Presented at IAHS Conference
Destructive Water: Water-Caused Natural Disasters - Their Abatement and Control
June 24-28, 1996
National Weather Service Advanced Capabilities
in Flash Flood Forecasting
Lee W. Larson
Chief, Hydrologic Research Laboratory
Office of Hydrology
NOAA/National Weather Service
1325 East-West Highway
Silver Spring, Maryland 20910
Introduction. The modernization of the National Weather Service (NWS) includes three
major systems. These systems are the Doppler Weather Surveillance Radar (WSR-88D), the
Advanced Weather Interactive Processing System (AWIPS), and the Automated Surface
Observing System (ASOS). These advanced technologies are providing significant data and
processing capabilities which are directly applicable to the flash flood problem.
In particular, the Weather Forecast Office (WFO) Hydrologic Forecast System (WHFS)
provides the forecasters with unparalleled access to real-time data and the capability to
process and identify potential flash flood situations. All of these technologies contribute to
improved capabilities of the NWS to provide early and useful flash flood products to
cooperators and the public.
On July 31, 1976, more than 12 inches of rain fell in the Big Thompson Canyon in
Colorado. The resulting flood left more than 140 people dead and destroyed homes and
businesses. There has been a significant effort within the NWS over the last 20 years to
improve our capabilities to respond to these types of events.
Background. Following the Big Thompson flash flood, in an effort to provide the WFO with
the tools necessary to effectively forecast these types of events, the Forecast Systems Lab
(FSL), with funding support from the NWS, began the Program for Regional Observing and
Forecast Services (PROFS) which was a proof of concept project for an NWS field office
advanced workstation environment. The PROFS workstations added considerable
functionality in data handling, graphical display capability, and additional local model
generation capability. The NWS and FSL, as a risk-reduction activity, then cooperated in
the Denver AWIPS Risk Reduction and Requirements Evaluation (DAR3E) in the mid-1980's. The DAR3E project was designed to put a series of the PROF developed
workstations in an operational Weather Service Forecast Office (WFSO). PROFS and the
early DAR3E implementations had limited functionality to address the hydrologic operations
at the WSFO. Over the next few years, through 1992, some limited success was achieved in
adding hydrologic displays and applications to the DAR3E system and a later pre-AWIPS
system. By mid-1993, the hydrologic application development on the pre-AWIPS system and
the WSFO hydrologic development effort was moved to the Office of Hydrology (OH).
Weather Forecast Office Hydrologic Forecast System. The WHFS features an integrated
data management approach, employing a relational database management system (RDBMS)
for storing the large volume of data necessary for hydrologic forecast operations. The
WHFS database incorporates many data elements ranging from modernized data sets such as
NEXRAD precipitation estimates and GOES satellite imagery, to more traditional hydrologic
data sets provided by automated reporting stations and cooperative observers.
The supporting River Forecast Center (RFC) is the primary source of hydrologic guidance
for the WFO, providing river stage forecasts on a daily and event-oriented basis. RFC
guidance is also provided in the form of modernized flash flood guidance products that
indicate current soil moisture conditions and associated rainfall thresholds necessary to induce
A collection of tools is provided within WHFS to allow the WFO hydrologic program
manager to manage through a series of graphical user interfaces. HydroBase, one of these
tools, provides a method of managing station data, allowing for definition of various station
attributes (Figure 1). Much of the data utilized by forecast applications are also defined
through HydroBase. Program management tools, such as automated generation of monthly
flood stage reports, are also part of HydroBase.
WHFS Capabilities. During a typical hydrologic situation, the forecaster may employ many
aspects of the WHFS in combination to evaluate the current hydrologic conditions, evaluate
data, and issue products notifying the public of flood situations.
The Stage and Display (HydroView) application provides the forecaster with a method of
monitoring and tracking the hydrologic situation in real-time (Figure 2). This application
provides a geographic depiction of the WFO County Warning Area (CWA) with the ability
to overlay an array of Hydrometeorological data. Station icons may be overlaid in
combination with hydrologic or geopolitical boundaries such as rivers, river basins, county
outlines, or major towns and highways. River station icons are color-coded to indicate the
proximity of the latest observation to action or flood stage. Precipitation stations are color-coded to represent a precipitation accumulation for a selected time duration. This display is
automatically refreshed at 15 minute intervals, using the most recent observations and
forecasts available. The forecaster may view a time-series display of river stage and
precipitation observations for a period of up to 21 days. Forecast data is provided for a
5-day time period.
The Area-Wide Hydrologic Prediction System (AWHPS) provides the forecaster at the WFO
with an analysis of a flash flood threat in the WFO forecast area (Figure 3). AWHPS uses
data from NEXRAD and gridded flash flood guidance from the servicing RFC to provide a
graphical depiction of (1) Critical Rainfall Probability (CRP), (2) 1-hour rainfall projection,
and (3) a difference display.
The NEXRAD product that is used in the AWHPS system is the Hourly Digital Precipitation
(HDP) product, which provides a gridded accumulation of precipitation for the previous hour
each volume scan of the radar. The modernized flash flood guidance from the RFC
indicates, for each HRAP grid, the amount of rainfall required in a particular duration to
cause over-bank flood of small streams. The common durations for the rainfall in the flash
flood guidance computations are 1, 3, and 6 hours.
Two CRPs are computed for each duration: the first is the CRP based on the radar estimated
rainfall, and the second is the CRP based on the radar estimated rainfall plus the 1-hour
projection. The CRP gives a statistical probability that the rainfall in a particular HRAP grid
has exceeded the flash flood guidance for that grid square. The difference fields are a
graphical depiction of the quantitative difference between the flash flood guidance and the
radar estimated rainfall for each duration. A second difference graphic will depict the same
information for the radar estimated plus 1-hour projected rainfall totals. Utilizing the CRP
products, the forecaster will be able to outline the potential flash flood area. At that point,
the forecaster can issue the appropriate public product, either a flash flood watch or a flash
The Site-Specific Hydrologic Prediction System (SSHPS) is a local hydrologic model
provided to allow the WFO forecaster to supplement RFC river forecast guidance by
generating forecast river stages for fast-response headwater and river basins (Figure 4).
River stage observations and precipitation estimates are provided as input to a simplified
rainfall-runoff model, which produces an estimate of streamflow rise due to runoff reaching
the river channel. Initial soil moisture conditions are accounted for through model state
variables provided by the RFC. Dependent upon the model definition, other inputs such as
snowmelt runoff and potential evapotranspiration may also be considered. Model definitionsfor individual basins are calibrated by the RFC, employing the NWS River Forecast System
(NWSRFS) hydrologic models as a baseline.
Gridded or point precipitation estimates may be used as model input, and may be selected by
the forecaster prior to the execution of the model. Each of these forms of estimates is
ingested through a precipitation preprocessor that calculates basin average precipitation values
for a time duration specified by the model definition. Gridded estimates are utilized on a
best-available basis employing Stage III, Stage II, and Stage I NEXRAD estimates. Future
precipitation estimates may be incorporated through the assimilation of gridded Quantitative
Precipitation Forecasts (QPF) products generated by the WFO, the RFC, or a national
center. The forecaster interacts with the SSHPS through a graphical user interface that
allows for interactive review and adjustment of model results and input.
Conclusions. WFO Hydrologic forecast operations in the AWIPS-era will differ dramatically
from those in the pre-modernized NWS. The advent of more powerful computing
technologies provides the opportunity to implement sophisticated hydrologic modeling,
analysis, and forecast tools in a manner suitable for use of dealing with the wide range of
possible hydrologic conditions and situations. Significant portions of the initial WHFS
capability will be fielded to WFO's beginning in the fall of 1996, with full hydrologic
forecast capability available shortly thereafter. This WHFS implementation will provide the
WFO forecaster with the tools necessary to meet the goals of the NWS hydrologic services
program, serve as the baseline for future enhancements, and dramatically enhance the WFO's
ability to identify and respond to short-lived hydrologic events such as flash floods.
Acknowledgments. Significant contributions by Edwin L. May, NWS, Fort Worth, and Dale
R. Shelton, formerly of the NWS, is gratefully acknowledged.
Shelton, Dale R. and Edwin L. Mays (1996). "Modernized Hydrologic Forecast Operations
at National Weather Service Forecast Offices," 12th International Conference on Interactive
Information and Processing System (IIPS) for Meteorology, Oceanography, and Hydrology,
American Meteorological Society, Atlanta, Georgia, January 28-February 2, 1996.