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May 11th, 2003

photograph showing rotating low clouds associated with thunderstorm. Strong to severe storms moved across parts of Central New York and Northeast Pennsylvania on May 11th, 2003. The photograph to the right shows rotating low clouds associated with a thunderstorm moving across parts of southwest Luzerne County Pennsylvania. Time of the photo was between 700 to 710 PM EDT. The photograph is courtesy of Jeremy Ervin. Click on the image for a larger view.

Below are radar images around the same time as the photograph. The radar site is the WSR-88D radar at the Binghamton airport. If you have a javascript enabled browser, clicking on the image with your mouse will toggle the image between reflectivity and storm relative velocity. If the toggle does not work in your browser or you want to see the images individually, just click on the links provided in the desciption of the radar images.

Here are some definitions,

Base Reflectivity
  • Used extensively to identify the location, intensity, pattern, and movement of precipitation, including light snow flurries to heavy rain and hail producing thunderstorms. Reflectivity data provide a wealth of crucial information needed to issue accurate severe weather warnings. High reflectivity values denote that heavy rain and possibly hail are occurring within thunderstorms. The radar generates reflectivity data at various altitudes, which permits an evaluation of thunderstorms in the lower, middle, and upper portions of the storms in order to determine their vertical structure. In winter storms, reflectivity data clearly can slow banded precipitation, including snow that can lead to narrow zones of heavier snowfall amounts.
Storm Relative Velocity
  • Measures winds relative to a moving precipitation entity (e.g., a thunderstorm). In other words, it is the wind that a thunderstorm actually "feels" as it moves through the environment. It is calculated by subtracting out the motion of the storm from the actual wind. Storm-relative velocity is critical for detecting shear zones and circulations/mesocyclones within severe thunderstorms that could result in damaging surface winds, hail, and even tornadoes. Converging and diverging wind fields can be detected using storm-relative data as well, which also have important implications on the structure and evolution of thunderstorms and their potential severity. To better conceptualize storm-relative velocity, consider this example: if a person rode a bike outside (i.e., was moving), the wind velocity that person felt while moving would be "storm-relative" or "system-relative" (combines the actual wind with the movement of the person), i.e., different from the actual or "ground-relative" wind.

Additional information can be found here WSR-88D Overview page and from the WSR-88D Radar FAQ's.

The image below occurred at 23:07 UTC or 7:07 PM EDT and shows reflectivity (and if toggled the Storm Relative Velocity Map) at 0.5 degrees elevation. A thunderstorm was moving across the southwest part of Luzerne County, just east of Mifflinville, along highway 80. The storm relative velocity image shows that there was weak rotation, shown within the faint yellow circle, associated with the storm. To view each of the images individually click on one of the following, reflectivity image at 23:07 UTC or 7:07 PM EDT and storm relative velocity map at 23:07 UTC or 7:07 PM EDT.

kbgm radar reflectivity image at 23:07 UTC or 7:07 PM EDT.


The image below was at 23:12 UTC or 7:12 PM EDT and shows reflectivity (and if toggled the Storm Relative Velocity Map) at 0.5 degrees elevation. These images are 5 minutes after the previous image and show that the storm had weakened and moved a little farther to the northeast. The storm relative velocity image continues to show weak rotation, shown within the faint yellow circle, associated with this storm. To view each of the images individually click on one of the following, reflectivity image at 23:12 UTC or 7:12 PM EDT and storm relative velocity map at 23:12 UTC or 7:12 PM EDT.

kbgm radar reflectivity image at 23:12 UTC or 7:12 PM EDT.


The National Weather Service at Binghamton, New York has short term warning and forecast responsibility for 17 counties in New York and 7 counties in Pennsylvania.

Questions or comments about this page? Send to Ron Murphy

Posted May 21, 2003