Crawfish Tales

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

David Reed, Hydrologist In Charge Vol. 1 No. 2, Winter 1997 Suzanne Van Cooten, Editor


Features Expanded


From the HIC
Happy New Year ! 1997 has been an exciting year. LMRFC has finally reached its modernized staffing level with 16 people. We started re-calibrating our soil moisture accounting model and have added a significant number of forecasts. Through hard work at the office, significant progress has been made in providing this information and data to the users via Internet.

I am excited about the possibilities and

opportunities for 1998. With the start of 1998, LMRFC began a second shift and will be open at least 16 hours a day. The Advanced Weather Interactive Processing System (AWIPS) will be installed in February and the increase in computing capabilities is welcome. We plan to continue our effort to re-calibrate our soil moisture accounting model. As you will see in this issue of Crawfish Tales, these are exciting times.
- Dave Reed


Expanded Operations
On January 4, 1998, LMRFC expanded its daily operations to two shifts per day and now provides 16 hours of continuous hydrometeorological coverage. During the winter/spring flood season from January through Alarm ClockMay, LMRFC will continue two HAS/two hydrologic forecasting shifts. Daily duty hours for the HAS function run from 6:00 AM to 10:00 PM with hydrologic forecast hours from 7:00 AM to 11:00 PM.

Beginning June ,

1998, the evening hydrology shift will be dropped while the evening HAS forecaster will continue full hydrometeorological coverage through 10:00 PM. The two HAS/one hydrologic shift will continue from June 8 until January, 1999 when dual hydrologic shifts will once again be implemented.

This expansion of service hours af Stage III afords LMRFC the opportunity to routinely update hydrometeorological/river forecasts as needed and presents an easy transition to 24 hour duty when required. As is normally done during flood situations, additional forecasters will be scheduled during evenings and through the night when deemed necessary. Furthermore, extended duty hours significantly increases continuous forecasting service to our family of WFOs and to external agencies especially during potential/existing flooding situations.


Stage III
Stage III is a precipitation software system used by the Hydrometeorological Analysis and Support (HAS) function to merge radar precipitation estimates and gage data across the entire LMRFC area. Stage III allows the forecaster to interactively quality control gridded precipitation data before it is processed into the hydrologic models. The output is a grid of hourly precipitation with a grid spacing of approximately 4 km X 4 km.

Stage III has two separate processing modes, the hourly and the post analysis mode. The hourly mode generates a merged field of gridded precipitation estimates for radar and

ground truth on a 1 hour time step. The post analysis mode compares summed hourly precipitation estimates from NEXRAD against observed 6 and 24 hourly precipitation. Features available in the post analysis mode allows the forecaster to merge summed hourly gage data into the final hourly precipitation product. Stage III at the LMRFC currently uses 26 WSR-88D Radars and combines this data with hourly gage data from several sources such as: GOES DCP, Alert, IFLOWS , TVA DCP, and ASOS. The output from Stage III is used in the NWSRFS hydrologic model and is being currently tested in southeast Mississippi on the Pascagoula River System.


Basin Calibration
The LMRFC is in the early stages of a major effort to calibrate the Sacramento Soil Moisture Accounting Model (SAC-SMA) for the nearly 250 basins defined and modeled in the LMRFC hydrologic forecast area. This effort will take advantage of new technology that will allow forecasters toquickly and accurately calibrate basins , resulting in more accurate river treesforecasts. Basin calibration involves the determination of 17 model parameters used by the SAC-SMA model to simulate runoff depth used in daily river forecasts. These model parameters are averaged over drainage areas of 100 to 200 square miles and can vary with time as changes in land use such as urbanization or deforestation occur within the basin. The model parameters are used to imitate the complex soil moisture process that must take into account the amount of precipitation available to the soil, the movement of the moisture through the soil, the amount of precipitation runoff that goes directly into the river channel, and the resultant combination to the total channel streamflow.

Historically, the NWS has used labor intensive

river procedures and the Manual Calibration Program (MCP) to process the large volume of data needed and calibrate basins. These procedures have been automated and take advantage of the graphical capabilities of scientific workstations the NWS now uses. The Interactive Calibration Program (ICP) is a powerful new tool for calibration that allows the user to fine tune the SAC-SMA parameters by graphically displaying simulated hydrographs created using the derived SAC-SMA parameters alongside observed historical hydrographs and archived mean areal precipitation data for a significant time period. With a thorough understanding of the model structure and knowledge of the effects of each model parameter on the physical processes occurring, a comparison of the simulated and observed hydrographs over the entire time period allows the user to remove any biases that exist between the simulated and observed hydrographs through the manipulation of the various model parameters. The LMRFC plans to have at least 20 basins calibrated by the end of Summer 1998.


Forecast Points
waterfallOver the last year, the LMRFC has made several additions to our forecast package. We are now issuing 5 day forecasts for the Amite-Comite and Atchafalaya River System in Louisiana. We have also begun to issue routine 3 day forecasts on the Black River at Acme in North Central Louisiana, and the southern Mississippi points of Black Creek at Brooklyn and the Pascagoula River at Grahams Ferry.

We have increased the number of locations where we do hydrologic simulations. By simulating stages and flows at these new points, we can shorten the reach for our hydrologic routing, resulting in improved forecasts at downstream locations. We are now modeling flows and stage but not issuing forecasts for the following points: 1) Bogue Chitto at Bush, LA; 2) Wolf River at Landon, MS; 3) Amite River at Grangeville and Magnolia Bridge, LA; 4) South Fork Holston River at Damascus, VA; 5) Little River at Townsend and Maryville, TN; 6) Toccoa (GA)- Ocoee River (TN) System to Dam #3; 7) Tangipahoa River at

Kentwood and Robert, LA; 8) Bayou Darbonne River System, LA; 9) Red Creek at Vestry, MS; 10) Black Creek at Wiggins, MS.

The LMRFC has been actively working to expand the number of forecast points through coordination with the appropriate Service Hydrologists. The Service Hydrologists are responsible for establishment of flood stages and development of E-19s for numerous locations. When these actions are complete, the NWS and the LMRFC will begin issuing forecasts at the following locations : 1) Mississippi River at Birdspoint, IL; 2) Mississippi River at Price Landing, MO; 3) Tuckaseege at Bryson City, NC; 3) Sunflower River at Anguilla, MS; 4) Tensas River at Newlight, Tendal, and Clayton, LA; 5) Boeuf River at Fort Necessity and Alto, LA; 6) Bayou Macon near Como, LA. Routine forecasts for these locations should start in 3-6 months.

cliffsWe will continue to enhance our forecast package and expand forecast segments in our model. The LMRFC will evaluate requests for additional forecast points in our area. Each request will be considered on a case by case basis.


MAP Climatology
Click here to go to the MAP Climatology technical attachment...

For the last 1 1/2 years, Quantitative Precipitation Forecasts (QPFs) have been provided to LMRFC by all of the Weather Forecast Offices (WFOs) to which we provide service. These QPFs are areal precipitation averages for drainage basins of 100 to 300 square miles. Since forecasters at WFOs are more familiar forecasting with point precipitation instead of mean areal precipitation (MAP) , the HAS unit at the LMRFC embarked on a MAP climatology project last year to help forecasters better understand the range of MAP values over a particular basin.

Using techniques available in the NWS River Forecast System (NWSRFS) , point precipitation is being collected, quality controlled, and processed over a period of 39 years, 1955 to 1993. For this climatological period, MAP is being computed from the point precipitation values using the Theissen method. Over the next year, MAP statistics and graphics will be generated for all of the basins in the

LMRFC area. Presently, frequency distributions for 6 and 24 hour MAPs as well as maximum/minimum MAP values are completed for basins in southeast LA and coastal MS. Each WFO will receive the statistical package for their HSA as the results are finished.

Mean Areal Precipitation statistics can be very useful to forecasters who issue 6 hour QPFs on a daily basis. Using the climatology as a guideline, forecasters will be able to see the probability of exceeding a particular MAP value and the return period of a specific MAP value for a basin. This climatology will allow forecasters to see how their QPF compares with previous events.

An example of a table generated for MAP Climatology is shown on the right. A complete overview of the LMRFC Climatology project, including MAP Climatology findings for two basins, can be seen in the LMRFC MAP Climatology Technical Attachment published in the Southern Region Topics dated October 15, 1997 or visit our web site link, Technical Attachments.


Internet Evolution

  The LMRFC home page continues to develop with an expanded range of products and new internet enhancements. New items now appearing online include two Hydrometeorological Analysis and Support (HAS) products, past issues of Crawfish Tales, and technical attachments produced by the LMRFC staff. Our internet enhancements make it even easier to access this new information through the development of a virtual domain and an interactive map of LMRFC associated Weather Forecast Offices.

Stage III hourly precipitation mosaics and the latest Hydrometeorological Discussion (HMD) are now available. The Stage III Gif images are produced by combining hourly gage reports and WSR-88D precipitation data. The Stage III online image product provides a visual representation of hourly precipitation patterns by allowing users to quickly identify areas of concern. The precipitation information is

  supplemented by the HMD text product which outlines areas of past and future 24 hour precipitation and river conditions including flood status.

Our new virtual domain allows you to locate the LMRFC home page easier. You can now find our site at The old internet addressing will continue to work, but we suggest that you update your bookmarks accordingly.

These improvements are only a few of many coming your way. Two areas of development are underway: one centers on an interactive basin map with point and click capabilities to acquire river forecasts and the other focuses on development of an online registration form (now online). Both areas of development center on feedback from our user community with all comments and suggestions highly encouraged. Hope to see you online. is the U.S. government's official web portal to all federal, state and local government web resources and services.