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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I 3.0 Poothills Thunderstor. Por.ation

The non-random initiation of cumulus clouds and thunderstorm
syste.s over elevated portions of the Colorado Pront Range has
been reasonably well-documented over the past 15 years. Initial
studies by Henz,1974 of thunderstorm for.ation on severe weather
days identified the existence of preferred thunderstorm formation
zones Dr "hot spots" related to topographic features. Pigure 1
shows the location of the hot spots while Table 1 lists their
physical characteristics. Karr and Wooten, 1976 perfor.ed a radar
cli.atology of eastern Colorado which confir.ed the hot SpDtS of
Henz and further elucidated on the diurnsl and areal tendencies
of radar echo formation over eastern Colorado and the Pront
Range.

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Recent studies by Weaver and Kelly, 1982, Banta,1984 and
Barker and Banta, 1985 used satellite photos to track the
formation of cumulus clouds over the Pront Range and their
develop.ent into thunderator. systems as they .igrated eastward.
In each case preferred cumulus cloud and thunders tor. generation
zones were found. Banta, 1984 discusses the causes of these
preferred zones and Barker and Banta, 1985 indicate repeated
develDp.ent cycles related to the wind directiDn above aountain
top level. In each case the satellite derived thunderstorm
genesis site ot hot SpDt was lDcated at elevations above 7500
feet.

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Henz(1974) identified the physical characteristics of the hOlt
SPDts, Portions of the Colorado Pront Range foothills with
elevations from 5100 feet to 12,431 feet are located in
thunderstDrm hot spots. The mean elevation of these hot SPDtS
falls around 7500 feet. Jarrett, 1986 incorrectly indicates that
these hot SpDtS are lDcated at or below 7500 feet. He also
indicates that the storms generally mDve frDm west to east to
imply the lack Df strong storm for.ation at elevations above 7500
feet. In fact the combined satellite and radar studies of
thunderstorm fDrmation support the initial formation of many
strDng storms at elevations abDve 7500 feet.

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The assumed storm move.ent frDm west tD east does not generally
apply tD storms producing heavy rainfall along a topographic
barrier. Both the Rapid City Plash Plood of June, 1972 and the
Big ThDmpson Plash Plood of 1976 (Maddox, et aI, 1978) were
caused by storms that .oved either parallel to or remained
stationary over the topographic bsrrier during the flood-
producing rainfall period. Indeed .ountain top level and above
winds tend to be from the south-southeast to sDuth-sDuthwest
during periods when heavy thunderstorm rainfall is produced in
the Colorado Pront Range foothills(Henz,1979).

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