FLOOD05189

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Survey of Thunderstorm Rainfall Characteristics - 1979 to 1988
• Date: 9/1/1989
• Prepared For: CWCB
• Prepared By: Henz Kelly Associates
• Floodplain - Doc Type: Educational/Technical/Reference Information

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indicated in Storm Data and noting the highest and lowest
elevation within 2 miles Df the report, If s specific site was
noted then the site elevation was used. By listing the highest
and lowest elevation near the report, potential biases inherent
in locating the rain center without the use Df radar informatiDn
were partially addressed. In some cases use was made of the UDPCD
flash flood prediction program lDg to assign the most reasDnable
location with a conservative bias towards lower elevation
locations.

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Pigures 2 and 3 show plots of the total thunderstorm rainfall
amDunt and the lowest and highest elevatiDn near the repDrt, When
the lDwest elevation is considered 10 of 24 or about 40% of the
reports occurred above 8,000 feet. When the highest elevation is
considered 20 of 24 reports or 83% occurred with portion of the
rainfall above 8,000 feet. Clearly rainfall of 2,00 inches or mDre
tDtal or intensity is no stranger to elevatiDns above 7500 feet.

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Another perspective on the rainfall data is apparent when the
amount of the thunderstorm rainfall is plotted against the
duration. It is interesting to note in Pigure 4 most reports
had durations of 1 hDur or less. High intensity rainfall of 2-3
inches in 30-60 minutes made up almost 50% of the sample. If the
hourly rainfall rate is calculated using the amount and duratiDn,
two-thirds Df the sample hDurly rates are 3.00 inches per hour Dr
more and a third of the sample support a hourly rate or 4.00 inches
per hDur or mDre as seen in Pigure 5, Two reports suggested
extreme rstes of over 10 inches per hour.

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If the DbservatiDns from the 24 storm events in the sample are
used tD create an average storm, the fDllowing characteristics
are dervived:

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a. The average storm rainfall is 2.06 inches falling in a
average time period of 38 minutes.

b. The average hourly intensity is 3.25 inches per hour.

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c. Steering winds in the cloud layer are sDutherly at 20 mph
while sub-cloud layer winds are easterly at 15 mph. StDrm
movement is nDrtherly at 10 mph.

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d. The center of the stDrm will be at 8,200 feet and the
storm's aresl coverage will extend dDwn tD elevations Df
7,500 feet and up tD elevations Df 9,000 feet.

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It is difficult to place undue significance on the results
Df this survey withDut the benefits Df a mDre complete study. A
.Dre cD.plete study should use radar data tD pinpoint storm
location, amount and coverage, weather data to estimate
thunderstDrm rainfall pDtential and movement. It should alsD
include an investigation of the nDrthern and southern portiDns Df

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