echo region of the broken-S radar signature. The severe weather event of 21 July 2003 is presented to illustrate the appearance of the broken-S radar signature in association with supercell thunderstorms.

The multi-cellular line event of 8 November 1996 contained 5 broken-S signatures associated with weak tornadoes. Several other multi-cellular events revealed a clear association with the broken-S radar signature and weak tornadoes. These events will show that the apparent broken-S signatures in these multi-cellular events were associated strong inflow, such as a rear-inflow jet, into the line of thunderstorms.

The severe weather event of 21 July 2003 contained at least 6 broken-S signatures over central Pennsylvania. Each of these signatures was associated with severe weather at some point and at least one was associated with an F3 tornado. Preliminary results suggest that two of these broken-S signatures were associated with persistent and rotating thunderstorms with well developed rear-flank downdrafts (RFD). The velocity surge which produced the broken-S signature in the reflectivity was clearly associated with the RFD in at least two instances. The remaining four broken-S signatures were associated with more linear features, similar to those observed in the 8 November 1996 case, with a strong rear-inflow jet into the line of thunderstorms.

The results suggest that the broken-S may be a useful reflectivity signature to focus the radar operator’s attention to a storm worthy of further investigation. However, the storm relative velocity (SRM) data suggest that two different storm types can produce the so called broken-S signature. This talk will focus on the storms types which may produce a broken-S in the reflectivity data and the importance of SRM data in identifying the storm type. It appears that a surge of high strong winds, produced by either a rear-inflow jet or an RFD may result in the apparent the broken-S signature in the reflectivity data.