FLOOD01529

• County: Larimer
• Community: Big Thompson Canyon
• Stream Name: Big Thompson
• Title: Natural Disaster Survey Report 76-1: Big Thompson Canyon Flash Flood of July 31 - August 1, 1976
• Date: 10/1/1976
• Prepared For: Department of Commerce
• Prepared By: NOAA
• Floodplain - Doc Type: Flood Documentation Report

Figure 19 shows the NMC surface analysis at 6:00 a.m. and 6:00 p.m. on
July 31. Figure 20 shows the 500 mb (50 kPa) analysis for the same times.

These synoptic features--well above normal moisture, conditionally unstable
air mass, marked low level convergence and strong upslope flow--all pointed
toward thunderstorm activity out of the ordinary for a summer afternoon
and evening in Colorado. The light winds aloft indicated that the storms

would move slowly, concentrating the rainfall in small areas. Despite these
clues, there was no basis to predict the 12+ inches of rainfall and the
disastrous flash flood that followed.

Guidance Received at WSFO Denver

To carry out its responsibility for warning of severe thunderstorms and
flash floods, the National Weather Service has established forecast echelons
at national and at state and local levels. The National Meteorological
Center (NMC) in Camp Springs, Md., provides initial guidance on the location
of synoptic scale features including expected rainfall. The National
Severe Storms Forecast Center (NSSFC) in Kansas City provides guidance on
severe thunderstorms on a national scale. Weather Service Forecast Offices
(WSFOs) and Weather Service Offices (WSOs) use this guidance to prepare
state and local forecasts and issue warnings of severe thunderstorms,
tornadoes and flash floods. WSFOs also issue flash flood watches.

NMC, a largely computerized facility, provides a variety of analyses and
prediction products covering the Northern Hemisphere with emphasis on the
United States and surrounding waters. Subjective forecasts showing frontal
positions, cloudiness, and precipitation areas and amounts are also prepared.
This basic guidance material, covering periods out to 48 hours in the
future, is distributed to NSSFC, WSFOs and WSOs over facsimile and tele-
typewriter networks.

NSSFC uses all available surface, upper air, radar and satellite data to
provide 24-hour surveillance of the development of severe thunderstorms
within the contiguous 48 states. Severe weather outlooks are issued on
a scheduled basis at 3:00 and 9:00 a.m. Watches are issued as required
to advise of areas of greatest potential for severe thunderstorm or tornado
development. These products are distributed mainly over the Radar Report
and Warning Coordination Circuit (RAWARC) to WSFOs and WSOs. A graphic
version of the severe weather outlook is transmitted via the National
Facsimile Network (NAFAX). There is no guidance issued on the potential
for very heavy rainfall associated with localized thunderstorms. Predictions
of this type are beyond the current state of the art and ,research and
development will be necessary before they could be provided.

River Forecast Centers (RFCs) supply WSFOs and WSOs with river stage fore-
casts and with flash flood guidance specifying those threshold rainfall
amounts which will produce minor flooding. This latter guidance covers
zones or parts of states and in some cases contains information for a

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