FLOOD00419

• County: Jefferson
• County: Park
• Community: Buffalo Creek
• Title: The Buffalo Creek Flash Flood of July 12, 1996 Draft - A Reconstruction of Rainfall and Meteorology
• Date: 7/12/1996
• Prepared For: CWCB
• Prepared By: Henz Meteorological Services
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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HMS radar records indicated that four storm periods occurred across the Buffalo
Creek watershed on July 12, 1996:

1. Storm Period 1: 210PM until 228PM, a light shower across the lower third
of the basin.

2, Storm Period 2: 612PM until 710PM, rain showers and two moderate
thunderstorms across the western third of the basin,

3. Storm Period 3: 739PM until 913PM, a line of severe thunderstorms cross
and go stationary over the watershed producing the flash flood.

4, Storm Period 4: 918PM unti11023PM, post-flash flood thundershowers and
showers cross the lower half of the'watershed.

The first storm period produced only minor rainfall accumulations of 0.18"
to 0,24" over the lower third of the basin from light rain showers, No radar
indication of thunderstorm activity over the basin was noted. The radar echoes
of these showers appeared to form over the burn area of the May 1996 fire,
These showers failed to develop further because of a strong inversion capping
their development above 25,000 feet. Further east on the plains of Arapahoe
and Douglas Counties, warmer temperatures and the converging winds of a
Denver Cyclone spawned a brief tornado southeast of Centennial Airport during
this time period,

The second storm period was more vigorous and preceded the line of
storms which produced the flash flooding by about one hour. These storms
formed over the higher terrain of Buffalo and Windy Peaks in the far western
portion of the watershed. Rain accumulation from these storms reached about
0,15" to 0,65" in the western third of the basin. The storms moved quickly from
west to east at almost 25 mph and cleared the basin shortly after 700PM. No
hail was reported with these storms but is quite possible that small hail of up to
0,75 inches in diameter could have occurred with these storms.

The low rainfall totals produced by these early storms did not reflect the
true potential of the atmosphere. Table 2 shows a comparison of the peak 30-
minute rainfall rates, the depth of the warm layer of the thunderstorm updrafts
and the Precipitable Water Index (PWI) from 530PM until 91 OPM. This temporal
variation in rainfall rates and PWI is quite common and underscores the
problems in using standard radar reflectivity-rainfall relationships, Note the
dramatic increase in the PWI by 805PM and the nearly doubling of the peak 30-
minute rainfall rates from the earlier second period of storms which concluded
shortly after 700PM,

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