JP3.17INFORMATION AND PRODUCTS DERIVED FROM ENSEMBLE STREAMFLOW FORECASTS

Mary Mullusky1*, Mike DeWeese2, Edwin Welles1, John Schaake1

1 Hydrology Laboratory, Office of Hydrologic Development , National Weather Service, NOAA, Silver Spr ing, MD2 North Central River Forecast Center, National Weather Service, NOAA, Chanhassen, MN

1. INTRODUCTION1

In an effort to generate more informative streamflowforecasts, the National Weather Service (NWS) hasdeveloped an ensemble forecasting system. Numerousmethods of displaying the probabilistic forecasts havebeen examined for both their advantages anddisadvantages. The methods include those currentlyavailable to forecasters at the River Forecast Centers(RFCs) via the Ensemble Streamflow Prediction Analysisand Display Program (ESPADP) and those requested byforecasters and end users that are being used in ademonstration mode. Additionally, other methods ofdisplaying the forecasts have been analyzed. The endproduct of this research is a suite of displays that will bemade available to end users to assist in their decisionmaking process.

2. EXISTING OPERATIONAL PRODUCTS

ESPADP is used in conjunction with the NWS RiverForecast System (NWSRFS) Ensemble StreamflowPrediction (ESP) Model to provide interactive analysis ofESP time series. ESPADP has been operationallyavailable to the RFCs since August, 1997. ESPADPoriginally provided displays of four different plot types toanalyze the ESP time series; Trace Ensemble Plot,Expected Value Plot, Exceedance Histogram and theExceedance Probability Plot. Within each plot type theforecaster controls the accumulation settings and thefrequency settings for their specific application. Theaccumulation settings include the display window, thedaily accumulation, the accumulation interval and theaccumulation variable. The frequency settings include theprobability distribution and the probability levels foranalysis. With the four plot types and the range ofaccumulation and frequency settings, forecasters havethe ability to make a suite of ESP products for anystreamflow forecast point.

With the September 2002 release of NWSRFS R22,two new plot types were made operationally availablewithin ESPADP. The new plot types include a Non-exceedance Histogram and a Non-exceedance ProbabilityPlot. These new plot types allow the forecaster toanalyze and display low flow or drought conditions more

intuitively. Figure 1 shows the Non ExceedanceHistogram of weekly minimum river stages for theMinnesota River at Jordan, Minnesota. This exampleshows that in the second week the minimum flow has a25% chance of falling below 5.5 feet and a 75% chanceof falling below 6.3 feet. Figure 2 shows the Non-exceedance Probability Plot for the Minnesota River atJordan Minnesota. This example shows the probability ofthe minimum stage over the next 90 days falling belowcertain river levels for both the forecast simulation(triangles) and the calibration simulation (circles). Theforecast simulation begins each ensemble forecast withthe current conditions and the calibration simulationbegins at the beginning of the period of record. Thecalibration simulation is a method used by forecasters toassess model errors and biases. In this example, itrepresents an approximation of the probability distributionunder "average" conditions.

Figure 1: Non-Exceedance Histogram

Figure 2: Non-Exceedance Probability Plot

ESPADP can also be used to analyze precipitationand temperature ensembles. In this way the forecaster

* Corresponding author address: Mary Mullusky,

NWS/OHD/Hydrology Lab, 1325 East-West

High way , Silve r Spr ing, M D 20 910 ; e-m ail:

Mary.Mullusky@noaa.gov

can analyze the meteorological inputs that force therainfall/runoff models within NWSRFS and view theresulting streamflows. This can be especially useful whenforecasters integrate meteorological forecasts and climateoutlooks into the ESP system (Perica, 1998). All plot typesoperationally available in ESPADP for stage andstreamflow are available for precipitation and temperature.

3. DEMONSTRATION PRODUCTS

In addition to the operational products available inESPADP other products have been created to meet thespecific needs of forecasters and end users and areavailable as demonstration products. Thesedemonstration products include a River ForecastInterface, Probabilistic River Stages Plot, StageExceedance Probabilities Plot, and Probabilistic FloodForecasting Maps.

One early attempt to clearly display the complexity ofinformation available in the ensembles was the RiverForecast Interface developed at the University of Virginiaas a part of an NWS Eastern Region Probabilistic RiverStage Forecasting Risk Reduction demonstration project.This display, shown in Figure 3, provides the user threeviews of similar information color coded to makeidentification of likely flooding as simple as possible. Thedistributions are first displayed, followed by a bar chart ofthe distributions and then a mapping of the distributioninto the cross section of the river at the forecast point.Creative solutions such as this one are needed to makethe information in the ensembles accessible.

Figure 3: The River Forecast Interface

The Probabilistic River Stages Plot and the StageExceedance Probabilities Plot are simply two differentways of viewing the information provided by the ESP timeseries of stages at a certain forecast point. TheProbabilistic River Stages Plot for the Juniata River atHuntingdon, PA is shown in Figure 4. The plot shows therange of river stage exceedance probabilities at each

forecast time interval. Figure 5 shows the companionpicture, a Historical Flood Data Plot, which gives ahistorical perspective to the selected probabilistic riverstages. In this figure the forecasted 10-90% exceedanceprobability stages for the selected day are shown on astaff gage with the different action levels and record floodfor that forecast location. The Stage ExceedanceProbabilities Plot is shown in Figure 6 for the Sabine Riverat Fort Smith. This figure shows the probability ofexceeding predetermined river stages. For this examplethe predetermined river stages are the action stage andthe flood stage.

Figure 4: Probabilistic River Stages Plot

Figure 5: Historical Flood Stages

Another demonstration plot type used by the NWSare Probabilistic Flood Forecasting Maps. Probabilisticwater surface profiles are computed using the one-dimensional unsteady flow hydraulic model FLDWAV withthe ESP methodology. These water surface profiles arethen mapped with the NWS flood forecasting mappingapplication FLDVIEW to produce probabilistic maps of theexpected flood extent. Given an appropriate digital terrainmodel, a water surface profile, and the river centerline,

FLDVIEW will generate a flood forecast raster in shapefileformat which can be overlaid with other information (e.g.,orthophoto images, tiger maps, etc.). FLDVIEW cangenerate either a deterministic (instantaneous or peakcondition) or probabilistic flood map (Cajina et al., 2002).A probabilistic flood forecasting map was created for theJuniata River near Lewistown, PA and is shown in Figure7.

Figure 6: Stage Exccedance Probabilities Plot

Figure 7: Probabilistic Flood Forecasting Map

4. AHPS PRODUCTS INFORMATION TEAM

The NWS has established an agency wide team todefine a set of core products for the initial AdvancedHydrologic Prediction Service (AHPS) implementation.NWS managers leading AHPS have a vision of easyaccess to a consistent core suite of AHPS graphicproducts and information. The team's mission is to definea consistent core suite of graphical NWS hydrologicproducts and information which support the full spectrumof hydrologic services from flash floods throughextended-range river forecasts. Hydrologic informationincludes observed and forecast data. The team is also toestablish standards to ensure a) the content and format ofthe core suite of products and information are consistent

and b) consistent and easy access to these products andinformation.

Because the hydrology of the nation variessubstantially from region to region the admirable goal ofa single suite of products delivered at all forecast pointsfor all seasons may not be realistic. For example, watermanagers using reservoir inflow forecasts upstream of areservoir to make release decisions require different typesof forecast information than do emergency managersfighting a flood on an urban headwater basin. The initialcore suite of AHPS products will provide the flexibility tomeet user needs with appropriately selected information.Additionally, RFCs can customize products above andbeyond the core suite of AHPS products to meet theneeds of their unique customers.

NWS regional managers have already concurred ona single format for AHPS web pages and theimplementation across all NWS regions is underway. Thisformat follows that developed in the NWS Central regionand can be viewed at http://www.crh.noaa.gov/ahps/ . Asingle map of the entire region is displayed and users canselect the region of interest to see a more detailed maplisting forecast points. From the map of forecast points,products can be selected. In addition, after selecting aforecast point, the user can navigate up and down a riverreach by selecting the 'upstream' or 'downstream' buttons.This feature simplifies the navigation up and down streamfor the user and makes the boundaries between forecastoffices transparent to the users. By following a river fromup to down stream an interested user can follow thepassage of a flood wave through a region.

5. TRAINING

Numerous methods of displaying probabilisticstreamflow forecast information have been demonstrated.However, this by itself is not enough. The NWS must takean active role in training hydrologic forecasters onanalyzing and interpreting the resulting probabilisticdisplays. To accomplish this, hydrologic forecasters needa basic understanding of probability and statistics, and abasic understanding of ensemble forecasting includinghow climatological adjustment tools affect forecast results.Additionally, there is a need for forecasters and end usersto understand how to take advantage of the probabilisticforecasts to make the risk based decisions needed. TheNWS has been addressing this need for training in anumber of ways. Statistical and ESP workshops areavailable to forecasters yearly. A web based trainingmodule on ensembles is currently being developed. NWShas been active in outreach efforts including participatingin professional conferences and working with theacademic and research community on these issues. 6. CONCLUSIONS

Numerous methods of displaying probabilisticforecasts have been developed by the NWS in an effort to

generate more informative streamflow forecasts andassist end users in understanding and making risk-baseddecisions from these forecasts. The operational programESPADP, gives forecasters the ability to create a suite ofdisplays of probabilistic streamflow, stages, precipitationand temperature. At the request of forecasters and endusers, demonstration graphics have also been developedin an effort to make the probabilistic information moreunderstandable. In an effort to provide a consistent coresuite of graphical NWS hydrologic products andinformation, which support the full spectrum of hydrologicservices, the NWS has created the AHPS Products andInformation Team. Additionally, the NWS is involved intraining and outreach efforts to assist forecasters and endusers on interpreting and using probabilistic forecasts.

Another complicating factor in assessing the value ofcertain probabilistic displays is the distinction betweenforecasters and end users. Ultimately, the needs of theforecaster and the needs of the end user when viewingprobabilistic displays are inherently different. The NWSmust recognize these differing needs and create the toolsnecessary to accommodate both to provide successfulprobabilistic information.

7. REFERENCES

Cajina, N, J. Sylvestre, E. Henderson, M. Logan, and M.Richardson, 2002: FLDVIEW: The NWS FloodForecast Mapping Application. Preprints of the 18th

International Conference on Interactive InformationProcessing Systems (IIPS) for Meteorology,Oceanography and Hydrology, 82nd AMS AnnualMeeting, Orlando, Florida.

Perica S., 1998: Integration of MeteorologicalForecasts/Climate Outlooks into an EnsembleStreamflow Prediction System. Preprints of the 14thConference on Probability and Statistics in theAtmospheric Sciences, 78th AMS Annual Meeting,Phoenix, Arizona, 130-133.