FLOOD04806

• County: Larimer
• Stream Name: Big Thompson/Cache la Poudre
• Basin: South Platte
• Title: Storm and Flood of July 31-August 1, 1976 in the Big Thompson River and Cache la Poudre River Basins
• Date: 1/1/1979
• Prepared For: Colroado Geological Survey
• Prepared By: USGS/NOAA
• Floodplain - Doc Type: Flood Documentation Report

40

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~ 600
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~ 500

w'
~ 400
~
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~ 300

~ 200
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""
~ 100
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w
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90
85
80
>- 75
w
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z 70
w
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"
"" 65
~
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w 60
w
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'" 55
w
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50
45
40
1630

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FLOOD, JULY 31-AUGUST 1,1976, BIG THOMPSON RIVER, COLORADO

l~ I\~~Y\'--- \ -.18\.-2~~?\j11Ii \~\~2Z~~// \ \ -IV/(i//::\
~I IV(\ \V ~ \ \ -18 lG:~~1 ~~-.v~ '--12-- 14., ^-(! 1\\
\: _,,-14 -16 \ i7~ J;y~I416LVI ~t~:i!J7(j ~~
~ -10 \~ l,,~'-JI 1\\,)(0 I '.
O -'., -, \ -I I-~~...'.,.'" ~:~~ II \J \
~ ~"2'''''' \ -r \,)
14 ..;;..."'....... ~ I);
\W~'i__ ~b~\j \

Surface air temperature

Surface dewpoint temperature

----------

1700

1730

1800 1830 1900
TIME IN HOURS IMOT)

1930

2000

2030

EXPLANATION

--8- LINE OF EQUAL EASTERLY COMPONENT
OF WINO SPEED. -Interval 2 meters
per second

-+2- LINE OF EQUAL WESTERLY COMPO-
NENT OF WINO SPEED. -Interval 2
meters per second

FIGURE 35.-Time series of winds. from the surface to a height of 2,000 feet, and surface-air and dewpoint temperatures, Table Mountain
north of Boulder, Colo" 1630-2030 MDT, July 31,1976.

the trailing front passed about 1730 MDT with a max-
imum easterly component of about 48 knots, 2,000 feet
above the surface, The easterly surface wind was tem-
porarily interrupted about 1900 MDT by a shallow
region of westerly winds-probably weak outflow from
a large thunderstorm cell located a few miles west of
the site.
In order to provide an estimate of upper-air condi-
tions just prior to the development of the severe
thunderstorms, a sounding was interpolated for
Loveland, Colo., at 1800 MDT. This sounding, shown
in figure 37, was based on rawinsonde data from Ster-
ling, Denver, and Grand Junction, Colo., surface obser-
vations, and Table Mountain wind data. The sounding
data yielded a Lifted Index of -6 and a mean mixing
ratio below the frontal inversion of 14.8 g/kg. The
lifted condensation level was at the 730-millibar level

which agrees with observed low cloud heights at Fort
Collins of 7,000-9,000 feet above mean sea level. The
data further indicated that the air near the surface re-
quired a lift of about 2,300 feet to reach the level of free
convection.

PHYSICAL MODELS OF THUNDERSTORMS

The lO-centimeter radar at Grover, Colo., scanned
the storms along the northeastern Colorado foothills
until a few minutes after 1900 MDT. Storm intensity
peaked about 1845 MDT and then temporarily
decreased. A comparison of Limon and Grover radar
echoes for this period is shown in figure 38,
Limon radar operated at O-degree elevation angle
during most of the evening with the center of the radar
beam intercepting storms over the Big Thompson area