Crawfish Tales

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

David Reed, Hydrologist In Charge Vol. 3No. 1, Autumn 1999 Ethan A. Jolly, Editor

Features Y2K Readiness XSETS - New Forecast Format The Great Mississippi River Flood of 1927 National Climate Data Online

From the HIC

Thanks to all our readers for completing our survey on the Crawfish Tales. We appreciate all the comments. This will help us to provide you more of the information you need. We will begin to highlight flood events, both immediately after they occur and historical floods. The Great Mississippi Flood of 1927 is highlighted in this edition.



We are nearing the end of hurricane season and have been fortunate to not have had a hurricane in the LMRFC area. We are preparing for the upcoming flood season by making adjustments to our forecast model. We are also making the final changes to use AWIPS as our primary forecast tool. These changes will allow us to make forecasts more accurately and in a more timely manner.
- Dave Reed

Y2k Readiness at the LMRFC

The entire National Weather Service (NWS) including the Lower Mississippi River Forecast Center (LMRFC)have been working since 1996 to ensure all its systems are Y2K-ready so there is no disruptions to operations when the new millennium arrives. In addition, other Y2K date problems such as the leap day (February 29 in 2000) have been tested and found to cause no problems.

The systems which collect hydrologic and meteorologic data and the systems which generate and disseminate forecasts at the LMRFC and NWS-wide have all been tested and are Y2K compliant. This includes the Advanced Weather Interactive Processing System (AWIPS), NWS River Forecast System (NWSRFS) and numerous local programs at the LMRFC. Both the LMRFC and NWS have a great deal of confidence there will be no interruption of our services to the public or other customers.

The NWS has worked with telecommunications and power companies which the NWS rely upon to ensure they are also Y2K ready. All NWS offices are equipped with HAM radios, cell phones, and emergency power generators as back up.

In addition to successful tests on individual systems, the NWS conducted a series of integrated end-to-end tests in early 1999. In this test, all of its systems were connected to ensure all systems are able to exchange and process hydrologic and meteorologic data properly when Y2K occurs. The tests included many of the LMRFC and NWS partners and customers and were successful as all systems functioned properly, with no anomalies found.

- Eric Jones

XSETS - the new standard for river forecast products

On November 15, 1999, LMRFC officially changed to a new RVF (river forecast) product format using the XSETS (Shef Encoded Time Series) software. XSETS, developed by the Arkansas Basin RFC, is supported nationally and makes full use of the AWIPS hydrologic software. This eliminates the need to maintain site-specific software at LMRFC and helps to standardize RVF products across River Forecast Centers.


The format of each forecast has changed but the content remains the same. At this time, forecasts will continue to be issued in 24-hour time steps. However, river forecasts are now being .ER SHEF encoded in contrast to the previous .B code.


The following changes became effective on the above date: 1.) the SHEF-encoded 7AM observed river stages in the RVF product have been dropped. The latest available observed stage, is, however, provided in commented form; 2.) multiple versions of the same RVF product per WFO (e.g., JANRVFJAN) are transmitted each day. The content of each RVF version now contains forecasts for only a specific river basin or Forecast Group (FGROUP) as is defined in the LMRFC NWSRFS model. Users will have to review previous versions to see all river/flood forecasts issued to a single

WFO. For example for WFO JAN, we issue a separate JANRVFJAN product with forecasts for only the Lower Yazoo basin; other versions contain forecasts for the Upper Pearl, Lower Pearl, Pascagola, Big Black, and Lower Ouachita basins (i.e., 6 versions); 3.) flood only points are not included in the RVF unless that location is expected to reach or exceed flood stage within the forecast period; 4.) crest information is shown on a separate line in .AR SHEF code. The date/time of above/below flood stage are provided as comments; 5.) flood stages and warning stages (if available) are shown as comments; 6.) basin average QPF is .E SHEF encoded in 6-hour amounts for the periods ending at 18Z, 00Z, 06Z, and 12Z; and, 7.) a Y2K compliant Date Creation (DC) code is now used having the form DCyyyymmddhhmm.


This software change enables us to prepare and disseminate forecasts in a more timely fashion; and, allow us to better serve you, our customers. The hypothetical flood forecast below is for the FGROUP Upper Pearl Basin in Mississippi showing proper flood coding and with appropriate comments. The last line of code shows the 6 hour QPF values for the Jackson basin.

- Bob Stucky

Example of XSETS for the Pearl River at Jackson, MS:
:LATEST STAGE 25.86 FT AT 1048AM CDT ON 1228
.AR : CREST : JACM6 1230 C DC199912281115/DH13/HGIFFX 29.0
.ER JACM6 1228 C DC199912281115/DH07/HGIFF/DIH24
:FORECAST at 7AM ON 1229 1230 1231
.E1 / 28.6/ 28.9/ 27.8
.ER JACM6 1228 C DC199912281115/DH13/PPQFZ/DIH6 0.53/0.32/0.19/0.00

The Great Mississippi River Flood of 1927 - A Historical Perspective

Introduction. In 1927, one of the greatest natural disasters of the United States occurred with the great Mississippi River Flood. This flood, a result of above average rainfall from August, 1926, through April, 1927, over the entire Mississippi River drainage of over 1.25 million square miles, had a tremendous impact socially and on the commerce of the United States and was a watershed event in flood control.


The Rainfall. Above average rainfall in late Summer of 1926 soaked much of the Mississippi Basin. During the Fall, heavy rains produced above average flows on the Mississippi during a period of normally low flows. In December through April, an average of over 12 inches of rain fell over the entire Mississippi Basin, more than 2 inches above normal. The climax came in the second week of April when southern Missouri and most of Arkansas received over 9 inches of rainfall. During one day that week, New Orleans received over 14 inches of rain in just 19 hours. This excessive rainfall fell on already saturated grounds. The runoff produced added to already swollen rivers to produce one of the most devastating floods of all time.


Flood Crests and Flooding. Above average flows were recorded during the Fall of 1926 on the Mississippi River. In December, record flooding occurred in the Tennessee and Cumberland Rivers. The Ohio River and the Red River flooded in January. A flood crest from the Upper Mississippi moved past the confluence of the Mississippi and Ohio Rivers in March and now all major tributaries were contributing above normal flows. From excessive rainfall the first three weeks of April, a second crest from the Tennessee and Ohio Rivers that was lower than those earlier in the year occurred in mid-April and caused Cairo, IL, to crest at 56.4 feet. The stage at Cairo was the highest measured

since records began in 1844 and has only been exceeded 3 times since. As this flood wave moved downstream, it combined with record flows on the Arkansas and high flows on the Red Rivers to produce disastrous flooding to the Gulf of Mexico. The flood waters passed New Orleans on May 15 when the Mississippi River crested at 20.7 feet there. Note that New Orleans had reached a stage of 21.0 feet about 3 weeks prior to that date when the Caernarvon Levee south of New Orleans was dynamited to allow the river to get to the Gulf faster.


During the height of the flood, the Mississippi River in northern Louisiana was over 80 miles wide. The levee system, thought to be sufficient to control any flood on the Mississippi River, was decimated with over 120 crevasses or breaks. These crevasses played an important role in reducing flood levels downstream and are estimated to have reduced the peak stage at New Orleans by over 6 feet. Over 600,000 people were driven from their homes and damages totaled over $230 million dollars in 1927. It is estimated that 246 people died in the flood.


Changes as a Result. This flood changed the nation's approach to flood control. Prior to 1927, it was believed that levees alone could control floods on the Mississippi and reservoirs upstream were not needed. The Federal Government had provided some money for flood control but there had never been a comprehensive plan to protect the country from floods. As a result of this flood, the Flood Control Act of 1928 was enacted giving the Corps of Engineers responsibility for flood control on the Mississippi River. Upstream reservoirs were built on the major tributaries, levees were to be constructed using engineering principles under the direction of the Corps of Engineers, and floodways were built to divert flood waters from the main channel. All these measures were completed before the flood of 1937. The levees held during this flood even though the flows during that flood exceeded those of the flood of 1927.


Significant social and economic changes resulted from the flood. Author John Barry wrote The Rising Tide detailing the events surrounding the flood and the social and economic impacts. See this book for more details on the flood and the changes the it wrought.

- Dave Reed

National Climate Data Online

The NOAA/National Climatic Data Center announces the Climatic Data Online (CDO) system, with full period of record digital data for 5 types of climatic data:

  • U.S. daily surface data
  • U.S. monthly surface data
  • U.S. hourly precipitation data
  • U.S. 15-minute precipitation data
  • Global monthly surface data

Data from as early as the late 1800's are available, and each dataset is updated as soon as the latest data are available. Users can select data by country, region, state, climate division, county, and station; and by time period (e.g., year, month, day). Data can be provided in a space or comma-delimited format (easy import into spreadsheet or database). A help system provides information to assist users.

CDO provides data previously only available via off- line order (e.g., place order by phone). Data are currently provided at no charge to ".edu" and ".noaa" users, with charges by credit card for others — but charges are significantly reduced as compared to off-line orders. Other datasets will be added to the system later this year and next year, including hourly surface data.

The Internet address for the NCDC CDO is:

If users have questions or experience problems with this new CDO system, they can contact NCDC by e-mail at:

- Taken from the National Weather Association's July Newsletter is the U.S. government's official web portal to all federal, state and local government web resources and services.