FLOOD01375

• County: Larimer
• Title: Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997
• Date: 2/1/1999
• Prepared By: American Meteorological Society
• Floodplain - Doc Type: Flood Documentation Report

A chronology of events beginning at
0000 MDT (mountain daylight time) on
28 July 1997 is presented in Fig, I rela-
tive to a time series of rainfall measured
at Christman Airport in northwestern
FCL (cL Figs, 2-3), Earlier, on the
evening of 27 July, up to 2.42 in, of rain
fell in the foothills immediately west and
northwest of FCL (Fig, 2), These rains
were followed in the early morning of28
July (e,g" 0300-1000 MDT; Fig, I) by
another episode of heavy rain on the west
side of FCL, extending into the foothills
and to the northwest of the city, At 0500
MDT on 28 July ("A" in Fig, I) the Na-
tional Weather Service (NWS) in Den-
ver, Colorado, issued a special weather
statement outlining the potential for heavy rains and
flash flooding in and along the foothills of the north-
ern Colorado Front Range in association with the de-
velopment of moist 10w,Ievel upslope flow and weak
upper, level winds (cL section 4), By 0730 MDT ("B"
in Fig, I) flooding had occurred in parts of Laporte and
in locations northwest of Laporte (cL Fig, 3) where 7-
9 in, of rain had fallen, As moderate to heavy rain con,
tinued to fall along the foothills, the NWS
issued an urban and small stream flood advisory
(USF) at 0940 MDT ("C"; Fig, I), The rainfall along
the foothills west and northwest of FCL ended
around 1200 MDT, and at 1400 MDT
the USF was canceled ("D"; Fig, I),
However, as the afternoon progressed,
heavy rainfall redeveloped over the
central and northern foothills west and
northwest ofFCL (Fig, 2), leading to the
issuance of a flash flood watch at 1700
MDT ("E" in Fig, I),
Between 1700 and 2000 MDT,
two small convective systems moved
over FCL producing brief heavy rains
(e,g" 0,5-D,75 in, of rain in 30 min) be,
fore moving off to the north,northeast
of the city, During the second rain
episode (1900-1930 MDT; Fig, I),
the NWS issued a USF for FCL
(1936 MDT; "F" in Fig, I), Following
issuance of the USF, a third area of
heavy rain moved over the city from
the southwest between 2000 and
2030 MDT and remained quasi-
stationary over the western side of

7/28/97; Event Chronology and Accumulated Precipitation al Christman Field

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Time (MDT)

FIG. 1. Chronology of events for 28 July 1997 vs accumulated precipitation
(inches) at Christman Field (cf. Fig. 3). Time (MDT) is indicated on the abscissa.
Events labeled A-M arc discussed in section 1 a.

under 6 h, By comparison, the Fort Collins 100'yr 6,h
and 24-h rainfall amounts as indicated in the National
Oceanic and Atmospheric Administration (NOAA)
Atlas,2 (Miller et al. 1973) are 3.50 and 4,80 in" re-
spectively, Approximately 4 km southwest of the CSU
weather station, a maximum of 10,2 in, of rain was re,
corded by a gauge in a 6,h period,

Q, Event chronology
The FCL flood was the result of a series of rain
events that occurred over a 30,h period ending in an
extremely heavy downpour on the evening of 28 July,

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fIG. 2. Map of regional topography with the locations of Fort Collins, CSU-CHILL,
Cheyenne, Denver, and the Big Thompson canyon indicated. Elevation intervals (ft)
are shaded as indicated at the bottom of the figure.

192

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Vol, 80, No.2, Februory 1999