Methods for Forecasts
Forecasts for the Red River of the North are produced at the North Central River Forecast
Center (NCRFC) using the Operational Forecast System (OFS) of the NWS River Forecast
System (NWSRFS). The hydrology of a typical downstream sub-basin in the Red River of the
North watershed is modeled with the following NWSRFS operations:
Table 2. Typical Operations in River Forecast Model
||Accounts for snow accumulation
||Determines the runoff from the
||Converts runoff from the basin
into stream channel discharge at the forecast point
||Determines the baseflow
contribution to the hydrograph
||Adds together the various flows
coming into the forecast point (including the local runoff from the UNIT-HG)
||Storage routing procedure which
routes water coming from upstream to the forecast point
||Adds the baseflow into the routed
||Converts observed stage values to
discharge using the rating curve
||Updates the simulated discharge
values using observed values
||Converts simulated discharge to
simulated stage using the rating curve
||Displays selected results of the
operations including the outflow hydrograph
A headwater basin would use all of the above operations except the TATUM routing
for Long-Range Outlooks
The conceptual methodology for developing numerical spring snowmelt outlooks (peak
stage forecasts) in the upper Midwest has remained the same for decades. The process was
originally developed and used by the Missouri Basin RFC prior to the creation of the North
Central RFC in 1979. The methodology utilizes the same forecasting system (the OFS
subsystem of NWSRFS) used to model and develop short-range flood forecast guidance. As
such, all watershed components (headwaters and locals) and routing reaches are identical
with respect to parameterization, data requirement, physical state, and output.
Spring snowmelt outlooks are based on one scenario of future temperature and two
scenarios of future precipitation. The scenario of future temperature reflects a normal
spring warm-up that generates a single snowmelt peak during the month of April. The two
scenarios of future precipitation are (1) zero future precipitation and (2) climatological
Procedurally, the outlook simulations are generated by making deterministic
"runs" that begin with the current operational model states and extend 60 days
into the future. Since the OFS can only support a 30-day forecast run, the process must be
run in two steps of 30 days each. While this is awkward, it does not affect the accuracy
of the guidance. For simplicity, climatological normal precipitation (scenario #2) through
the end of the forecast period is added to the current model-state snowpack at the start
of the run. Since these runs are only used to assess the potential spring peak rather than
the time-series of spring runoff, this simplification is reasonable. Once the two
simulations are made, RFC hydrologists review the model guidance for each forecast point.
1997 Spring Outlooks for the
Red River of the North
The 1997 outlooks for the Red River of the North are presented in Figure
1; values shown are the second scenario; i.e., they include climatologically normal
future precipitation. Note that later outlooks did not change the values from the late
February outlooks. The observed crest at all points except East Grand Forks is within 1-2
feet of the outlook and generally 1-2 feet above the previous record stage. At East Grand
Forks, the observed crest stage of 54.35 feet is 5.35 feet above the outlook and 5.55 feet
above the previous record stage.
History of Outlooks for the Red River of the North
Spring Outlook peaks issued for Wahpeton, North Dakota; Fargo, North Dakota; and East
Grand Forks, Minnesota, for years 1980 through 1997 are shown below (Table
3). These are the outlooks issued around March 15 (or earlier) each year. Note that in
very low snow years, no outlook value was provided. The last three columns are (1)
forecast snowmelt peak with zero future precipitation , (2) forecast snowmelt peak with
climatologically average future precipitation, and (3) observed peak stage.
Table 3. History of Spring Flood Outlooks
for the Red River of the North
No Future Precipitation
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
E. Grand Forks, MN
Although the outlooks in Table 3 represent a fairly small sample, a
few simple observations are possible: First, the outlooks that assume zero future
precipitation have a high likelihood of being exceeded. (As an historical footnote, the
"no future precipitation" outlook scenarios were originally developed to aid in
evaluation of river transportation issues.) Second, snowmelt outlook peaks that assume
normal future precipitation are approximately at the median; i.e., they have approximately
a 50 percent chance of being equaled or exceeded. Since the outlook process does not
produce an explicit probability of exceedance and the historical sample of outlooks is
fairly small, attribution of exceedance probabilities involves a degree of judgement.
The NCRFC has developed the ability to provide probabilistic spring flood outlooks for
the Des Moines Basin using the Ensemble Streamflow Prediction (ESP) procedure of NWSRFS.
Both traditional (as were done for the Red River of the North) and ESP forecasts for the
Des Moines Basin were generated during the Spring of 1997. The ESP procedure was not
implemented for the Red River of the North during the 1997 floods.
While hydrologic modeling is based on flow volumes, most public forecasts are made for
river levels or stages. The relationship between river stage and flow volume rate
(discharge) is called a rating curve or stage-discharge relation. These rating curves are
critical to forecasting the river stages at gaged locations along rivers.
The USGS is responsible for measuring streamflow throughout the United States. The USGS
makes discharge measurements and develops most of the official rating curves used by the
NWS. When significant rises occur, the USGS makes additional discharge measurements and
routinely provides the information to the NWS, the USACE, and other cooperators. These
measurements are used to update rating curves. The rating curves are developed based on
actual flow measurements; the USGS does not normally extend them beyond observed flows.
Rating curves are used extensively in the OFS to convert observed stage data into
discharge for routing water to downstream points. They are also used to convert discharge
at a forecast point into stage values for issuing public forecasts. In some events, the
forecast flow or stage is beyond the uppermost value in the rating curve for that
location. For those cases, OFS will extend the rating curve by one of three methods:
linear extrapolation, logarithmic extrapolation, or hydraulic extension. The method used
to extend the curve is defined by the forecaster when setting up the rating curve in OFS.
These automatic extensions work well in many situations and are used to provide estimated
values until an updated rating curve is available. The extensions may not work well in
other situations, especially when significant backwater effects are present. The rating
curve in use at the NCRFC for forecasting at the East Grand Forks gage is shown in Figure 2; it includes the portion of the rating curve based on previous
(to the flood) observations of the USGS (noted as USGS Rating No. 18), the logarithmic
extension within OFS used for forecasting by the NCRFC, and the extension of the rating
based on the observations taken during the flood.
The hydraulic characteristics of the Red River of the North at Grand Forks and East Grand
Forks are very complex. The slope of the river channel upstream and downstream from Grand
Forks, inflow from the Red Lake River, and the presence of five bridges connecting Grand
Forks and East Grand Forks contribute to a variable relation between river stage and
discharge. The official rating curve at a gaged location is a single value function that
describes a one-to-one relationship between stage and discharge. Unfortunately, the
discharge associated with a given stage may differ depending on whether the river is
rising or falling. This effect is particularly dramatic on mildly sloping rivers where
significant backwater exists; such as, the Red River of the North. The gradient of the
river at flood stage varies from 0.58 foot per mile between Wahpeton and Fargo, to 0.48
foot per mile between Fargo and East Grand Forks, to 0.32 foot per mile between East Grand
Forks and Drayton, and to 0.14 foot per mile between Drayton and the International
Boundary. The very flat slope downstream from East Grand Forks results in variable
backwater conditions depending on ice conditions, debris accumulation, and the volume of
runoff flowing into that reach of the river. Under these conditions, small changes in
discharge can lead to large differences in stage. This variable stage-discharge relation
often appears as a "loop" in the rating curve. The USGS has developed a looped
rating curve for East Grand Forks based on the April 1997 flood event, which is shown as Figure 3.
Finally, it is possible to use hydraulic models in engineering studies to produce a rating
curve for hypothetical flood control works or hypothetical flood events. The St. Paul
District of the USACE had such a rating curve for East Grand Forks developed as part of a
flood control design study for a project which was not built. This rating curve became the
subject of some controversy in this event and is included in this report as Figure 4.
Rating at Grand
The variable stage-discharge relation at East Grand Forks makes it inherently difficult
to provide a highly accurate prediction of flood crests (especially for extreme events)
there. The discharge measurements made by the USGS during the April 1997 flooding show
that the stage varied as much as 6 feet for a discharge of 30,000 cubic feet per second
(cfs). Historically, the stage at a discharge of 30,000 cfs has varied between 36 and 40
feet but was affected this year by backwater from downstream ice on the rising limb of the
hydrograph to increase the range to 6 feet. Near the peak, the discharge varied from
100,000 to 137,000 cfs while the stage varied between 52 and 54 feet. Measurements made
during previous floods show similar variation in the relation between stage and discharge.
The discharge measurements made by the USGS during the April flooding define a loop
rating for the East Grand Forks forecast location based on the April 1997 event, but this
rating is only representative of conditions during the 1997 flood. The shape and relative
position of rating (relation of stage to discharge) varies considerably from year to year
depending on ice conditions, debris accumulation, and the volume of runoff flowing into
the river at and downstream from Grand Forks. The rating for any individual flood can only
be defined precisely with discharge measurements during the flood, which severely
restricts the NWS ability to provide an accurate long-range forecast of the flood crest.
Using the existing methodology, future flood forecasts should be based on the most recent
stage-discharge rating but presented in the context of the range of stages that have been
experienced at the predicted peak discharge.
Extensions of the rating curves, based on hydraulic analysis, are routinely done for
flood insurance studies by the Federal Emergency Management Agency (FEMA) or during
project design by the USACE, city engineers, or other planning agencies. While these
engineering analyses may not be directly applicable to a real-time forecast situation,
they should be considered on a case-by-case basis to aid NWS forecasters, especially for
record and near-record floods. The USACE had such a rating curve for an unbuilt project in
Grand Forks. In a post-analysis of the flood forecasting at Grand Forks, the St. Paul
District Office of the Corps concluded:
||"The Corps rating curve for Grand Forks was not furnished to the
NWS because the curve was a draft working tool used in our ongoing feasibility study for
the Grand Forks flood control project to determine top-of-levee elevations. It was not
intended to be used as a predictive tool in forecasting flood stages during an emergency.
... The Corps had no reason to believe that their rating curve was more accurate than the
official extended rating curve used by the NWS, until after measured flows at Grand Forks
exceeded the 1979 flow of 82,000 cfs, (stage 48.8 feet), which occurred early on the
morning of 17 April. ... The cities of Grand Forks and East Grand Forks were given the
maximum possible time known by either Federal agency to raise their levees, which was on
An interagency review by the NWS, USGS, and USACE to examine the extension of the
existing rating curves at all forecast points in the Red River Basin could provide an
efficient mechanism to improve forecast methods. Since most rating curves provided by the
USGS are based on historical records, an effort should be made to review all critical
rating curves and any additional information to see if a rating curve can be extended
based on information in flood insurance studies, USACE project studies, etc. This review
should also confirm that the RFC has current copies of all relevant engineering studies,
etc., for the basins in their forecast area.
Although a single-valued rating curve cannot define conditions completely, existing
single-valued rating curves for all points along the Red River of the North should
certainly be updated with new ones developed from the 1997 data. This should be
accomplished through the usual process of updates by the USGS based on the 1997 data.
To achieve maximum accuracy in stage forecasts at East Grand Forks, a considerably more
complex hydraulic routing model than the one available during the 1997 flood would be
needed. The implementation of such a model needs to establish the performance of the new
routing for a variety of historical floods, including the 1997 flood, and the sensitivity
of the routing model to real-time observations to account for ice effects and other
year-to-year variations. Since it will be used in real-time forecasting, stability of the
solution procedures under a wide range of flows must be assured as well.
An hydraulic analysis (see Appendix B) has been performed to
understand the hydraulic characteristics that led to the difference between the NWS
forecast stage at East Grand Forks based on the NWS simulated hydrograph using the initial
extended rating curve used by the NWS and the actual stage at East Grand Forks. The
analysis concludes that this difference (which totals ~3.8 feet) is due to the following
factors: the effects of the very mild channel slope which produced an unsteady loop rating
effect (2.0 feet), the effects of the bridges (0.8 foot), the fact that the NWS simulated
discharge hydrograph did not capture the peak discharge which occurred early in the event
(well before the peak stage) (0.4 foot), effects from the overtopping of the levees in the
Grand Forks area (negligible), and unexplained effects (0.6 foot). A detailed discussion
of these effects is contained in Appendix B.
|Finding 1: The complex hydraulic characteristics of the Red
River of the North at Grand Forks and East Grand Forks were difficult to model with the
NWS forecast methods in place during the April 1997 flood. This was the primary reason for
the forecast error at that location.
The NCRFC should immediately include new rating curves in the NCRFC modeling system as
soon as the USGS updates all single-valued rating curves for the Red River of the North to
reflect data from the 1997 flood.
The NCRFC should conduct an interagency review of all available data that might be
applicable to the hydraulics of the Red River of the North.
1C: The NCRFC should review the hydraulic study of this report for its
applicability to the existing forecast procedures at East Grand Forks and use it as the
basis to develop a more sophisticated model for the rating at East Grand Forks.
The NCRFC should develop a plan for implementation of dynamic routing procedures for
real-time forecasting for the entire Red River of the North.
The rating curve extensions performed automatically within OFS are required to complete
the forecast model execution but may lead to inaccurate stage forecasts. For example, on
April 14, NCRFC forecast a peak discharge of 110,000 cfs at East Grand Forks. Because this
was above the top of the USGS rating curve available at that time, the OFS used the
log-log extrapolation technique (as selected by NCRFC) to translate that discharge to
stage. Manual adjustments to the rating curve in East Grand Forks were made by RFC
forecasters later during the flood event (see Figure 2). The
Tulsa-Plot of the API Model used by the NCRFC to display forecast conditions on the Red
River of the North during the 1997 flood does provide a visual cue that the rating has
been extended but not an explicit warning message.
The rating curve extensions performed automatically within OFS are required to complete
the forecast model execution but may lead to inaccurate stage forecasts.
2: The OFS should provide a clear warning when a forecast goes beyond the top of
a rating curve so the forecaster is aware of it and can determine whether the extension is
The variable stage-discharge relation for the Red River of the North at East Grand
Forks is not unique. Such variable relations can occur on any river with very flat slopes
or reaches of rivers affected by inflows from major tributaries or tides. Acting together,
the NWS and USGS have the ability to conduct a joint review of all NWS forecast locations
to determine which ones could be affected significantly by variable stage-discharge
relations in future flood forecasts and whether the use of a looped rating numerical model
is warranted on the basis of improved forecast accuracy.
The potential for impacts of variable stage-discharge relationships on NWS river forecast
accuracy is not unique to Grand Forks.
3: The NWS and the USGS should add a parameter to their joint review of the
relationship between NWS forecast locations and USGS stream gage sites to identify those
sites where development of a looped rating is warranted for use in flood forecast
While the two scenarios of snowmelt peaks have been provided for many years, they lack
information that allows NWS users to better assess a reasonable level of risk. Further,
the user is provided with scenarios of zero and normal future precipitation; but a
scenario that reflects above average future precipitation is not provided.
NWSRFS supports the ability to develop probabilistic long-range forecasts through the
Ensemble Streamflow Prediction (ESP) process. ESP provides a frequency distribution of
future outcomes (e.g., peak streamflows) that can be sampled at any desired level of
exceedance and associated risk . The streamflow simulation model currently deployed in the
Red River of the North (Kansas City API) cannot provide an objective estimate of forecast
uncertainty and is not compatible with ESP.
Although it will require a multi-year effort and significant resources, the best method
to improve the Spring Outlook process is to make a series of significant investments in
NWS forecast procedures. These all point toward implementing ESP to produce the spring
snowmelt outlooks at several levels of exceedance probability. This would allow NWS
customers to have an objective basis to assess a reasonable level of risk and better
understand the uncertainty associated with the guidance values. Implementation of ESP will
require the calibration of a continuous streamflow simulation model; such as, the
Sacramento Soil Moisture Accounting (SMA) model. The NCRFC should develop a phased plan
for recalibration of the Red River of the North using a model that is compatible with ESP.
Consideration of river ice effects on stage will also have to be considered within an ESP
implementation used for Spring Outlooks.
Implementation of the Advanced Hydrologic Prediction System (AHPS) (demonstrated in
1997 in the Des Moines River Basin) which includes ESP and probabilistic flood inundation
mapping would provide additional information to users as to where flooding might occur in
cities along the Red River of the North.
The streamflow simulation model currently deployed in the Red River of the North (Kansas
City API) is not compatible with ESP.
4: The NCRFC should develop and execute a phased plan for recalibration of the
Red River of the North using a model that is compatible with ESP.
Surface Water Not Modeled
It was observed that much of the meltwater remained ponded on fields due to the very
flat terrain and snow and ice that initially blocked culverts until these temporary
obstructions melted. NWSRFS does not contain a hydrologic operation that will model the
temporary storage of meltwater that accumulates before conveyances open and allow movement
to the stream channel. This inadequacy was circumvented subjectively by manually reducing
the melt rate prior to runoff and then enhancing the melt rate once runoff began.
NWSRFS does not contain a hydrologic operation that will model the temporary storage of
meltwater that accumulates before conveyances open and allow movement to the stream
5: The Office of Hydrology should develop a method to model the temporary storage
of meltwater and add it to NWSRFS.
River Forecast Site Information (E-19) Needs Review
The NWS Form E-19 conveys site-specific information regarding river forecast locations
to forecasters who have often never seen the site for which they are forecasting. Accurate
and complete information in the E-19s is critical to the forecast process at RFCs and
WFOs. The E-19 information for the Red River of the North forecast points could be
improved and expanded to include additional information to aid forecasting (which is a
common issue for the entire NWS hydrologic services program). E-19s focus on the history
of floods and their impacts and on information about measurements at the site. E-19s
usually do not include detailed information on local structures and topography that might
affect floods at levels above the historical floods; e.g., at the time of the flood, the
E-19 for East Grand Forks, Minnesota, did not include a description of the five bridges
that cross the Red River of the North within the city. (The only reference to a bridge was
that the railroad bridge becomes inoperative at 50 feet.) The established flood stage of
28 feet at East Grand Forks is well below the level that causes damage in Grand Forks
itself, due to the levee protection that is in place, although at the 28-foot level
certain actions are needed in the city infrastructure and minor flooding may begin
The Eastern North Dakota office in Grand Forks should review and update the existing
E-19s for the Red River Basin, including the defined flood stages. These should include
updated impacts and photographs of the surrounding area. The plans for future use of the
E-19 information in digital form to control product formatting provides added impetus to
The E-19 information for the Red River of the North forecast points could be improved and
6: The Eastern North Dakota office in Grand Forks should review and update the
existing E-19s for the Red River Basin, including the defined flood stages.
Did Not Include Discharge
The NCRFC provided stage but not discharge values in its forecast products. In
conferences with the USACE and others, both stage and discharge values were discussed by
NCRFC staff (at least at some points). Including discharge in selected forecast products
would provide additional information to sophisticated users (emergency managers, city
engineers, USACE, etc.) to independently evaluate the assumed stage-discharge
7: The NCRFC provided stage but not discharge values in its forecast products.
||Recommendation 7A: NCRFC should include both stage
and discharge in information exchanged with water managers.
7B: All NWS RFCs should include both stage and discharge in information exchanged
with water managers.
Flows Not Modeled
At record flood levels, water may flow in areas where it has never been observed to
flow before, including overland flows of water across low points between two streams.
Transbasin flows were reported by observers in numerous locations during the 1997 Red
River of the North flood event. This information was useful qualitatively, but the
condition made forecasting more difficult. At a few locations where transbasin flows are
fairly common, the current forecast system has empirical procedures that estimate these
flows, but the current forecasting system cannot estimate these flows at locations where
they rarely (in some cases never before 1997) occur, and there was no quantitative
estimate of the amount of transbasin flow that could have been used to make adjustments in
the current forecast system. Even if a dynamic wave model were installed for the Red River
of the North, it would require significant additional effort to model transbasin flows
with geographic information system methods applied to high-resolution topographic
information (which is not currently available) to extend the model to include definition
of topographic features that allow transbasin flows to occur.
8: The current forecasting system for the Red River of the North cannot simulate
transbasin flows that occurred at the very high flood levels in the 1997 flood.
8A: NCRFC cannot realistically add physically-based transbasin flow simulation to
its current forecast procedures and should, therefore, continue to rely on empirical /
subjective estimates of the effects of transbasin flows for the near term.
8B: The feasibility of adding physically-based transbasin flow procedures to a
future advanced hydraulic model should be included in the plan for dynamic routing
recommended above. (See recommendation 1D.)
Surveys and April Blizzard Effects
The National Operational Hydrologic Remote Sensing Center (NOHRSC) conducts airborne
snow surveys which provide critical information for forecasting in the upper Midwest. In
1997, NOHRSC conducted seven airborne surveys over the Red River of the North Basin (see
timeline in Appendix A). This unusually large number of surveys
was the result of the exceptional snow conditions in the area. Two of these surveys are of
special note: The first survey, conducted on February 6-9, 1997, provided much of the
justification for the earliest outlook that characterized the potential for spring
flooding as "severe."
The blizzard that struck the Red River of the North Basin in early April dramatically
altered the hydrologic situation. Data collection and communications were seriously
hampered, and full assessment of the storm's impact on the hydrology was not possible for
nearly a week after the storm ended. Generally, crest outlook values would not have been
exceeded if this blizzard had not occurred. Responding to a request by the NCRFC, the
National Operational Hydrologic Remote Sensing Center (NOHRSC) of the NWS Office of
Hydrology terminated an airborne snow survey in progress elsewhere and collected snow
water data for the Red River of the North on April 9-12. The basin estimates provided to
the NCRFC on April 13 provided critical information. This was especially true since much
of precipitation data required by the model to simulate snow cover was unavailable as a
result of communication failures caused by the severe blizzard conditions.
The airborne snow survey program provides unique information that is critical to accurate
snowmelt outlooks and flood forecasts in the Red River of the North. The program was able
to respond quickly to a critical, special request to collect snow water observations.
9: The NWS should continue to set aside funds to support routine and special
airborne snow surveys for the Red River of the North Basin.
Current spatial snow water estimation procedures do not make full and optimal use of
all available information. Further, the spatial estimates for basins are provided to the
model as "perfect" data without regard to the potential for introduced bias or
variance. The NOHRSC is developing a Snow Estimation and Updating System for the Eastern
U.S. that will allow integration of the ground-based, airborne, radar, and satellite snow
data to derive snow water equivalent estimates and optimally update the snow model.
Current methods used by NCRFC to integrate snow observations into forecast models could be
10: The NCRFC and the NOHRSC should develop an implementation plan for the SEUS-E
procedure for the Red River of the North.