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


David Reed, Hydrologist In Charge Vol. 5 No. 2, Winter 2002 Ethan A. Jolly, Editor


Features Flooding in the Yazoo River Basin Flood Outlook Products New Forecast Services

From the HIC

2001 was a GREAT year! LMRFC continued to provide a high level of products and services. To confirm this, a quote from the Tropical Storm Allison Services Assessment Report stated that, “The LMRFC provided outstanding service to its customers and partners.” Congratulations to the LMRFC staff for their efforts!

While continuing to meet our operational requirements, LMRFC accomplished many other goals in 2001. We continued expanding our presence on the web by adding additional rainfall graphics and making forecasts available thru a hand-held Personal Digital Assistant (PDA). We have implemented the River Flood Outlook (RFO) and Significant Flood Outlook Product (FOP) to give cooperators and users an idea of possible flooding out as far as five

days into the future. Numerous papers were presented at professional conferences. As you can see, 2001 was a GREAT year!

2002 is expected to be even better. Based on high soil moisture values and full reservoirs over much of the LMRFC area, we expect an active spring flood season. We are prepared to meet this challenge. In addition to our forecast activities, we expect an active year in developing new products and procedures to meet our customers and partners needs. Our web presence will continue to increase and we are planning to implement new forecast services at three locations. Yes, 2002 is expected to be an active and exciting year at LMRFC.

- Dave Reed


Heavy Rainfall and River Flooding in the Yazoo River Basin of Mississippi

The fall season across the LMRFC area started quietly with only seasonal flooding occurring over parts of the area. The Yazoo River basin was dry and streamflows were near normal Fall conditions until the last week in November. Rainfall events during the end of November through December brought significant flooding to the Yazoo River basin. Flooding occurred at most forecast points in the Yazoo River basin with record flooding occurring on the Little Tallahatchie River.

Rainfall Events
The first rainfall event occurred on November 26th through November 30th. Upper level winds became parallel to a surface front creating a stationaly boundary across the LMRFC area.

Figure 1 shows heavy rainfall occurring from west Tennessee extending southwest through east Arkansas and north Mississippi to northeast Louisiana. Radar derived precipitation estimates were between 8 to 12 inches over the Yazoo River basin with localized 12 to 15 inches over the Big Sunflower and Tallahatchie Rivers as well as Grenada Lake.

Figure 1: Stage III Precipitation Estimates from 11-26-2001 through 11-30-2001
Stage III Precipitation Estimates from 11-26-2001 through 11-30-2001
Click for larger image

The second rainfall event occurred on December 13th through December 15th. A slow moving cold front produced another round of heavy rainfall over saturated soils from the late November event. Figure 2 shows heavy rainfall occurring over southeast Arkansas and north Mississippi. Radar derived precipitation estimates were between 4 to 6 inches over the Yazoo River basin with localized 6 to 8 inches occurring over the Big Sunflower and Tallahatchie Basins.

Figure 2 : Stage III Precipitation Estimates from 12-13-2001 through 12-15-2001
Stage III Precipitation Estimates from 12-13-2001 through 12-15-2001
Click for larger image

The third and last rainfall event occurred on January 18th through January 20th. Another slow moving frontal system produced heavy rainfall over north Mississippi but the heaviest rainfall occurred over the north portion of the Yazoo River basin. Radar derived precipitation estimates were between 3 to 5 inches with isolated 6 to 7 inches occurring over Sardis Lake.

River and Flood Conditions
Heavy rainfall during November, December, and January caused significant rises on all rivers and streams in the Yazoo River basin. The first rainfall event at the end of November caused minor to moderate flooding at many locations along the Tallahatchie, Big Sunflower, Yalobusha, Coldwater, and Yazoo Rivers. The rainfall event also caused an all time record stage of 29.41 feet to occur at Etta, Mississippi on the Little Tallahatchie River on November 30th breaking the old record of 29.35 feet set in 1955. The flood stage at Etta is 25 feet.

The second rainfall event continued the flood conditions along the Yazoo River Drainage during the entire month of December. The Big Sunflower River at Sunflower crested at 27.66 feet on December 18th which was less than one foot away from breaking its all time high stage. At the end of December, the Yalobusha, Yazoo, Big Sunflower, and Tallahatchie remained above flood stage. The third rainfall event combined with smaller events during January and February kept the Yalobusha and Tallahatchie Rivers above flood stage through the end of February.

Figure 3 highlights flood categories for specific LMRFC river forecast points in the Yazoo river basin.

Figure 3 : Yazoo River Flood Categories 11-26-2001 through 01-20-2002
Yazoo River Flood Categories 11-26-2001 through 01-20-2002
Click for larger image

In March, soil moisture conditions and streamflows still remain much above seasonal levels. Minor flooding continued along the Yalobusha and Tallahatchie Rivers. The Lake control structures on the Yazoo Drainage continue at 25 to 50 % of flood control storage. Based on normal rainfall patterns during winter/spring, a much above average flood potential exists for the Yazoo River drainage.

- Jeff Graschel and Keith Stellman

 


Flood Outlook Products

RFC river forecasts routinely include rain that has already fallen and forecasted rainfall or quantitative precipitation forecasts (QPF) for 12 to 24 hours in the future. To provide information on potential flooding based on forecasted rainfall over the next five days, RFCs began disseminating two new flood outlooks last fall, the River Flood Outlook (RFO) and Significant Flood Outlook Product (FOP).

The RFO is being prepared by Southern Region (SR) RFCs only. The LMRFC runs their hydrologic model with 24 hours of QPF and compares forecasted river levels with routine forecasts with an additional 12 hours of QPF. For locations that flood with the additional QPF, a RFO text product will be issued by the appropriate WFO in an ESF product. In addition to the text product, RFO web graphics are generated for SR (www.srh.noaa.gov/ftproot/hsd/html/rfo.htm), each RFC (www.srh.noaa.gov/lmrfc/rfo/lmrfcrfo.shtml for LMRFC as an example), and each WFO (www.srh.noaa.gov/lmrfc/rfo/megrfo.shtml for WFO Memphis as an example). The product is prepared twice daily at 10AM and 9PM with updates at 2PM and 2AM if conditions

warrant.

The FOP is a national initiative and uses a potential for rainfall for the next five days. This product highlights areas where significant moderate or higher flooding is possible based on predicted rainfall and soil moisture conditions. The product may be based on a forecasters knowledge of soil moisture conditions and possible rainfall resulting from meteorological model forecasts. The FOP from each of the 13 RFCs is mosaicked into a national product. LMRFC’s FOP is available at www.srh.noaa.gov/lmrfc/fop/index.shtml; the national product at www.hpc.ncep.noaa.gov/nationalfloodoutlook/. These products are prepared once each day by 3PM.

These products have been designed based on user feedback. Each of these graphics on the web has the capability to accept feedback which will be used to improve the product and this feedback is encouraged.

- Dave Reed


New Forecast Services

In response to NWS user needs, the NWS has expanded forecast service at Ross Barnett Reservoir and will implement river forecasts on the Pearl River near Lena, MS, and the Escatawpa River near Orange Grove, MS.

On February 8th, 2002, LMRFC and WFO Jackson began issuing a routine 5-day inflow forecast at Ross Barnett Reservoir replacing our old 3-day forecast. This increase in service will assist the Pearl River Valley Water Supply District in operations at Ross Barnett Reservoir by giving more lead time for significant inflows into the reservoir.

On March 19, 2002, LMRFC and WFO New Orleans/Baton Rouge will begin issuing flood only forecasts for the Escatawpa River near Orange Grove, MS (ORAM6) and is located at Interstate 10.

Forecasts will provide residents in the area and downstream to Moss Point, MS, information on river conditions. The flood stage at the I-10 gage is 10.0 feet. Forecast services at this location were requested by local residents after major flooding from Hurricane Georges in September, 1998.

Again on March 19, 2002, LMRFC and WFO Jackson will begin issuing routine daily 3-day forecasts for the Pearl River near Lena, MS. This location, also referred to as the Pearl River near Good Hope, is upstream of Ross Barnett Reservoir and will provide valuable information on reservoir inflow forecasts. This routine forecast will be beneficial to residents in the area and also provide additional support to the Pearl River Valley Water Supply District in operations of Ross Barnett Reservoir. The flood stage at the Lena gage is 24 feet.

- Laurie Hall


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