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. 4, Summer 1998 Ethan A. Jolly, Editor


 

Features AWIPS
Activities
Part II
DWOPER Flash Flood
Guidance
New Forecast
Points


 

From the HIC
LMRFC continues to work to implement AWIPS and take advantage of the new technology. Many of the needed applications to support operations are available on AWIPS. After establishing reliable communications, LMRFC is able to prepare forecasts on AWIPS and will run parallel systems for a year.

In the LMRFC area, forecasts are now being issued at more locations. Additional forecasts are now availbale for northeastern LA and northern MS and we now issue 5-day

forecasts at many locations we previously only issued 3-day forecasts. We expect to add more forecast locations to meet public needs. LMRFC will also begin using precipitation estimates from the WSR-88D this fall. These estimates, when combined with ground-truth data, should improve our forecasts.

As you can see, this summer has been busy. We look forward to continuing to provide high quality products and services to all our users.

- Dave Reed


 

AWIPS  Activities - Part II
In the last issue of "Crawfish Tales" we discussed the forecast components in AWIPS that the Hydrologist uses exclusivly at the River Forecast Center (RFC). In this issue we will go over two more aspects of AWIPS, the forecast components that both the RFCs and Weather Forecast Offices (WFO) use together to create hydrologic products, and the communication network of AWIPS.

THE FORECAST COMPONENTS

At the River Forecast Centers, the work stations will be the main interface between hydrologists and the rest of the AWIPS system. NWS hydrologic forecasters will spend the majority of their time at the work stations interpreting and analyzing data, and preparing river forecast products for transmission. These forecasters will view a wide variety of hydrometeorological data through the WFO Hydrologic Forecast System (WHFS) program.

This program was developed by the Office of Hydrology (OH) to help the hydrologists and meteorologists at the RFCs and WFOs manage and apply operationally the vast array of data through a series of graphical user interfaces associated with AWIPS - back officeseveral different applications. AWIPS/WHFS currently includes an integrated environment which provides a database maintenance tool (HydroBase), a data viewer application (HydroView), and an application for automatically generating the public hydrological forecast products (RiverPro). Future versions of WHFS will include applications for modeling flash flood areas (Area Wide), calculating stage data and allowing input of QPF information (Site Specific), and providing dam break information (Dam Break/Dam Catalog).

HydroBase is an interface to help manage database information used by the various WFO Hydrologic Forecast System Programs. Access to HydroBase is restricted because of the importance of these managed data. The Service Hydrologist and/or Hydrologic Focal Point is responsible for maintaining the database. Much of the parametric data utilized by the forecast applications are also defined through HydroBase, eliminating the requirement for forecasters to manipulate these data during operations. Since the same information is needed at both the WFO and RFC it is logical that they both utilize the same applications to view and update data. The major difference is that the RFC database will contain information from all the WFOs in their service area.

HydroView is the primary operational interface to the data contained in the hydrological database. It provides the

forecaster with a geographical window showing the river stage and precipitation at locations in the WFO area of responsibility. Operational data such as observed and forecast values can be displayed in graphical or tabular form. Data are updated every 15 minutes from the Hydrologic Database so the forecaster is always working with the most recent information. The data can be manually refreshed more often, particularly during rapidly changing hydrologic conditions.

RiverPro analyzes forecast Awipsand observed data, then determines derived values from stage time-series data. Using this derived information, the program automatically determines a recommended product to issue (for example, FLW) and the recommended forecast points to include.

The forecaster can either accept the recommendations or specify which product to issue and which forecast points to include in the statement or warning. RiverPro reads the instructions, which can be customized for generating the product. Then, the product can be generated, reviewed, edited and issued.

THE COMMUNICATIONS NETWORK

A communications network will feed data to each AWIPS site, distribute information among the AWIPS site, and provide for dissemination of information to the public and other outside users. A one-way, point-to-multipoint satellite broadcast service called NOAAPORT will be used to distribute the very large amounts of data products which are collected/produced at NOAA central facilities. Data distributed via NOAAPORT will be accessible not only at all NWS sites but also by any appropriately equipped ground station operated by private sector organizations, universities, etc.

In addition to NOAAPORT, the AWIPS sites will be interconnected by a high speed data network of land based communications lines. This network will allow two-way, point-to-point communications between the River Forecast Centers (RFCs) and the Weather Forecast offices (WFOs) thus facilitating the generation and dissemination of hydrologic warnings and forecasts in a rapid, highly reliable environment.


 

DWOPER
Dynamic Wave OPERational (DWOPER) Model for stage/flow forecasting along the lower Ohio/Mississippi River complex. DWOPER was developed in 1976 by Dr. Danny L. Fread, Director of the NWS Office of Hydrology, and has been used operationally at LMRFC since 1977. The one-dimensional, unsteady flow model produces solutions to the conservation of mass and conservation of energy equations. DWOPER allows for simultaneous propagation of waves both upstream and downstream; and, provides solutions to complex backwater and hurricane surge problems.

There are a total of 1,202 river miles modeled; consisting of 141 channel cross-sections and 40 forecast-to-forecast channel sections or reaches. Upstream model boundaries are observed and forecasted discharges located at: 1) Ohio R. at Smithland Dam, IL, 2) Cumberland R. at Barkley Dam, KY, 3) Tennessee R. at Kentucky Dam, KY, and 4) the upper Mississippi R. at Chester, IL. The main channel is defined as running from the Ohio River at Smithland Dam, down the Mississippi River, and terminating in the Gulf of Mexico. The three remaining channels are modeled as dynamic tributaries; allowing for complex backwater interactions between the four river systems.

The downstream boundary is stage and is set to 0.0 feet MSL, arbitrarily positioned in the Gulf of Mexico. The upper Mississippi from Chester to the Ohio River confluence is modeled as a dynamic tributary to the main channel. Other tributary inflows and diversionary outflows, such as the Arkansas River and the Old River Control Structure, are currently modeled as channel flow additions/subtractions. Off-channel or dead storage areas are also defined and utilized by DWOPER.

Five-day forecasts for 23 locations are produced daily using guidance from DWOPER simulations. Daily mainstem forecasts are issued to the public under NWS product, NEWRVASIL. LMRFC is in the process of re-calibrating DWOPER using 1995 observed stage/flow data and updated channel surveys provided by the Lower Mississippi Division and Ohio Division of the Corps of Engineers. Plans are to expand DWOPER to include the Arkansas River, at and below Pine Bluff, AR, and the Yazoo River, at and below Greenwood, MS; as dynamic tributaries. Daily coordination with external agencies and upstream RFCs play a vital role in the lower Ohio Mississippi River forecast process.


 

Flash Flood Guidance
Flash Flood Guidance (FFG) is an index representing the average amount of rainfall required over an area during a specified time interval to initiate flooding on small watersheds. FFG, which was first produced by RFCs in the mid 1970's, is routinely issued for 1,3,and 6 hour time periods for most areas of the country. For some locations, FFG is issued for 12 and 24 hour time periods. Pre-determined threshold runoff values are derived from regional studies or site specific basins and estimate the amount of runoff required to produce flooding in small streams. Using existing soil moisture conditions, the amount of rainfall necessary during a certain time interval to generate the threshold runoff for a particular area is computed. The LMRFC issues FFG products twice a day by late morning and late evening for every county/parish in the hydrologic forecast area.

FFG forecasts produced by RFCs are used by forecasters at National Weather Service Forecast Offices as the initial step in determining if conditions warrant the issuance of flash flood

watches or warnings. Forecasters at these offices compare FFG with Quantitative Precipitation Forecasts (QPF) to determine if a flash flood watch should be issued. If observed or estimated rainfall for a specified time period exceeds FFG values, the office will issue a Flash Flood Warning.

Currently each RFC has its own method and format for computing and distributing flash flood guidance values. In the modernized National Weather Service, a nationally supported flash flood guidance system will calculate FFG values for a uniform grid. Gridded threshold runoff values will be calculated for each RFC using a geographic information system and a simplified hydrologic model. The result will be improved gridded FFG values that are consistent across RFC boundaries. These can be used in conjunction with WSR-88D Doppler Radar precipitation data to evaluate the flash flood using both observed precipitation accumulations and precipitation projections.


 

New Forecast Points and Model Enhancements
Since the first of the year, the LMRFC has made several additions to our forecast package. We are now issuing routine 5 day stage and flow forecasts for the Red River mainstem in Louisiana. We also began issuing routine 3 day forecasts for six points in northeast LA. They are as follows: Boeuf River at Alto (ALGL1) and Fort Necessity (NECL1); Tensas River at Tendal (TDGL1), Newlight (NLGL1), and Clayton (CYNL1); and Bayou Macon near Como (COGL1). We have also begun to issue routine 3 day forecasts for the Big Sunflower at Anguilla, MS (ANGM6).

Expansion of our forecast model has continued at a rapid pace. We are now modeling flows and stages but not issuing forecast for the following points: Tuckasegee at Bryson City, NC (TKSN7); Oconaluftee at Birdtown, NC (BIRN7); Bogue Phalia at Leland, MS (LBGM6); Big Sunflower at Little Callao Landing (LCGM6) ; Bogue Homo River at Richton, MS (BHRM6); White Sand Creek at Oakvale, MS (OVWM6); East Hobolochitto

Creek at Carrier, MS (MNLM6) and the West Hobolochitto near McNeil, MS (CREM6); Escatawapa River near Agricola, MS (AGRM6); North Fork Obion at Palmersville, TN (PLMT1); Middle Fork Obion at Dresden, TN (DRST1); South Fork Obion at Mckenzie, TN (MKGT1); Rutherford Fork Obion at Milan, TN (MLGT1); Obion at Obion, TN (OBNT1); Big Black at Kilmichael, MS (KILM6) ; Pelahatchie Creek at Fannin, MS (PECM6); Bayou Macon at Eudora, AR (EUDA4); Tchefuncta River at Folsom, LA (FLSL1); and Little Missouri at Murphreesboro, AR (MLMA4). Quantitative precipitation forecasts (QPF) and mean areal precipitation (MAP) are now computed for the entire Tennessee Valley.

The LMRFC will continue to expand forecast services and enhance modeling efforts. Evaluation of requests for additional forecast points in the LMRFC area will be on a case by case basis.


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