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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Unfortunately, its arrival and that of the gust front from the earlier storm in central
Jefferson County coincided, Figure 7 shows the path of the storm complex from
715PM to 900PM. It is very possible that the storm would not have gone
stationary if the gust front had not provided a focus for a strong inflow of moist,
unstable air into the storm complex,

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The radar-rainfall maps in Appendix B should be consulted for the
incremental rainfall by time for the flash flooding period, Review of these maps
suggests that the peak rainfall production occurred from 815PM until 900PM
over the lower third of the basin. During this period over 3.00 inches of rain fell
over the lower third of the basin, The rain was accompanied by hail at times,
especially before 830PM, By 913PM the core of the storm had moved eastward
over the Spring Creek watershed which experienced the same amounts of rain
as the lower third of Buffalo Creek between 900PM and 945PM. '

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Figure 8 shows the HMS radar-estimated storm rainfall from 739PM until
913PM across the Buffalo Creek basin, The basin average rainfall during this
period was 2,16 inches of rain with a peak 0,5 square mile rainfall of 5.13
inches. During this period, an 18 square mile area of the lower third of the basin
received over 3.25 inches of rain while a core area of 6 square miles just
above the town of Buffalo Creek was inundated by over 4 inches of rain and
hail. HMS removed hail contamination from its rainfall estimates by not including
the peak 10 minute rainfall estimates in its precipitation mapping. In effect, this
process eliminated just over an inch of rain from the mapping which may have
been used for hail production by the storm,

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Figure 9 shows the July 12, 1996 storm total rainfall over the basin for the
entire period from 21 OPM until 1 023PM when the last showers of consequence
cleared the basin. The basin average rain increases to 2.49 inches while the
peak 0.5 square mile increases to 6.29 inches, The lower third of the basin
shows an average rainfall of just over 4.00 inches. Clearly, this thunderstorm
rainfall was focussed by the stationary movement of the storm over the
basin for about 30-45 minutes.

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Another contributing factor to the flash flood may have been the slow
movement down the basin of the rainfall core which could have aided in
producing a runoff maximum that traveled with the storm. This storm was the
only one which went stationary over northeastern Colorado, All the other storm
complexes moved at speeds of 25 to 40 mph while producing severe weather. It
appears that the gust front which aided storm intensification and inhibited storm
motion was a major contributing weather feature to the storm's severity along
with the very moist condition of the atmosphere.

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