NSSL SWAT Case Study - 30 April 1994 Pennsylvania

Northeast U.S. Low Topped Supercell

The afternoon of April 30, 1994, provided one of many scientifically important tornado events to have been witnessed by the WSR-88D radars since their nationwide deployment.  The particular supercell of interest impacted the town of Pond Bank, PA.  This was the first Northeast U.S. low topped supercell ever recognized in WSR-88D data and archived for analysis.  Such events are critical to advancing our understanding of various tornadic storms.  As meteorologists, we do not have the luxury of creating a particular weather event in a laboratory.  The atmosphere serves as our laboratory, and we must continuously monitor the atmosphere to make use of the times when events create themselves.  Low topped supercells are rare, so the Pond Bank storm is one of many prized possessions in the NSSL radar database.

Presented here are various WSR-88D radar images from the Sterling, VA (KLWX) radar. Included in some of the images is output from NSSL's Mesocyclone Detection Algorithm (MDA) and Tornado Detection Algorithm (TDA).  A yellow circle represents a mesocyclone as detected by the MDA.  The red-in-yellow circle represents a mesocyclone whose base is at the lowest radar scan (where it is a more likely tornado threat).  A red triangle indicates the location of a tornado as detected by the TDA.

Description:   In this case, the tornadic storms of interest are not obvious on Sweep 1 (or base) reflectivity.  A higher angle radar beam, Sweep 5, however, overshoots the echos of lesser concern, and reveals two supercells with taller, denser cores and extensive anvils.  See Sweep1 vs. Sweep5.  The Storm Evolution Loop, then, focuses on these two cells as the southern one becomes tornadic and approaches Pond Bank (north of Hagerstown, MD). Description:   Compared to the Great Plains supercell, the Pond Bank storm is very sheered over.  It also has a much lower top (by 5 km) and a somewhat less extensive core.  See the Cross Section.
The two storms do not look radically different at Base Reflectivity, but the Pond Bank storm does have slightly smaller features.  The real differences show up around 11 km which is Sweep 6 for Pond Bank but Sweep 5 for the Great Plains storm due to their respective distances from the radars.  The Upper Region images reveal in the Great Plains storm high reflectivities, a large core, and far-reaching anvil material.  At this same height in the Pond Bank storm, the radar beam lands slightly above the anvil, near the top of the overshoot, and reflectivities are less expansive or intense.  The Four Panel image further examines the difference in reflectivities at various heights.
  Description:   A significant mesocyclone developed and produced an F1 tornado in the township of Antrim, PA, at 0030 UTC.  The storm continued to strengthen as it approached Pond Bank, and at 0059 UTC a second tornado formed.  This tornado tracked roughly six miles, doing significant damage to property before dissipating in Pond Bank.  Unfortunately, there were also two serious injuries. Valuable data was obtained from this Northeast U.S. Low Topped Supercell.


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