Crawfish Tales

A Quarterly Publication of the National Weather Service Lower Mississippi River Forecast Center
Slidell, Louisiana

All future issues of the LMRFC’s Crawfish Tales will only be posted to our web site.
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David Reed, Hydrologist In Charge Vol. 9No. 3, Summer 2010 Glenn Carrin, Editor

Conversion from NWSRFS to CHPS at the LMRFC GFE and its Role at the LMRFC Flooding Concerns on the Pearl River Near Walkiah Bluff, MS

From the HIC

From the Hydrologist in Charge

The Hydrology Program and LMRFC are working to implement many new technologies to improve the services we provide. In this issue, we will focus on some of those changes. CHPS will fundamentally alter our forecast process and make it easier to collaborate and work with other federal agencies and the academic community. We have made and continue to make significant changes in our processing of rainfall. We are also looking in more detail at flood problems to provide additional services to those we serve. These changes promise to provide us the capabilities to provide better forecasts and services to our customers.



I would be remiss in not mentioning an upcoming major milestone for the LMRFC and local area. The five year anniversary of Katrina is later this month. I approach this milestone with the knowledge that the LMRFC is better prepared to continue operations when another major hurricane affects the local area. I am also hopeful that the pace of re-building along the Gulf Coast will continue.

We would love to hear from you. If there are additional services you need, please do not hesitate to contact us or your local NWS office to let them know of your needs.



Dave Reed

Conversion from NWSRFS to CHPS at the LMRFC

    The Lower Mississippi River Forecast Center (LMRFC) uses the National Weather Service River Forecast System (NWSRFS). This system includes a variety of hydraulic and hydrologic techniques and programs that handle everything from the initial processing of historical data to the preparation of river forecasts. When NWSRFS was developed in 1971, it was run on a mainframe computer and the code was streamlined to function with the limited computer resources of the day. Computer hardware and software development architecture have advanced in the last 40 years so that much of NWSRFS functionality is no longer necessary. NWSRFS requires a large amount of maintenance and is no longer cost effective to keep it in service.

Fig. 1. NWSRFS forecast display

      In 1997 the Office of Hydrologic Development (OHD) began the process of investigating solutions for the aging NWSRFS. After much research on the subject and testing of software, FEWS (the Flood Early Warning System), which was developed and maintained by the Dutch Company, Deltares, was chosen as the replacement for NWSRFS. The FEWS software is platform independent and offers a service oriented architecture that is modular like NWSRFS, but lends itself to more readily incorporate new modules and techniques. To make it usable for the River Forecast Centers, FEWS adapters were developed to use many of the existing hydrologic operations, techniques, and models from NWSRFS.

Currently CHPS is installed at the LMRFC but is not fully operational. Our staff is migrating all of the hydrologic and hydraulic data that is necessary to make a forecast from NWSRFS to CHPS. This process is largely done automatically by scripts but some local customization is necessary. Once migration to CHPS is complete and the model is running and stable, the LMRFC will begin parallel operations where we will produce forecasts using NWSRFS (figure 1) and CHPS (figure 2). During the period of parallel operations, model simulations from NWSRFS and CHPS will be compared to verify that similar results are being achieved. After a period of evaluation (approximately 1 year), NWSRFS will be retired and CHPS will be the operational forecast system used at the LMRFC.

Fig. 2. CHPS forecast display

After years of testing, evaluation, and customization, the River Forecast Center modeling system is getting an overhaul. NWSRFS is being replaced with CHPS, a newer modeling system that is used worldwide, that can be adapted to meet new needs, and can more easily incorporate new models to improve our forecasts in support of our mission to protecting life and property.

- Kai Roth

GFE and its Role at the LMRFC

    Each day, meteorologists at the LMRFC use software called the Graphical Forecast Editor (GFE) to produce a Quantitative Precipitation Forecast (QPF). For our purposes, QPF is an areal forecast of how much rainfall is expected over the next few days, in six-hour time steps. GFE ingests gridded data from several sources, including the Hydrometeorological Prediction Center and several meteorological models (Figure 1). The actual values for each grid point are then viewed using the graphical on-screen editor. A set of tools included in the GFE software allows the forecaster to manipulate the gridded data to produce detailed graphical depictions of future precipitation events. The great detail that this software provides, combined with its editing capabilities, allows the forecaster to easily modify the QPF using their judgment of current and forecasted meteorological conditions.

Fig. 1. GFS model data being viewed in the GFE.

The LMRFC routinely issues QPF out to 72 hours at the 00Z and 12Z synoptic times, and out to 54 hours for the 06Z and 18Z synoptic times. It is these GFE-generated QPFs that drive the hydrologic forecasting process. The QPFs are used to initialize our river models. Normally, our river forecasts use the first 12 hours of QPF. However, in anticipation of significant precipitation events, the models may be initialized using the first 24

hours of QPF, or more. Thus, having a high-quality, accurate QPF is very important to better defining and improving our hydrologic forecasts.

Data produced by the GFE is also being used in the next generation of NWS hydrologic modeling, the Community Hydrologic Prediction System (CHPS). Each morning, a local computer script runs and creates the necessary fields (i.e., the average 24-hour temperature and dew point, etc.) needed to calculate a potential evapotranspiration value. Since this is done in the GFE, there is a value calculated for each point in the grid (Figure 2). These gridded data are sent to CHPS, where internal functions within the system are used to create mean areal values for each of the basins. These values are then used as forcings for the hydrologic model.

Fig 2. Potential evapotranspiration display in GFE.

GFE is an essential piece of software used in the LMRFC’s day-to-day operations. Its gridding capabilities allow the forecasters to enhance and improve the forecast process as science and technology continue to evolve. If you have any questions on the GFE or QPF process, please contact one of the HAS forecasters at (985) 641-4343.

- Jessica Smith


Flooding Concerns on the Pearl River Near Walkiah Bluff, MS

    Walkiah Bluff is located in southern Mississippi on the Pearl River, which splits at Walkiah Bluff. The east branch becomes the East Pearl River, forms the state boundary between Mississippi and Louisiana, and empties into Lake Borgne. The west branch feeds Wilson Slough which soon joins the Bogue Chitto River to form the West Pearl River and empties into the Rigolets (Fig. 1).

Approximately 175 residents along the Pearl River near Walkiah Bluff experience flooding due to high water from the Pearl River. The LMRFC has agreed to provide guidance on flooding in this area and has developed several tools to assist with such guidance.

Historically at low flows, approximately 75% of the flow in the Pearl River passed through Wilson Slough to the West Pearl River, with only about 25% feeding the East Pearl. During extended periods of low flow, the East Pearl would have little or no flow. The Walkiah Bluff diversion was constructed in 1999 to increase discharge and water levels in the East Pearl near Walkiah Bluff and decrease flows in Wilson Slough and the West Pearl during low flow conditions. The structure is an earthen, trapezoidal weir.

Fifteen historical flood events were analyzed to obtain a better understanding of how Walkiah Bluff reacts in relationship to flooding upstream on the Pearl River at Bogalusa, LA and the Bogue Chitto River at Bush, LA, as well as downstream impacts on the Pearl River at Pearl River, LA.

The main contributor of flow at Walkiah Bluff is the water routed down from the mainstem of the Pearl River. However when the Bogue Chitto River provides a significant portion of the flow, it can have considerable effects. The amount, timing, and distribution of routed water from the Pearl River at Bogalusa and the Bogue Chitto at Bush control the shape of the Walkiah Bluff hydrograph.

The Bogue Chitto River flows roughly southward, paralleling the Pearl River to the east and Pearl River canal to the west. Small sloughs and bayous connect the Pearl and Bogue Chitto Rivers above Wilson Slough allowing some water to be transferred between the two rivers, especially when the Bogue Chitto River is extremely high. During these events, the Bogue Chitto can cause a rise in the tailwater behind the weir structure, which lessens the flow through the weir and in turn allows more water down the East Pearl River. If the weir structure is overtopped, open river conditions exist on the Pearl and the water level differential between Wilson Slough and the Pearl River is reduced or absent. The water will spread beyond the channel, and a greater volume of water is needed in order to see a significant rise in stage.

The affects of the Bogue Chitto on the crest at Walkiah Bluff are complicated. To alleviate this complication, forecasts at Walkiah Bluff are based on the relationship of the crests at Walkiah Bluff and the West Pearl River at Pearl River, LA downstream of Walkiah Bluff.

The crest at Walkiah Bluff generally occurs a day and a half to two days prior to the crest at Pearl River, LA on the West Pearl River. The Walkiah Bluff hydrograph strongly mimics that of Bogalusa, but additional rises have also been noted when water from the Bogue Chitto causes a rise in the tailwater at the weir or when the weir is overtopped. A significant difference is noted in the response of the river when the top of the bank at the weir is overtopped (42.65’ NGVD 29).

WFO New Orleans/Baton Rouge has flood warning responsibility for Walkiah Bluff and the Pearl River. The LMRFC is working with the WFO to provide forecast and warning services to those in the local area.




-Amanda Roberts


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