My WebLink
|
Help
|
About
|
Sign Out
Home
Browse
Search
FLOOD01529
CWCB
>
Floodplain Documents
>
Backfile
>
1001-2000
>
FLOOD01529
Metadata
Thumbnails
Annotations
Entry Properties
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
There are no annotations on this page.
Document management portal powered by Laserfiche WebLink 9 © 1998-2015
Laserfiche.
All rights reserved.
/
48
PDF
Print
Pages to print
Enter page numbers and/or page ranges separated by commas. For example, 1,3,5-12.
After downloading, print the document using a PDF reader (e.g. Adobe Reader).
Show annotations
View images
View plain text
<br /> <br />The intensity (reflectivity) of weather radar echoes is used operationally <br />to estimate rainfall rates. The accuracy of this technique decreases with <br />increasing distance of the precipitating cloud from the radar. This is due <br />largely to earth curvature causing the radar beam to intersect increasingly <br />higher levels in precipitating clouds as ranges increase. For this reason, <br />no quantitative estimates of rainfall rates are made for ranges beyond 125 <br />miles (230 km) from WSR-57 radars. Thus, the distance of the Big Thompson <br />Canyon from Limon (100-115 miles) approaches the effective limits for <br />estimating rainfall from WSR-57 radars, particularly in the absence of <br />"ground truth" rainfall observation to calibrate or verify the radar <br />estimates. It should also be noted that other factors, such as the structure <br />of the cloud (liquid water versus water-covered hail) affect reflectivity <br />and the accuracy of rainfall estimates. In practice, the following table, <br />which was available to both the radar operator and the lead forecaster, <br />is used to convert reflectivity levels to estimated rainfall rates: <br /> <br />Levell - less than 0.1 inch (2.5 mm) per hour <br />Level 2 - 0.1 to 0.5 inch (2.5 to 12.7 mm) per hour <br />Level 3 - 0.5 to 1 inch (12.7 to 25.4 mm) per hour <br />Level 4 - 1 to 2 inches (25.4 to 50.8 mm) per hour <br />Level 5 - 2 to 5 inches (50.8 to 127 mm) per hour <br />Level 6 - greater than 5 inches (127 mm) per hour <br /> <br />Both the lead forecaster and the radar operator have stated that they were <br />puzzled by an apparent discrepancy between the very high radar echo tops <br />(62,000 feet) and relatively low reflectivity (level 3) of the storm <br />centered 10 miles southwest of Fort Collins about 7:30 p.m. Storms with <br />tops this high normally have reflectivity values greater than level 3. <br />The radar operator stated that he concluded from the high tops and vertical <br />structure of the storm cells that the area was receiving large amounts of <br />rain and so notified the forecaster about 7:15 p.m., when he also suggested <br />a severe thunderstorm warning with a mention of flooding. This was the basis <br />for the severe thunderstorm warning issued by WSFO Denver at 7:35 p.m. <br /> <br />The radar operator was maintaining a log of Manually Digitized Radar (MDR) <br />values indicative of cumulative rainfall shown by the radar. The MDR total <br />for the four hourly observations from 6:30 to 9:30 p.m. was 18, slightly <br />below the NWS standard flash flood alerting threshold of 20. <br /> <br />Experience gathered at the National Severe Storms Laboratory and from the <br />Digitized Radar Experiment (D/Radex) indicates that these standard values <br />are not always valid. A recent study* of cool season precipitation <br />(generally stratiform clouds) and MDR values in the southern Appalachians <br />shows widely differing radar/rain gage relationships for mountainous and <br />flat terrain. <br /> <br />*NOAA Technical Memorandum SR-84 "A Comparison of Manually Digitized Radar <br />Data and Observed Cool Season Precipitation over the Southern Appalachians." <br />January 1976. <br /> <br />17 <br />
The URL can be used to link to this page
Your browser does not support the video tag.