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

David Reed, Hydrologist In Charge Vol. 7 No. 2, Winter 2004 Ethan A. Jolly, Editor

Features Spring Flood Outlook LMRFC Service Changes Flood Safety Checklist Know Your River Systems:
Yazoo River System

From the HIC

This flood season started off slowly but heavy rainfall this past month has resulted in widespread flooding with some major levels being reached. Our goal is to provide timely and accurate river forecasts and information to our cooperators and partners. We are ready to meet any of the operational challenges that might face us this year.

This issue of the Crawfish Tales will document some of our operational procedures and highlight some changes in our forecast products. A description of the Spring Flood Outlooks is provided in this issue. We are implementing a new forecast point on the Duck River at Centerville, TN (CNVT1) and there will be changes in the flood stage at the Big Black River at West, MS. In this issue, we

are also providing some guidance on preparing for a flood.

We have other development efforts underway. We have began calibrating some of our river basins to improve our model performance and expect a contractor to assist us in this effort. This should result in improved model simulations allowing us to provide improved services to our partners and customers.

Feedback is always welcome. Please let us know if there are additional products or services you need form the LMRFC. We always enjoy hearing from our cooperators and partners.

- Dave Reed

Spring Flood Outlook

Each Winter/Spring flood season, the LMRFC prepares and issues a Spring flood potential outlook product describing the potential and relative magnitude for Spring flooding over our river basins. This outlook product considers expected snowmelt, river ice conditions, existing soil moisture and streamflow conditions relative to long term averages, as well as available reservoir flood control stages and future normal precipitation. The outlook also includes any ongoing flooding, along with the current meteorological situation and the National Weather Service 30-day and 90-day precipitation and temperature outlooks over the LMRFC area. Information on flooding potential for the LMRFC area basins includes the lower Mississippi River and its tributaries, as well as all of the Louisiana and Mississippi coastal drainages.

Generally, this seasonal outlook product is first issued during late February and is updated again in mid March. An outlook is issued in April if meteorological and river conditions warrant. The LMRFC releases this outlook to its servicing

Weather Forecast Offices (WFOs) on Thursdays where it is repackaged for dissemination to the public on Fridays. The WFO versions contain flood potential information for their local river basins within their Hydrologic Service Area. Links to the WFO websites can be found on the LMRFC website at:

The primary objective of this product is to provide flood potential outlooks to our servicing WFOs so they can alert the general public to the likelihood for Spring flooding over their local basins. For 2004, the flood potential outlooks covering the entire LMRFC will be issued February 26th and March 11th. A national river flood outlook is scheduled for March 18th and will be released during a press conference hosted by National Weather Service Headquarters.

- Angelo Dalessandro

Service Changes

Two new service changes are happening at the Lower Mississippi River Forecast Center. On December 15, 2003, the Duck River at Centerville, Tennessee, became a routine daily forecast point. This river gauge is located on the Highway 100/48 bridge and has a flood stage of 17.0 feet. This service will provide emergency managers and the public advanced warning of impending floods. Official forecasts along with flood warnings and statements are issued by the National Weather Service Forecast Office in Nashville Tennessee. The Nashville Office of the U.S. Geological Survey will operate and maintain the river gauge.

The other service change is on the Big Black River at West, Mississippi. On February 24, 2004, flood stage will be raised from 12.0 feet to 15.0 feet. At flood stage, agricultural interests begin to be impacted along the river. Official river forecasts, warnings, and statements are issued by the National Weather Service Forecast Office in Jackson Mississippi.

- Eric Jones

Flood Safety Checklist
Flood season has begun and the LMRFC is doing our part by providing the public with the best river forecasts possible. The greatest forecast in the world is usless unless you take actions to aviod loss of life and property. That is why the National Weather Service has developed this flood safety checklist.

Before the Flood

* Know your risk and elevation above flood stage.
* Know your evacuation routes.
* Keep your automobile fueled.
* Store drinking water in clean containers.
* Keep a stock of food that requires little cooking and no refrigeration.
* Keep first aid supplies on hand.
* Keep a NOAA Weather Radio, a battery-powered portable radio, emergency cooking equipment, and flashlights in working order.
* Listen to local radio or television for flood info.
* Floods can take several hours to days to develop.

During the Flood

* Get out of areas subject to flooding. This includes dips. low spots, canyons, washes, etc.
* Avoid already flooded and high velocity flow areas. Do not attempt to cross flowing water.
* If driving, be aware that the road bed may not be intact under flood waters.
* If the vehicle stalls, leave it immediately and seek higher ground.
* Children should NEVER play around high water, storm drains, viaducts, or arroyos.

After the Flood

* If fresh food has come in contact with flood waters, throw it out.
* Boil drinking water before using.
* Do not visit disaster areas.
* Electrical equipment should be checked and dried before being returned to service.
* Use flashlights to examine buildings.
* Report downed utility lines to your local power company.

Know Your River Systems: Central Tennessee

Yazoo - (ya-’zü). What’s in a name? Although there is no definitive meaning for the term yazoo, the term generally applies to any stream with a belated confluence to the main river. How appropriate, since the Yazoo River slowly meanders parallel to the Mississippi River for approximately 175 miles before joining it. The Yazoo was named by French Explorer René-Robert Cavelier, Sieur de La Salle in 1692.

Figure 1: Yazoo River System in Northern MS
Central Tennessee and Northern Alabama
Click for larger image

The Yazoo River, a tributary of the Mississippi River, flows 188 miles from the confluence of the Tallahatchie and the Yalobusha Rivers to the Mississippi River, north of Vicksburg, MS. The basin, an area of about 13,400 square miles, is protected from flooding by an extensive system of levees and reservoirs and covers Mississippi’s northwest. Headwaters for the Yazoo include the Coldwater, Tallahatchie, Yocona, Skuna, Yalobusha, and Big Sunflower sub-basins. The Delta, a fertile plain and major agricultural region, lies between the Yazoo and Mississippi Rivers. This relatively flat plain has a gentle slope and covers the western half of the drainage basin. Flooding in the Delta can occur from rainfall upstream or backwater flooding when the Mississippi River is in flood. The basin’s eastern half is characterized by rolling hills and steep slopes. On average, the Yazoo basin receives 52 inches of annual rainfall, a third of which becomes runoff.

Severe backwater flooding from the record 1927 flood on the lower Mississippi River and disastrous headwater flooding in 1932 and 1933 served as a catalyst for many flood control programs, such as the Yazoo Headwater and the Yazoo Backwater Projects. The Yazoo Headwater project authorized the four lake reservoirs, Sardis, Arkabutla, Enid, and Grenada, with the last completed in 1955. The reservoirs, completed in 1955, control 60% of the hill country runoff and provide a benefit/cost ratio of $2 benefit for every $1 invested.

Reservoir operations by the US Army Corps of Engineers maximize flood control benefits while considering other water resource purposes. Gate are operated to prevent or lessen downstream area flooding. Despite the complex network of levees and reservoirs, flood potential in the Yazoo basin still exists as two lives were lost to the floodwaters last year.

In the spring of 1973, over 20% of the Yazoo basin was inundated from Mississippi River backwater, with some areas saturated for as long as four months. This event caused the record flood for the Sunflower, MS forecast points, as seen in the table below showing all of the forecasted points in the Yazoo basin.

Table 1: Record and Recent Flooding
Upper Yazoo River Basin
River Location NWS ID Flood Stage (ft) Record Recent Flood (stage/date)
Coldwater Olive Branch, MS OLVM6 11.0 13.6
  Sarah, MS SARM6 18.0 25.0
  Marks, MS MKSM6 39.0 41.9
Tallahatchie Lambert, MS LMGM6 32.0 36.8
  Locopolis, MS LOGM6 33.0 36.4
  Swan Lake, MS SWNM6 26.0 37.0
Little Tallahatchie Etta, MS ETAM6 25.0 29.4
Yocona Oxford, MS OXDM6 25.0 28.7
Lower Yazoo River Basin
River Location NWS ID Flood Stage (ft) Record Recent Flood (stage/date)
Skuna Bruce, MS BRCM6 31.0 34.4
Yalobusha Calhoun City, MS CCTM6 23.0 25.8
  Whaley, MS YWGM6 21.0 27.8
Yazoo Greenwood, MS GREM6 35.0 40.1
  Belzoni, MS BELM6 34.0 37.9
  Yazoo City, MS YZOM6 29.0 37.7
Big Sunflower Sunflower, MS SUNM6 21.0 28.4
  Anguilla, MS ANGM6 45.0 50.5

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