Case Review: 28 July 1997
THE FORT COLLINS FLOOD: UNUSUAL PRECIPITATION
EFFICIENCY COMBINED WITH COMPLEX URBAN
HYDROLOGY.
Learning Objectives:
(1) To understand the unusually efficient precipitation production of this
storm; how it developed, and how to recognize the "warm rain"
signature.
(2) To examine what may have anchored the storm in the Fort Collins area.
(3) To consider the radar performance and the scientific reasons for certain
changes in radar parameter settings.
(4) To begin to recognize the complex runoff characteristics of urban
environments.
Materials:
(1) Hand analyzed maps from 2100 UTC 28 July and 0000 UTC 29 July of
surface isobars and dewpoints.
(2) Maps of 700 mb dewpoints & plots, 500 mb heights, and 250 mb plots
and isotachs, all at 0000 UTC 29 July.
(3) Isohyet analysis of previous night's excessive rainfall centered mainly
northwest of Fort Collins.
(4) Surface plot for 0300 UTC.
Assignment: After a weather discussion about the situation:
(1) Review data from roughly 2100 through 0300 UTC 28-29 July.
 What is the general movement of storm elements?
 What about the storm complex in the Fort Collins vicinity?
(2) Now look at data through 0430 UTC and examine the Fort Collins storm
complex. Make sure to examine satellite and radar data.
 Did the storm-scale details seem to change at all?
 What seems to be anchoring the storm?
 What finally moved the storm complex off to the northeast?
(3) Might there be atypically high rainfall rates?
 look for warm rain signature such as relatively warm cloud tops
and low storm centroid (precipitation concentrated in lower part of
the storm).
Hint: looks at BREF1 and BREF3 from both the KCYS and KFTG
radars.
 What environmental conditions may be contributing to warm rain
processes (consider both meteorological and geographical).
(4) Look at the radar derived precipitation products.
 How does the warm rain (tropical) radar setting (KFTG2)
compare with the standard (KFTG)? Which is likely to be more
accurate?
 A precipitation bullseye of similar magnitude can be seen to the
southeast of Fort Collins. Which is likely to be more
representative (KFTG or KFTG2) for that precipitation
accumulation bullseye?
(5) Consider three important contributions to flash flooding:
 Precipitation Duration
 Precipitation Intensity
 Hydrologic Response
How important were these contributions in Fort Collins?
Answers:
Isohyet analysis in the Fort Collins area for the evening of 28 July 1997
provided.
A summary of this event can be found in the March 1998 FSL Forum
produced by the NOAA Forecast Systems Lab.
http://www.fsl.noaa.gov/~vondaust/f398/
Information from the Colorado State University research radar
(CHILL) for this event can be found on their website.
http://olympic.atmos.colostate.edu/CHILL/flood97/index.html
(1) Storm elements were moving to the north-northeast throughout the
northeastern part of Colorado. In the Fort Collins vicinity the storms
began moving south to north repeatedly by 0300. This is likely due to
the combination of regeneration westward toward the foothills and the
steering current toward the north-northeast.
(2) During the period of 0300-0430 UTC 29 July, the activity over the Fort
Collins area became more of a quasi-stationary storm system with some
tendency for elements to move northeastward. The lowest radar tilt
from either KFTG or KCYS could not accurately depict the low-level
flow supporting this system. However, the Colorado State University
research Doppler radar was located much closer to the activity. This
depicted strong outflow from the east and into Fort Collins that quite
likely enhanced the low-level support for new cell generation against
the foothills. Regeneration of storm cells appears to have balanced the
movement of cells to the northeast. Eventually, this enhanced upslope
flow ended and the storm system moved off to the northeast as it
dissipated.
(3) Conditions on 28-29 July showed unusually moist, tropical conditions
for the region as seen in the atmospheric sounding (especially the
exceptional precipitable water values), and the atypical low-level
dewpoint temperatures. As the storms were in progress certain
characteristics, particularly of the Fort Collins activity, were noteworthy
from the perspective of precipitation efficiency:
 High reflectivity, but low centroid. A four-panel view of the lowest
radar tilts from KFTG shows the low-centroid nature to this storm.
The four panels clockwise from top left show data from radar tilt
angles of 0.5o, 1.5 o, 2.4 o, and 3.4 o. Much of the storm intensity (and
thus the precipitation production) was below the radar beam at 2.4 o,
or about 4.5 km (15,000 ft) AGL. With storm bases about 500 m
AGL, a large area of the precipitation growth was in the abovefreezing layer of the cloud. Warm-rain processes were enhanced,
thus enhancing the precipitation efficiency.
 In support of the radar’s suggestion of an enhanced warm-rain
process, note the relatively warm cloud-top temperatures and the
lack of lightning activity when compared with nearby storms to the
east.
 Finally, the terrain focus increases greatly in Fort Collins where the
foothills of the Front Range rise abruptly immediately to the west of
the city. This would serve to enhance the low-level upward motion
in a cloud system already characterized by efficient precipitation
growth in the low-levels.
(4) Fortunately, excellent ground reports exist for this event. The radarderived precipitation guidance compared much better for the tropical ZR (Z=250R1.2) than for the default 88D settings (Z=300R1.4) in the
vicinity of Fort Collins. Other precipitation extrema occurred that day
that were not as well documented by ground-based measurements.
Although warm-rain processes appeared to have played a role in other
precipitation extrema, the Fort Collins complex exhibited the most
impressive precipitation efficiency. Reliable ground reports outside the
Fort Collins area suggest that the default Z-R conversion results in
underestimated accumulations (but not as much as the underestimation
in Fort Collins), and the tropical Z-R setting appears to result in some
radar overestimation. Radar parameters will not always be able to
represent all storms occurring within the radar domain. Forecasters are
faced with the difficult task of identifying and reacting to the signatures
that make a particular storm unique. Signatures of exceptional
precipitation efficiency have been seen in other major flood events in
recent years and thus it’s important to understand the physical processes
affecting rainfall rates. The radar is only one of several important tools.
(5) Rapid regeneration of echoes increased the duration of the heavy
rainfall period over Fort Collins. An unusually warm tropical
environment greatly increased the precipitation efficiency. Enhanced
duration and intensity of rainfall combined with urban runoff
complexities and resulted in a very major hydrologic response.
However, the intense rates and the urban environment were even more
important than the duration, as is often the case with urban flash floods.