Crawfish Tales

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

David Reed, Hydrologist In Charge Vol. 2 No. 3, Spring 1999 Ethan A. Jolly, Editor


Features LMRFC Station History Calibrating our River Basins Storm Surge Forecasting SHEF Coding


From the HIC

During the first quarter of 1999, LMRFC continued making progress to establish AWIPS as our primary forecast system. We have been able to accomplish a lot during this period since much of the LMRFC area has experienced below normal rainfall. All WFOs we provide forecasts to will have AWIPS by the end of April and changes in the way we support these offices will change to take advantage of AWIPS. We look forward to an exciting and challenging 1999 and the new millennium as AWIPS is commissioned. The staff at LMRFC and the NWS are up to the challenge. We look forward to serving all our cooperators and users during these exciting times.
- Dave Reed


LMRFC Station History

The LMRFC was commissioned and dedicated on January 11, 1972, as the eleventh CONUS River Forecast Center and began operations at the NASA Slidell Computer Complex. Initially, river forecasts were computed using the STANFORD soil moisture accounting model running on the NASA Sperry Rand mainframe computers. LMRFC was the first RFC to implement the continuous STANFORD Model area wide and later transitioned to the currently used Sacramento Soil Moisture Accounting (SACSMA) model during the late 1970s. Collection of river/rainfall data and river forecast dissemination was by a 60 word/min teletype system and by telephone. Model data entry was via an IBM keypunch/card reader. A network of cooperative observers with some limited telemark gaging provided the daily collection of data during this era

and included 450 rainfall and 125 river stage reports. During 1978, LMRFC became the first RFC to model a river system using the NWS Dynamic Wave OPERational (DWOPER) model for forecasting the lower Ohio/Mississippi .

 

During April, 1979, the LMRFC office moved to 1120 Old Spanish Trail becoming co-located with WSFO New Orleans. The NWSRFS forecast system execution progressed to a Remote Job Entry (RJE) system linked by a 9600 baud dedicated to three IBM 360/195 mainframe computers residing in Suitland, MD. AFOS was commissioned in 1981 and the DATACOL data collection system was brought online in 1982. Datacol eventually replaced the old keypunch/cardreader function and provided limited in-house batch processing with automated batch sends to the remote mainframes. Conversion of operations to total card image form was completed in 1986. During the mid-1980s, PCs were initially utilized for routine data collection, hydrologic development work, product generation, and dissemination to the AFOS system.

 

A change in LMRFC area of responsibility occured in 1979 with acquisition of the Sulphur/Cypress basins of NE TX from WGRFC. During the mid-1980s, construction of Smithland Dam on the Ohio River was completed and the LMRFC area of responsibility shifted downstream from Lock & Dam 50 (Fords Ferry, KY) to Smithland Dam, IL. These changes resulted in

the LMRFC area to remain approximately 200,000 square miles.

 

In January 1992, the RJE system was upgraded to a local LAN (Novell) system comprised of four 386 Pcs, replacing the old RJE hardware. On April 22, 1993, the modernized WSR-88D Principal User Processor (PUP) was installed and brought online. During August ,1993, the DATACOL system was replaced by the HYDROMET Data Collection and Dial-in/Dial-out system which resided on a Gateway 2000/486 PC.

 

On February 1, 1994, the LMRFC moved its operations to its current collocated site with WFO New Orleans/Baton Rouge (LIX) and the WSR-88D Doppler Radar; located at 62300 Airport Road. Dedication of the new site was on June 3, 1994, with featured guest speaker, the Honorable Robert Livingston.

 

The daily river/rainfall network has increased to over 2200 rainfall and 500 river gages, including automated and cooperative sites. NWSRFS model data input advanced to automated SHEF encoding/ decoding with incoming data supplied by AFOS, GOES data collection systems (DCPs), and automated dial-up gages. Product dissemination continues over AFOS with forecast generation advancing to automated digitized forecasts in SHEF code.

 

During June 1996, the RJE system was decommissioned and all programs/procedures were converted to in-house operations running on a local network of UNIX servers. The new Interactive Forecast Program (IFP) became the primary river forcasting tool. This local network would drive all LMRFC operations until January, 1999; when all operations were ported to the new AWIPS system.

 

On January 4, 1998, dual operational shifts commenced with the addition of an evening shift providing continuous coverage/support from 600am through 10pm, daily. Modernized staffing has been completed with a staff of sixteen consisting of the HIC, ASA, DOH, three HAS positions, plus a remaining staff of ten hydrologists.

 


Calibrating our River Basins

The Sacramento Soil Moisture Accounting Model (SACSMA) is a 16 parameter lumped hydrologic model used to estimate the runoff from a basin using basin average precipitation, evaporation, and snow data as model inputs. Calibration of the 16 parameters is necessary to get optimum model performance. In the calibration model, mean areal precipitation (MAP) and potential evapotranspiration (MAPE) are input values into the SACSMA. SACSMA model streamflow simulation is compared with observed stages or mean daily discharges.

 

The SACSMA requires mean areal precipitation for basin calibration. The National Climatic Data Center (NCDC) located in Asheville, NC is responsible for the long term archival of precipitation and synoptic data. Due to a large number of precipitation stations an inventory program is

 

used to assist in determining which precipitation stations are available in and around the chosen basin. All precipitation stations are considered, but only those who fit a specified criteria for location, period of record, missing data and data type are used in the actual calibration process. Daily MAP values are computed from daily and hourly point precipitation data in and around the basin. Pre-determined station weights are determined from either the grid point technique or Thiessen weighting method. Hourly precipitation is used to time distribute the daily MAP values to determine 6-hour MAP values for the basin.

 

Daily mean areal evaporation (MAPE) is also needed for basin calibration. MAPE is computed from point potential evapotranspiration (PET) estimated from meteorological parameters including wind speed and temperature. Like MAP, MAPE is an average of PET in and around the area.

PET and evaportranspiration (ET) are fairly conservative parameters in space. Only a few stations are required to accurately represent MAPE. Unlike PET and ET, point precipitation can vary greatlyspatially and many more stations are needed to accurately represent MAP when compared to MAPE. In the case of MAPE, a transfer program is used to retrieve meteorological parameters for synoptic stations and compute point PE using the Penman equation.

 

Stage and discharge data are used to determine the best fit of parameters. Mean daily discharge data are used to determine the best fit of parameters in the SACSMA model and ensure that the model performs a good water balance. Over the long term, the model simulates the same volume of runoff as is measured as flowing past a gaging site. In its simulation, the calibration program computes simulated instantaneous and mean discharges at the basin outlet to

compare available observed data obtained from the US Geological Survey. Soil moisture parameters of the model are then adjusted in a trial-and-error procedure until the set of parameters that provide the best simulation are determined.

 

The LMRFC is using an RFC area wide strategy to the calibration process. Thirteen headwater locations with a good set of precipitation data and long-term stage/discharge data are being calibrated to obtain good parameters. These parameters will then be used to calibrate hydrologically similar areas with an effort made to maintain consistency between these similar areas. Calibration of the SACSMA is a continuing process and with each complete calibration of a watershed the rainfall runoff model will perform more accurately.


Storm Surge Forecasting

As a hurricane makes landfall along the Louisiana Gulf Coast, its storm surge may inundate parishes along the Gulf Coast causing tidal waves to propagate upstream along coastal river systems. As a storm surge moves up the Mississippi River, stages can rapidly increase and significantly impact barge and shipping traffic along the Mississippi River. The Lower Mississippi River Forecast Center (LMRFC) in Slidell, LA, has teamed with the New Orleans/Baton Rouge Weather Office (WFO LIX) and the Tropical Prediction Center (TPC) to develop a procedure to automate the process for forecasting the effects of storm surge on the Mississippi River.

 

To support RFC storm surge forecasting, the TPC ran the Sea Lake Overland Surge Hurricane (SLOSH) model for 1640 different hurricane scenarios with varying storm speeds, directions, and intensities; producing an ensemble of forecast surge hydrographs for the Mississippi River at West Pointe ala Hache, LA. West Pointe ala Hache, 54 river miles below New Orleans, is the location where the continuous levee system ends on the Mississippi River. These hydrographs were placed on the LMRFC computer system in a format for use by the LMRFC hydrologic forecast models.

 

As a hurricane approaches the Louisiana coast and the area is placed under a hurricane watch, LIX selects five storm tracks from the TPC suite of tracks that best represent category, direction, speed, and time of landfall

 

of the approaching hurricane and provides this information to the LMRFC. These MSL stage hydrographs are used by the LMRFC as boundary conditions in the NWSRFS river forecast model and the Dynamic Wave Operation (DWOPER). For these five scenarios, LMRFC runs DWOPER and develops an envelop of forecast hydrographs at eight River locations from West Pointe ala Hache upstream to Red River Landing, LA, a total of 254 river miles. The major ports of New Orleans and Baton Rouge are included in this forecast list. The WFO LIX forecasters use this information to alert river interests along the lower Mississippi River on the possible effects of storm surge in LIX hurricane local statements.

 

If the Louisiana coast is upgraded to a hurricane warning, the TPC will provide the LMRFC a real-time surge hydrograph at West Pointe ala Hache based on the latest SLOSH model run and forecasted track of the hurricane; forward speed, direction, intensity, and time of landfall. The LMRFC will input this surge hydrograph into the DWOPER model to produce storm surge forecasts along the lower Mississippi River. The LMRFC will develop a set of stage/crest forecasts for the storm surge at the eight river locations and disseminate this information using routine forecast procedures. This information is provided to LIX forecasters and flood warnings may be issued which are consistent with LMRFC products. Coordinated forecast updates are provided to LIX when necessary.


Standard Hydrometeorological Exchange Format (SHEF)

The Standard Hydrometeorological Exchange Format (SHEF), developed by the National Weather Service in 1983, is a documented set of rules specifically for real-time use. It was designed for interagency sharing of data, visual and machine readability, and compatibility with anticipated receiving databases. The widespread implementation of SHEF allows the same decoding software to process data from various agencies. Decoded data are put into a standard record format so posting software can easily place the data in a database. New data sources and parameter codes can easily be added as they become available. The visual nature of SHEF allows users quickly to become familiar with it.

 

There are three formats that make up SHEF. Through the use of parameter code characters to identify the data, these three message formats have the flexibility to transmit a wide range of hydrometeorological information. The formats are as follows:

.A format is for single stations and is designed for transmission of one or more hydrometeorological parameters observed a various times.

.B format is for the transmission of one or more hydrometeorological parameters from several stations for which many or all of the parameters are the same and are observed at corresponding times.

.E format is used for the transmission of a single hydrometeorological parameter at an evenly spaced time interval for a single station.

 

River model databases at the LMRFC are populated by SHEF encoded data. Any agency or cooperator can download a complete copy of the SHEF documentation and/or a copy of the SHEF decoder from the following web site: http://hsp.nws.noaa.gov/oh/tt/soft/hsoft.shtml

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