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Last modified
11/23/2009 12:58:16 PM
Creation date
10/4/2006 10:05:18 PM
Metadata
Fields
Template:
Floodplain Documents
County
Larimer
Community
Big Thompson Canyon
Stream Name
Big Thompson
Title
Natural Disaster Survey Report 76-1: Big Thompson Canyon Flash Flood of July 31 - August 1, 1976
Date
10/1/1976
Prepared For
Department of Commerce
Prepared By
NOAA
Floodplain - Doc Type
Flood Documentation Report
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<br /> <br />was operating through the afternoon and night of July 31-August 1, although <br />at 7:30 p.m., the radar operator became concerned that the Video Integrator <br />Processor (VIP) might be reading low and had begun obtaining storm intensi- <br />ties in the LIN mode. * The VIP, which automatically provides contouring <br />of various levels of echo intensity, relies on relatively complex circuitry. <br />The LIN mode of operation relies on simpler circuits to obtain these <br />intensities. The radar operator informed the lead forecaster at WSFO Denver <br />of this action between 9:30 and 10:30 p.m. Based on a review of photographs <br />of the PPI scope made during the time in question, the team concluded that <br />the WSR-57, including the VIP, was operating satisfactorily on the evening <br />of July 31, 1976. <br /> <br />Reproduction of the WSR-57 radar's PPI scope presentation, plus operator <br />annotations, are provided to WSFO Denver by means of a facsimile system known <br />as WBRR (Weather Bureau Radar Remote). The WBRR system was not operating on <br />July 31, 1976. This equipment was logged out of operation at 3:30 p.m. on <br />Friday, July 30, and was restored to service at 1:00 p.m. on Sunday, August 1. <br /> <br />Without the WBRR picture, the lead forecaster at WSFO Denver was dependent <br />for his radar information upon a verbal description of radar echoes as given <br />by the radar operator at Limon over the direct telephone circuit connecting <br />the two offices. This circuit was used frequently during the evening of <br />the flood, and the lead forecaster plotted what he considered significant <br />information on blank radar overlays at approximately 7:30, 8:20, and 11:00 <br />p.m. Comparing these sketches with copies of the applicable portion of the <br />PPI scope tracings (figure 8) made by the radar operator at about the same <br />times supports the team's conclusion that the outage of the WBRR detracted <br />from the capability of the lead forecaster at WSFO Denver. This outage had <br />little effect on the issue time or content of the initial warning released <br />at 7:35 p.m., but the forecaster was subsequently unable to fully grasp the <br />situation as the radar was viewing it. The lead forecaster initially stated <br />to the team that he didn't believe that the WBRR outage had any significant <br />impact on his warning capabilities. After looking at the radar operator's <br />sketches, he revised his views on this point. He became aware of important <br />information on radar echo location and intensity he had not assimilated by <br />telephone. Figure 9 shows the PPI scope photographs and the radar operator's <br />tracings; persistence of the precipitation echoes over the Big Thompson <br />Canyon is evident in both. <br /> <br />*A check of the radar after the flood indicated that the VIP was operating <br />within acceptable tolerance. The reason the operator believed the VIP <br />might be reading low was the apparent discrepancy between the extremely <br />high radar echo tops (62,000 feet or about 18.9 km) and the relatively <br />low reflectivity (level 3) for the storm centered 10 miles southwest of <br />Fort Collins at 7:30 p.m. <br /> <br />14 <br />
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