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

FLOOD. JULY 3I-AUGUST 1,1976, BIG THOMPSON RIVER. COLORADO

the trailing front (fig. 8). In general, surface dewpoints
were 50_150F higher than normal over much of the in-
termountain west and central High Plains.
At the 700-millibar (fig. 9) and 500-millibar levels
(fig. 10), the dominant large-scale feature was a ridge
extending from southern Texas into southwestern
Canada. Moisture values were high over much of the
area west of the pressure ridge. A weak trough extend-
ed from Utah to southern New Mexico (fig. 9). At the
500-millibar level, the weak trough was farther south,
located over Arizona and New Mexico. A second weak
trough at the 500-millibar level was located over
northern Mexico (fig. 10).
The stability analysis (fig. 11) shows the Totals In-
dex and Lifted Index for 0600 MDT, July 31, 1976.
Values of the Totals Index equal to or greater than 46
indicate favorable conditions for convective develop-
ment and values greater than 50 indicate potential for
moderate to heavy thunderstorms (Miller, 1972). The
Lifted Index was computed for a parcel of air with
mean-thermodynamic characteristics of the lowest
100-millibar layer. Negative values indicate a condi-
tionally unstable environment. Both indices showed
the potential for moderate to heavy thunderstorms
over northern Arizona, most of Utah and Nevada,
western Kansas, and northeastern Colorado.
Rawinsonde data obtained during the early morning
of July 31, 1976, at Denver and Sterling, Colo., are
shown in figures 12 and 13. The data for Denver at
0600 MDT (fig. 21 indicate that the air was very moist
with an average mixing ratio for the lowest
100-millibar layer of 12 glkg (grams per kilogram)
below a temperature inversion at the 670-millibar
level. Winds above the inversion were light and
variable while winds in the cool airmass below the in-
version were generally easterly with speeds less than
10 knots. The Lifted Index was -1, but the level of
free convection was at the 530-millibar level indicating
considerable lifting and (or) heating would be needed to
initiate deep convection. The high moisture content of
the air was the most unusual feature of the rawinsonde
data. Precipitable water contents of 0.67 inch in the
lowest 150-millibar layer, and 1.00 inch in the layer
from the surface to the 500-millibar level were approx-
imately 50 percent greater than the means for July at
Denver of 0.40 inch and 0.69 inch, respectively (Lott,
1976). A low overcast at 1,200 feet was reported at
Denver at the time of the rawinsonde observation.
The rawinsonde data obtained at Sterling, Colo., at
0740 MDT (fig. 13) was part of the National Hail
Research Experiment. A pronounced radiational inver-
sion near the land surface was topped by a weaker in-
version at the 725-millibar level. Winds in the cool air-
mass were easterly with speeds less than 10 knots. The

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AREA ABOVE GIVEN ALTITUDE,
IN SQUARE MILES

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FIGURE 5.-Relation of area to altitude for approximate
storm area upstream from Drake.

responsible for creating the thunderstorms that caused
the floods. Surface and upper-air data, stability
analyses, rawinsonde data, radar summaries, and
satellite photographs are used in summarizing the
meteorological events. Geographic locations mention-
ed in the discussions are shown in figure 7.

CONDITIONS PRIOR TO STORM DEVELOPMENT

Atmospheric conditions for western North America
at 0600 MDT (Mountain Daylight Time) on July 31,
1976, are shown in figures 8-11. A strong polar high-
pressure area was centered in southern Canada. A dou-
ble frontal structure extended from the Great Lakes
through Kansas and then northwestward into central
Montana and defined the southern boundary of the
polar air. The leading front was characterized by a
wind shift and pressure trough while the trailing front
was characterized by a pressure trough with a strong
thermal gradient. To the west of the fronts, a weak
low-pressure area was located over western Colorado
(fig. 8).
Surface-dewpoint temperatures equal to or greater
than 600F extended northwestward from Kansas into
Colorado and Nebraska. A narrow band of very moist
surface air with dewpoints equal to or greater than
650F was moving into southwestern Nebraska behind