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

David Reed, Hydrologist In Charge Vol. 6 No. 3, Spring 2003 Ethan A. Jolly, Editor

Features New Products for the Web: Precipitation Analysis and Flash Flood Guidance Know Your River Systems: West Tenn. & St. Francis Rivers

From the HIC

We are nearing the end of an active flood season. Kudos to the LMRFC staff for their hard work and efforts during this period. They have done a GREAT job in providing quality products and services to our customers and partners during this busy time.

As we move out of our typical flood season, we will shift much of our focus to development activities to improve hydrologic model performance and enhance our methods of disseminating information and data to our users. We will continue to improve the hydrology defined in our hydrologic models and prepare data for contractors to calibrate our models through support from the Advanced Hydrologic Prediction Services (AHPS) in FY 2004.

As you can see in this issue, we have implemented some new and improved products on our website for all of Southern Region. A team of developers from all four Southern Region RFCs has joined together to prepare the two suites of products described in this issue. I urge you to review these products and provide feedback so we can continue to improve. Congratulations to the team members who have made this happen and will continue this work over the next few months on additional products.

While placing more emphasis on procedure development efforts, we will continue to keep a “sharp eye” on hydrometeorological conditions that may produce flooding. Hurricane season will start on June 1so the threat for flooding is always highduring this period.

- Dave Reed

New Products for the Web: Precipitation Analysis and Flash Flood Guidance

Southern Region (SR) River Forecast Centers teamed up to produce region-wide graphics of Flash Flood Guidance (FFG) and precipitation products. These products are currently experimental and show an overview of Southern Region with drill-down capability to each SR River Forecast Center and state (Note that in addition to a state-wide graphic for Texas, graphics are available for North, South, and West Texas.). The ultimate goal is to produce regional mosaics of all hydrologic products so that customers can find information quickly and easily without having to visit several web sites.

Figure 1: Southern Region (SR) 3 hour Flash Flood Guidance
Southern Region (SR) 3 hour Flash Flood Guidance
Click for larger image

For each of the areas listed above, the following graphics are created: 1, 3 and 6-hour FFG (figs. 1 & 2) with 12 and 24-hour FFG (where generated); radar-derived rainfall for the previous 24 hours ending at 12Z; the last 7 and 14 days; month-to-date and year-to-date (fig. 3); and an archival of monthly and yearly radar-derived rainfall. Graphics also produced include normals (fig. 4), percent normal (fig. 5) and departure from normal rainfall (figs. 6 & 7) for all periods except the past 24 hours ending at 12Z; and a 4-panel summary graphic for all periods except the 24 hour total at 12Z.

Figure 2 : 3 hour FFG drill down to state (AR)

Click for larger image

Precipitation graphics and derivations are automatically created using off the shelf GIS software and mosaiced from quality controlled Stage III gridded rainfall estimates. Stage III gridded datasets are derived from mosaiced NWS radar rainfall estimates that have been adjusted with hourly rain gage reports. Hydrometerologists at the River Forecast Centers continuously quality control incoming gage data and adjust radar biases to come up with the best possible Stage III rainfall estimates.

Departure from normal products are computed using GIS software by comparing gridded Stage III rainfall totals (4x4 km grid) to the normal rainfall grid (4x4 km grid). Grids of normal rainfall are obtained from the PRISM (Parameter-Elevation Regressions on Independent Slopes Model) data sets. More information on these data can be found at or Precipitation graphics are updated twice daily on the web site after 10:00AM and 4:00PM.

The Flash Flood Guidance graphics are also automatically generated in GIS software from the text-based products sent out to the web site. These graphics are updated as new FFG becomes available. Links to these graphics are available on each SR River Forecast Center front page or can be found directly at: and

Figure 3 : SR month-to-date observed precipitation
SR month-to-date observed precipitation
Click for larger image
Figure 4 : SR normal rainfall from PRISM data set
SR normal rainfall from PRISM data set
Click for larger image
Figure 5 : SR month to date % of normal rainfall
SR month to date % of normal rainfall
Click for larger image
Figure 6 : SR month-to-date departure from normal
SR month-to-date departure from normal
Click for larger image
Figure 7 : Louisiana (drill down) month-to-date departure from normal
Louisiana (drill down) month-to-date departure from normal
Click for larger image

- David Welch and Keith Stellman

Know Your River Systems: Central Tennessee

The rivers in West Tennessee cover a drainage area of 27,000 km2 bounded by the Mississippi River to the West and the Tennessee River divide to the east. The area lies within the Mississippi embayment and is part of the Gulf Coastal Plain Province. The upper portion of the St. Francis river system overlies the hilly terrain of the Ozark Plateau physiographic province, whereas the lower portion lies in low-lying flood plains of the Mississippi alluvial plain province.

Figure 1: West Tennessee and St. Francis River Systems
West Tennessee and St. Francis River Systems
Click for larger image

The figure to the right shows the major rivers in western TN including the Obion, Forked Deer, Hatchie, Loosahatchie, and Wolf rivers. Also shown are the St. Francis, Little and L’ Anguille rivers of the St. Francis river system in northeastern AR and southeastern MO. Much of the land in west TN is relatively flat and has been developed for agriculture. Rivers flow through loose, unconsolidated sediments and respond quickly in the upper reaches typically cresting within 12 to 24 hours. Forecast points in lower reaches respond quickly to local runoff, but take 4 to 6 days to crest with water coming in from upstream. Forecast points in lower reaches also experience backwater effects when the Mississippi river is high. The upper reach of the St. Francis River and its tributaries above Wappapello reservoir lie in hilly terrain and respond quickly cresting in about 24 hours or less. Forecast points downstream of Wappapello lie in flatter, agricultural lands and take longer to crest. River channels along these lower reaches are also affected by canals, ditches and irrigation structures developed for agriculture.

Table 1: Record and Recent Flooding
River Location Flood
Stage (ft)
Record Flood
Recent Flood
Obion (N. Fork) Martin, TN 20 23.1
Obion Obion, TN 34 40.4


  Bogota, TN 22 31.2
N. Forked Deer Dyersburg, TN 22 30.9
S. Forked Deer Jackson, TN 34 37.6
  Halls, TN 9 15.2
Hatchie Bolivar, TN 18 21.7
  Rialto, TN 20 25.1
Wolf Germantown, TN 20.5 28.0
St. Francis Patterson, MO 16 35.7
  Fisk, MO 20 28.0
  St. Francis, AR 22 28.2
  Lake City, AR 10 13.3
  Madison, AR 32 41.8
Little River Rivervale, AR 11 13.6
L’ Anguille Palestine, AR 25 39.7

- David Welch is the U.S. government's official web portal to all federal, state and local government web resources and services.