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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Caracena,et aI, 1977 indicated the inherent differences in the
vertical structure of the at.osphere on severe weather and
flash flooding days using the Big Thompson stDrm as an example.
Maddox and Chappell, 1978 further developed these differences by
studying twenty significant flash flooding eyents. The flash
flooding atmDspheric structure typically favored:

a. A deep layer of moist air in the troposphere
b. Weak to mDderate vertical shear of the horizontal wind
through the cloud depth
c. Convective storms or cells repeatedly formed Dr mDved over
the same area or basin
d. A weak mid-level disturbance helped to trigger and focus
the storms
e. MDst storms were aided by strong sub-cloud layer fDrcing

Henz, et aI, 1985 cD.pared the average ColoradD thunderstorm heavy
rainfall environment to that reported by MaddDx, et aI, 1979 for
average Western states flash flooding events. The co.parison in
Table 2 shows marked similarities in atmospheric structure. While
a mean cloud layer wind of 240 degrees or southwest was reported
fDr all flash flDDding days the same study noted that on fDDthills
flash flooding days the direction tended to be 190 degrees Dr
almost out of the south or parallel to the Pront Range mountain
barrier. The light wind speeds invDlved and the directiDn of the
steering wind of the storms wDuld tend to move stDrms either
parallel to Dr stationary over the tDpDgraphic barrier.

These studies indicate that contrary to Jarrett, 1986 heavy
thunderstDrms capable of producing flooding rainfalls cDuld form
at elevations above 7500 feet and would likely move either
parallel tD or remain stationary over their fDrmatiDn zone and
adjacent topographic barriers. The next sectiDn identifies the
characteristics of heavy thunderstorm rainfalls which occurred
at or above 7500 feet along the Colorado PrDnt Range from 1979 to
1988 as repDrted in Storm Data.

Heavy Thunderstorm Rainfall Characteristics

A survey was cDmpleted of the NOAA publication, Storm Data, for
all thunderstorm rainfall repDrts which Dccurred between April
and September for the years 1979 to 1988. A lieting is presented
in Table 3 which shows all days and reports where either 2 inches
per hDur rainfall intensity was equalled or exceeded or more than
2 inches total rainfall was reported due to thunderstorm activity
at elevatiDns at or above 7500 feet in the Colorado PrDnt Range.