Topographic Map, Colorado

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General summary

• The Fort Collins flood was the biggest natural
  disaster to ever affect the city (in nearly 130
  years of record).
• It was the largest 24-hour rainfall ever to fall
  on a Colorado urban drainage.
• There were a total of 5 deaths, more than 62
  injuries requiring hospitalization, 120 mobile
  homes and 22 houses destroyed, 2000 homes
  damaged, and 250m in total damage.

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Topographic Map of Colorado
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Some aspects of the flood were entirely expected

• The event occurred during a period in which
  severe weather is expected somewhere in the near
  vicinity (Weaver Doesken, Weather, 1990)
• It took place in an environment considered
  synoptically favorable for severe weather on the
  high Plains. (Doswell, Bull. Amer. Meteor. Soc.,
  1980)

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Monsoonal Flow began several days before the event
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500 mb Heights and Vertical Velocity
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1200 UTC 28July97 Surface
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1800 UTC 28July97 Surface
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Pre-flood setup

• Heavy rain fell to the northwest of Fort Collins
  the day before the flood
• Up to 9 of rain fell just north of Laporte,
  Colorado between 430 p.m. on 27 July 97 through
  noon on the 28th.
• West Fort Collins received up to 4 during this
  same period.

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Fort Collins arterials and collector streets
shown in red
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Some were not . . .

• The morning sounding found a tropical-oceanic
  airmass throughout the depth of the troposphere
• There were no severe storm indications on
  satellite
• Radar-estimated rainfall was as much as 50
  understated
• Sheeting runoff played as great a role in the
  event as did stream overflow

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Satellite Imagery

• Satellite imagery over Colorado showed a line of
  storms (probably along the returning cold
  front) moving eastward during the afternoon.
• Vis loops showed that upslope was accelerating
• Infrared loops showed that storm activity on the
  Plains was transitional in nature, while the Fort
  Collins activity was recurrent

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Multiple storm cells early on

• During the first half of the evening, a series of
  storms formed southwest of the city and traveled
  north-northeastward at about 8 ms-1 across
  western Fort Collins.
• 3 - 4 of rainfall in west Fort Collins between
  600 and 830 p.m. MDT.
• E-911 calls begin to increase dramatically after
  815 p.m.

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Overall nature of the rainfall

• Rainfall totals were extremely large in southwest
  Fort Collins the night of the 28th
• The rainfall was concentrated primarily within
  two large drainage basins
• The heaviest rains came near the end of the event
  which is unusual (according to NWS Office of
  Hydrology)

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Radar-estimated rainfall was 40-50 too low

• The underestimate by radar was due to the lack of
  frozen precipitation (i.e., the storm did not
  fit the normal upon which Z-R is based).
• Also, there were no reports from spotters,
  because a) the storms just looked like heavy
  rain, to them, or b) they were having problems
  of their own with flooding

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15-min lightning strikes ending 930 p.m.
(Relatively little lightning in the FCL
activity)
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Large, slow-moving storms late

• Storm motion changed abruptly at around 830 p.m.
• The storms after 830 moved toward the
  east-northeast at about 3 ms-1.
• The reason for the slowing and turning was
  outflow from a storm near Greeley, CO.

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A Developing surge of water

• Between 830 and 900 p.m. a surge of water
  developed.
• This was different from the deepening creeks and
  irrigation ditches -- it was a sudden deepening
  of runoff water on collector streets.

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Detection was a major problem

• A 50-acre detention area, designed to handle a
  500-year flood, filled to capacity by 1045 p.m.
  The areal extent is about 40 football fields, and
  the contained area 19-foot deep on its east side.
• There were no rain gauges in the region, and
  there was no human habitation nearby
• A 19-foot railroad bed over-topped, and a 12x14
  culvert failed

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Horizontally-striped region is the 50-acre
detention pond
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Summary

• Radar-estimated rainfall was 40-50 low,
• There were major problems with recognition
  detection - - weather spotters did not even
  report,
• Lightning activity was minimal due to a lack of
  ice processes in the storm,
• No large storms were observed on satellite in the
  vicinity of Fort Collins throughout the night,
• Sheeting urban runoff played the major role in
  the evolution of the event.

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Timing of Critical Events

• Flash Flood Watch at 500 p.m.
• Urban Small Stream Flood Advisory 736p.m.
• Spring Creek flooding problems begin at 830
  p.m.
• Drainage basin runoff problems begin at 900
  p.m.
• Urban Small Stream Advisory extended at 902
  p.m.
• Flash Flood Warning 940 p.m.
• Colorado State University problems begin 1000
  p.m.
• Converging of drainages 1030 p.m.
• Culvert breach at 1046 p.m.
• Deaths occur 1055 through 1105 p.m.

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Future Plans

• Fort Collins is installing an Emergency Managers
  Weather Information Network (EMWIN) ground
  station.
• The citys EM will be paged-out only when local
  events, exceeding a pre-determined, critical
  threshold are anticipated.
• Alerts to local officials and city/county
  departments can be carried out in a multi-tiered,
  preset order.

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Future Plans

• The city is also installing a Local Data
  Acquisition and Display (LDAD) system.
• Critical stream and precipitation gage
  information will reach the NWS in a timely
  fashion.
• Better local guidance for the citys EM will
  thereby be available from the NWS.

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Future Plans

• The city of Fort Collins has begun a project to
  develop a real-time flood inundation mapping
  capability. The project is funded primarily by
  FEMA.
• The first step will take place in late February
  or early March. At that time a one-foot
  resolution topographic data base will be
  developed for the FCL metro area.

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Future Plans

• The city of Fort Collins is
• a) purchasing a reverse 911 telephone dialing
  system, EDD cards for dispatch
• b) installing a low-power am radio station for
  re-broadcasting NOAA weather radio information
  during critical events.
• c) designing a two-level training program for
  natural disasters -- one for the general public,
  and one for emergency responders

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Future Plans

• The general public training will be based on
  so-called action videos which illustrate people
  making critical mistakes (fun to watch),
• The emergency responder program will include
  weather/spotter training, exercises and learning
  to look at false alarms as useful events.
  Weather spotters will be treated as part of the
  total emergency response force.

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Flood Photo Site http//www.verinet.com/rwf/floo
d/pfa3.htm Fort Collins OEM Site http//www.ci
.fort-collins.co.us/C_SAFETY/oem/index.htm
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Forecaster Tip

• If you are working in an environment that is
  characterized by a tropical, or near-tropical
  airmass, and are looking at either radar or
  satellite output, be especially alert for small,
  shallow storms which have little or no
  electrical activity. Chances are that WSR-88D
  radar-estimated rain rates are significantly
  understated . . .

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