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

'"

I
I
'I
I
I
I
I
I
I
I
il
I
I
I
I
I
I
I
I

weather stations and the District ALERT weather station at Evergreen to
calculate and assign a radar-rainfall relationships to the Buffalo Creek basin,

Figure 3 shows the HMS-plotted surface weather observations for the
PROFS mesonet for 805PM PM (02052) on July 12. Note the strong moist,
northeasterly winds flowing into Jefferson County, Next, HMS plotted the
surface weather station observations from the mesonet on a Skew-T, Log P
thermodynamic diagram (Figure 4) on which the evening Denver upper air
sounding had already been plotted, The vertical plotting of the surface weather
observations is called by HMS a mesosound and is used as a means of
calculating the changes in the stability and moisture content of the air over the
District.

Note on Figure 4 points A and B. Point A is where the cloud base has
been calculated using the surface weather observations from the mesonet and
point B is the point where the thunderstorm updraft cools to OC. The calculated
PWI is 1,31" adjusted for an elevation of 7,000 feet while the depth of the warm
updraft layer is point B (5Akm) minus point A (2,6 km) or 2,8 km, The next
step is to solve Equations 2 to 4 for the peak rainfall rates.

Next insert the values for PWI (1,31 ") and the depth of the warm layer (2.8
km) into Equation 2 and solve for the peak 60-minute rainfall rate which is 4,90",
Using this value in Equation 3 and solving the equation, the peak 30-minute
rainfall rate is calculated to be 3A3 inches, These rainfall rates are assigned to
the grid squares covered by radar reflectivity values of 5 level or greater. The
rainfall rates assigned to lower reflectivity values are found in Table 1. Note that
the peak 10-minute rain rates are not calculated to allow for the reduction of
rainfall by hail production in the storm,

Table 1

Relationship between peak 50-minute and peak
30-minute rainfall rates and radar reflectivity levels.

.,.. . Radar Peak 50-mill.... Peak30';mill Peak10"m.in
".Z - Level 4.90" . " .,' 3.43" 12.05"ll-lailr
.
,'. 2 O,10"/5min 0.22"/5min
). '.3 .....' O,22"/5min 0.30"/5min
:,.., 4 " O,30"/5min OA1 "/5min
..X.... . 5 ' . OAO"/5min 0.57"/5min * Hail
..'. 5 OAO"/5min 0.57"15min * Hail

HMS routinely archives 6 minute radar observations from the National
Weather Service (NWS) WSR-88D located at Watkins, Colorado. This radar is
located less than 60 miles from the Buffalo Creek basin and provides very
accurate radar reflectivity observations. The resolution of the radar reflectivity

6