FLOOD09830

• County: Statewide
• Community: Front Range
• Basin: Statewide
• Title: The Virginia Canyon Flash Flood
• Date: 8/18/1991
• Prepared For: UDFDC
• Prepared By: Henz Meteorological Services
• Floodplain - Doc Type: Flood Documentation Report

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Once east of the Divide, the terrain covered by the storm varied from 8,000-13,000 ft. in
Boulder County to 7,800-10,500 ft. in Gilpin and Clear Creek Counties where the storm
reached its peak intensity. As the storm dissipated over the western Jefferson County
foothills it covered elevations of 7,000-8,000 ft. Thus over 90 percent of the storm and its
flash flooding rainfall occurred at altitudes above 7,500 ft.

Clearly the thunderstorm was sufficiently strong to produce a significant mountain flash
flooding event along a populated mountain highway. The occurrence of this storm in an
area where it could be observed by both radar and surface observations was very
fortuitous. The remainder of this report will provide a narrative of the meteorological
factors influencing the formation of the thunderstorm complex and are-constitution of the
rainfall which caused the flash flooding in Virginia Canyon.

Narrative Of Key Meteorological Factors

The weather pattern which produced the Virginia Canyon flood is a fairly typical late
summer regime associated with heavy precipitation events. In general a moist and slow
moving monsoonal air mass covered all of Colorado, Utah, New Mexico and Arizona at
altitudes below 20,000 ft. At jet stream altitudes above 20,000 ft., a sub-tropical jet stream
arcs across portions of Arizona, Utah and northern Colorado. Disturbances moving in the
sub-tropical jet stream were associated with the triggering of general thunderstorm
outbreaks. The key to the occurrence of a flash flooding event in this weather pattern
usually is the timing of passage of one of these disturbances during an unstable time of
day, the availability of moisture to fuel the thunderstorm complex and a low level wind
field mechanism to focus the storm's rainfall production over a basin capable of flooding.

Satellite photo loops (not shown here) indicate that an upper level disturbance was
traveling along the Colorado-Wyoming border the evening of the Virginia Canyon flood.
In all likelihood the arrival of this disturbance played an important role in setting the stage
for the flash flood. First, the disturbance waS preceded during the day by a sinking mid-
level air mass or a subsidence field over most of the mountains and the eastern Colorado
plains. This subsidence field contributed two factor to the storms. First, it kept skies nearly
cloudless and allowed the higher foothills to warm substantially through mid-afternoon.
Next, the sinking motion of the subsidence delayed the formation of thunderstorm systems
during the daylight hours of August 19, 1991 over the foothills. In effect the subsidence
clamped a lid on the highly, unstable monsoonal air mass which covered the foothills.

An indication of what could happen once the lid was removed was noted in a brief flare-up
of thunderstorms just east and south of the Denver metro area. An arc of strong
thunderstorms formed over portions of Arapahoe, Douglas and Elbert Counties producing
brieflocally heavy rainfall, small hail and gusty winds. The storms formed in response to a
low level converging wind field which formed over the Douglas County central foothills.
The converging winds acted to lift the unstable monsoonal air above the capping
inversion.

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