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Jonathan Kegges
482 Consulting Meteorologist
The Pennsylvania State University
Introduction:
I have been requested to obtain and review data dealing with rainfall amounts over the Conocoheague Creek Basin located in and around Franklin County, Pennsylvania for Monday, May 12, 2008. Specifically, I was asked to report an opinion if the data obtained from the day of the event supported the claim of creeks and/or streams running over their banks.
Data Used for this Analysis:
Doppler radar data (reflectivity) from the State College NEXRAD (located in State College, PA) for Monday, May 12, 2008 acquired from the National Climate Data Center. (From 8:23 AM to 1:00 PM)
Attached are the color prints of the reflectivity from 8:00 AM, 1:00 PM and a zoomed in image after the precipitation had ended. (Figures 1, 2, and 3 respectively. The red lines outline Franklin County and the black line outlines Conocoheague Creek. A scale indicating rainfall intensity precedes these images.)
Methodology:
Doppler Radar Analysis:
The Doppler radar has the capability to scan the skies about every five minutes with a beam to detect precipitation. Since the Earth is a sphere and the beam angle is .5, the horizontal range of a WSR-88 D radar is about 125 miles in every direction from the radar site. Franklin County, Pennsylvania is approximately 80 miles south of State College, Pennsylvania. The Doppler Radar has the ability to determine an estimated amount of precipitation that has fallen over a given area based on reflectivity intensity.
Limitations:
Mountains and other obstructions can get in the way of the radar beam causing an area of precipitation to look more intense than it actually is.
Hail Stones in a thunderstorm will also increase the reflectivity intensity. An area on a map will appear brighter, indicating heavier precipitation has fallen over an area. Since hail is bigger and denser, the beam of the radar will be reflected back stronger, falsely indicating higher amounts of precipitation when the beam was actually hitting hail.
Given these limitation, the rainfall amounts predicted by the radar are still reasonable to use due to the lack of mountains in that part of the state. Conditions on this day did not favor thunderstorm development; therefore no hail stones were present and all data can be presumed as measurement of actual rainfall amounts.
SUMMARY of EVENTS:
Forecasted heavy rain prompted a Flood Watch for all of Central Pennsylvania including Franklin County issued by the National Weather Service in State College, Pennsylvania. The flood watch for Franklin County was quickly upgraded to a Flood Warning once the rain began to fall into the streams just north of the county. The rain began to fall in Northeastern Franklin County around 4:00AM EDT. By 8:00AM, 1 inch of rain had already fallen in the upper northeast part of the county. At this same time rain had been falling for a few hours throughout all of Franklin County. At this time Doppler Radar had estimated over .5 inches of rain had fallen over the county. The rain had stopped falling over Central Pennsylvania at approximately 1:00PM. When this rain event was all said and done, approximately 1.5 inches of rain had fallen in the extreme northeastern part of the county. The top half of Franklin County had received approximately 1 inch of rain and the southern half received approximately .6 inches of rain from this event. The heaviest rain from this rain event was centered just northeast of Franklin County into parts of Perry County and Cumberland County.
Conclusions:
Based on the meteorological data available, an average of about 1 inch of rain fell over the Conocoheague Creek Basin in approximately 9 hours. The Conocoheague Creek runs through Franklin County and through a part of Cumberland County (Lies due east of Franklin County). The mouth of the creek lies in Cumberland County, where approximately 1.5 inches fell over this time span. Smaller bodies of water such as creeks or streams tend to flood faster than a river since their volume of water is much lower. With the amount rain that fell and the short time in which it fell it is highly likely that streams and creeks over all of Franklin County ran over their banks on the morning and afternoon of May 12, 2008.
The above conclusions represent my professional evaluation of estimated rainfall amounts over the Conocoheague Creek Basin in Franklin County, Pennsylvania on Monday, May 12, 2008.
Jonathan Kegges
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