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1997 Toccoa Flood

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The 1977 Toccoa Flood

USGS Hydrologic Atlas 613
Meteorological conditions

Meteorological conditions. - The NWS (National Weather Service) described meteorological conditions in the report by the Federal Investigative Board(1977) as follows: "Before rain began the ground was already wet from heavy rainfall of 3 1/2 to 4 1/2 inches, which fell on October 25 - 26. The rainfall began on Wednesday morning, November 2, and ended by midnight on Saturday, November 5. A strong high pressure area, centered over New England, was bringing Atlantic moisture into Georgia on Wednesday and this produced the rainfall at the beginning of the storm period. Meanwhile, a more important development was taking place in Texas where a strong low pressure area was developing at upper levels in the atmosphere. This intense upper level low was located near Shreveport, Louisiana, on Thursday morning, November 3; New Orleans, Louisiana, on Friday morning, November 4, Mobile, Alabama, on Saturday morning, November 5; Centreville, Alabama, on Sunday, November 6. As this slow-moving upper level storm moved closer to Georgia, more and more moisture was brought in on southerly winds from the Gulf of Mexico. Lifting of the air by the higher terrain of north Georgia accentuated the lifting processes provided by the atmospheric storm patterns. A plentiful supply of moisture, lifting of the air, and slow-moving storm system are the ingredients of a long, heavy rain event.

Figure 8. -- Mass curves of rainfall at Burton Dam, Ga., and Long Creek, S.C., and estimated rainfall in upper basin of Toccoa Creek, November 2-6, 1977. (National Oceanic and Atmospheric Administration, 1977)

"Rainfall reports are received by the National Weather Service from radio stations WLET and WNEG in Toccoa. Since these stations are located some distance from the drainage area above Kelly Barnes Dam, a rainfall survey was made. Three additional storm total values were obtained . . .. [The location and storm totals of hourly and daily rainfall stations from the Federal Investigative Board (1977) and National Oceanic and Atmospheric Administration (1977) for the period November 2 - 6 are shown in fig. 1.] These measurements, interviews with local residents, radar echoes, and hourly values from nearby recording stations [at Burton Dam, Ga., and Long Creek, S.C.] were used in estimating the rainfall distribution for the drainage basin above Kelly Barnes Dam. [See fig. 8.] Although radar echoes indicated that the heaviest rainfall probably occurred between 6:30 pm. and 7:30 pm. on November 5, the estimated maximum 1-hour value is shown between 6:00 pm. and 7:00 pm. since points are plotted on the hour.

"The rainfall was light for the first two days with slightly over 1 inch falling in the Toccoa area by Friday morning, November 4. Rainfall intensity increased somewhat for the next 24 hours, with a storm total of approximately 2 1/2 inches by 8:00 am., Saturday morning, November 5. The storm total increased to approximately 3 1/2 inches by noon on Saturday and then the rainfall was apparently light until about 6:00 p.m., when showers moved into the area. From 6:30 pm. to 7:30 p.m., radar echoes [from Athens, Gal] indicated periods of torrential rainfall with frequent intense lightning and thunder. A small tornado apparently touched down about 5 miles southeast of Kelly Barnes Dam at approximately 8:00 pm. The damage to trees was limited to only 100 yards in width and about one-fourth mile in length, but the tornado was indicative of the severity of the weather. After 8:00 p.m. the showers were less frequent and had practically ceased by midnight. Estimated basin rainfall for the entire period is about 7 inches, with almost 3 1/2 inches occurring between 6:00 pm. and midnight on November 5."

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