FLOOD02053

• County: Weld
• County: Boulder
• Community: Weld, Boulder Counties
• Stream Name: Big Thompson River
• Title: Meteorological Aspects of the Big Thompson Flash Flood of 31 July 1976
• Date: 7/1/1977
• Prepared For: State of Colorado
• Prepared By: NOAA
• Floodplain - Doc Type: Flood Documentation Report

like that sampled hy the 1920 GMT Ster-
ling sonde. Strong easterly flow hehind the
trailing front carried this moist, condition-
ally unstahle hut strongly capped air mass
into the foothills of the Front R,mge. Oro-
graphic lifting triggered a large convective
storm complex, which hecame oriented
north to south along the foothills, and
which fed on the very moist air mass he-
neath the frontal inversion. \Veak south-
southeasterly winds ahove the inversion

3. Conditions During the
Storm Period

Meteorological conditions associated
with the evolution of the Big Thompson
storm complex were studied along the
Front Range from Denver to the area just
north and west of Ft. Collins. Data were
availahle from the following sites: Ft. Col-
lins (Colorado State University, Atmos-
pheric Science Building), Greeley (Uni-
versity of Northern Colorado, Ross Hall),
Table Mountain (NOAA-ERL), Boulder
(NOAA-ERL), Jefferson County Ail])ort
(FAA), Rocky Flats (ERDA), Denver
(Stapleton International Airport, )JOAA-
NWS), and Arapahoe County Airport
(FAA). Radar rellectivitv data were avail-
ahle for the entire ston~ period from the
NWS WSR-57 radar at Limon, Colorado,
(located approximately 205 km southeast of
the Big Thompson area). Reflectivity data
were also availahle from the NHRE 10 cm
radar (located at Grover, Colorado, approx-
imately lIS km east-northeast of the Big
Thompson "rea) for a 45 min period near
the heginningofthe storm. Thunderstorms
that produced flooding on the North Fork
of the Cache la Pondre River were gener-
ally heyond the area of radar coverage,

3.1 Local Area Analyses

At three sites (Stapleton International
Airport, Tahle Mountain, and Rocky Flats)

allowed the storm complex to remain
nearlv stationarv over the foothills. Fur-
then';ore, indi~idual cells continued to
form on the southeast flank of the storm and
moved north-northwestward into the com-
plex hefore dissipating. Thus a nearly
stationary rainstorm developed over the
Big Thompson drainage.

surface data were recorded on an almost
continuous hasis. At Tahle Mountain
(ahout 6 mi north of Boulder) the east-west
component of the wind was measured,
from the sUlface to 600 m, with '1 Doppler
acoustic echo sounder operated hy the
'Nave Propagation Lahoratorv of )JOAA-
ERL. Detailed time series' were con-
structed using these data and are presented
in Figs. 28-30. These time series hourlv
observations from the remaining si~es, an;!
Limon radar data were utilized to construct
the local-scale surElCe ,malvses shown in
Fig. 31. These analyses are ,{t \12 h intervals
for the time period 2,330-0100 GMT. Radar
echo contours for VIP (Video Integrator
Processor) levels 1, 2, and 3 are depicted.
These contours correspond to the
minimum detectahle signal, 30 dBZ and 41
dBZ respectively.
In the Ft. Collins, Loveland, and
Greeley areas an increase in wind speeds
and gustiness were the onlv indications of
the passage of the trailing fr~nt. These areas
had remained partly cloudy with upslope
southeasterly flow during the afternoon,
resulting in little temperature difference
across the front. In the Denver-Boulder
area, however, afternoon cloudiness was
minimal. The resulting heating and mixing
had elevated surface temperatures and
lowered de"1Joints, \Vinds were also more

33