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


David Reed, Hydrologist In Charge Vol. 4 No. 3, Spring 2001 Ethan A. Jolly, Editor


Features Spring Flood Events NWS Acronyms

From the HIC

2000 has been an eventful year so far. We had major and record flooding during February and March over most of the LMRFC area. During this period, LMRFC was open continuously for over 4 weeks keeping a constant hydromet watch for our area. The LMRFC staff performed well during this period. We are highlighting the flooding in this issue of the Crawfish Tales.

 

Our procedure development efforts are in full swing. We continue to implement ArcView in our operations and to improve our website. We are working to implement databasing technology to make our website more efficient and provide forecasts on hand-held devices such as personal data assistants and wireless phones. We are also working to improve our modeling with our Sacrament Soil Moisture Accounting Model calibration effort.

 

We will continue with the implementation of the river flood watch program. The target date for implementation is July 1. Each of you should have received a survey to provide feedback on the format of the graphic product and content of the text product. National Weather Service Headquarters is also looking into a flood potential product to address possible flooding out to 5 days.

 

As you can see, we expect a busy summer. We will continue with our development efforts while maintaining a hydromet watch. This is especially important as we move into hurricane season. Our number 1 priority will remain providing timely and accurate forecasts and quality service to our customers and partners. We look forward to continuing to serve you in 2001. Please do not hesitate to call with any questions or needs for additional services.

- Dave Reed


Spring Flood Events

Winter and spring months tend to be wet, as many locations across the LMRFC climatologically receive maximum rainfall during these months. However, the past two winter and spring seasons have been relatively dry, leading to continued drought conditions over much of the area. The Winter and Spring of 2001 saw several rainfall events, which helped alleviate some of the dry conditions. These rainfall events brought minor to significant flooding to many of LMRFC's basins, and in a few cases the floodingreached record levels.

 

Meteorological Synopsis
In most of the rainfall events these past few months, the upper-level pattern showed a slow-moving trough of low pressure, often cutoff from the main flow. With the low in this position, the southwesterly subtropical jet becomes active somewhere across the South, bringing plenty of moisture across the region. Disturbances in the subtropical jet, along with stationary surface frontal boundaries, generally produce a large area of overrunning precipitation, which was the case for many of the Winter/Spring 2001 rainfall events. In each of these events, the axis of heaviest rainfall occurred just to the north of the stationary frontal boundary, with the northward extent of the rainfall dependent on the frontal boundary position. These overrunning events can last for an extended period of time, and cover a very large area. In another rainfall event on April 4th, slow-moving convection developed in the warm sector, just to the west of a north-south oriented stationary front. Though the areal extent wasn't as great as for the overrunning events, the observed rainfall amounts were just as excessive.

 

Rainfall Analysis
Figure 1 shows the contoured gauge amounts for the four overrunning events. Table 1 gives a summary of the five rainfall events.

Figure 1: Gridded gauge analysis (inches) for the rainfall events of (a) 1/15/01-1/20/01 (b) 2/11/01-2/17/01 (c) 2/26/01-3/05/01 (d) 3/08/01-3/15/01
US Drought Monitor
Click for larger image

 

The first rainfall event, which occurred from January 15th through the 20th, 2001, produced rainfall over most of the LMRFC area, with the axis of heaviest rainfall (greater than 4.00" total) from north Alabama, southwest through north Mississippi, and extending across south Arkansas and north Louisiana. A secondary axis of maximum rainfall extended from southwest Mississippi through southwest Louisiana. Hardest hit were the Bayou Bodcau and Bayou Dorcheat basins of southwest Arkansas and northwest Louisiana, and the Yazoo, Yalobusha, and Skuna Rivers in Mississippi, with the largest rainfall amount of 6.80 inches at Yazoo City.

 

For the second rainfall event, which occurred from February 11th through the 17th, the axis of heaviest rainfall was located farther to the north, extending across much of Tennessee, northwest Mississippi, and south Arkansas. Rainfall amounts, in general, were much higher than the previous event, with several areas reporting in excess of 6.00 inches for the event. In Mississippi, the heaviest rainfall was near the Coldwater, Little Tallahatchie, Tallahatchie, and the upper portion of the Sunflower basins, and in western Tennessee, near the Hatchie basin. Bayou Bodcau and Bayou Dorcheat, along with the lower Red in southwest Arkansas, were also hardest hit.

 

The third rainfall event, from February 26th through March 5th, had most of the rain falling across south Arkansas, Louisiana, and central and south Mississippi while the remainder of LMRFC's area received at least some rainfall. The axis of heaviest rainfall was much farther to the south with this event, affecting the lower Red and Ouachita River systems in Louisiana, and the lower Pearl and Pascougoula River systems in Mississippi. On the lower Pearl, the heaviest rainfall of 9.45 inches occurred near Mize, Mississippi.

 

The final overrunning event, occurring from March 8th through 15th, was similar to the previous three events in that most of the LMRFC area received some rainfall, however the rainfall amounts were not as great as the earlier events. For this time period, the heaviest rainfall was focused across extreme south Mississippi, with Pascagoula 2ENE receiving 6.88 inches. Areas of maximum rainfall also occurred across central Mississippi, south Arkansas, and portions of north Louisiana.

 

Figure 2 shows the contoured gauge analysis, in inches, for the convective event of April 4th through 5th. While the areal extent of the rainfall wasn't as great as the overrunning events, the rainfall amounts were just as impressive. The heaviest rainfall occurred over the upper Big Black, upper Pearl, and Yockanookany Rivers, with Kosciusko receiving 9.97 inches of rainfall for the 24-hour event.

Figure 2 : Gridded gauge analysis for the convective event of 4/04/01-4/05/01
US Drought Monitor
Click to go to Stage III Cumulative Page

Table 1: Dates and maximum rainfall amounts
Fig Dates Max Amounts
1a

1/15/01-1/20/01

6.80" (Yazoo City, MS)
1b 2/11/01-2/17/01 8.89" (Clifton, TN)
1c 2/26/01-3/05/01 9.45" (Mize, MS)
1d 3/08/01-3/15/01 6.88" (Pascagoula 2ENE, MS)
2 4/04/01-4/05/01 9.97" (Kosciusko, MS)

 

Flooding Analysis
The heavy rainfall during the first few months of 2001 generated significant runoff across most of the LMRFC basins. This runoff caused flooding along a majority of the rivers in the LMRFC area. The Big Black and Yazoo basins in Mississippi, and the lower Red River tributaries were among the first to crest, coincident with the area of initial heavy rain. As the rainfall axis shifted northward with the second event, the Ouachita and White rivers in Arkansas began to rise, cresting during middle to late February. Flows made their way down the Ouachita, and with additional rainfall, points along the Ouachita in Louisiana reached their crests in mid-March. Rivers in the Tennessee Valley also crested during middle to late February, due to the first two rainfall events. The third rainfall event, which was focused across southern Mississippi, brought significant crests along the Pearl and Pascagoula systems in early to mid-March, with significant crests along the Yockanookany in early April due to the convective rainfall event.

 

The flooding proved to be record-breaking along some tributaries of the lower Red River system, in both Texas and Louisiana. Bayou Bodcau at Bayou Bodcau Dam, Louisiana, which has a flood stage of 172.0 feet, crested at 197.39 feet on March 18th, which represented the highest level since 1958 when the Bayou reached 196.67 feet on May 11th. The South Sulphur River at Cooper Dam, TX reached a record pool level of 446.04 feet on March 4th, a few tenths of a foot below the top of the flood control pool. Finally, the Sulphur River at Talco, TX, where the flood stage is 20 feet, crested at 29.10 feet on February 18th, which is the second highest crest, second only to the crest of 29.40 feet on December 11th, 1971.

 

Summary
Though the winter and spring seasons of the past two years have been relatively dry across the LMRFC area, the recent Winter and Spring saw much of the region receiving some much needed rainfall. Resultant runoff did cause significant flooding problems along several of the LMRFC's rivers, due to repetitive rainfall events and the large areal extent of the rainfall. In a few cases, the heavy rainfall/runoff brought some river and reservoir pool levels to record and near-record heights.

- Steve Listemaa


National Weather Service (NWS) Acronyms

AFOS - Automation of Field Operations and Services

ASOS - Automated Surface Observing
System

AWIPS - Advanced Weather Interactive
Processing System

CONUS - Continental United States

DCP - Data Collection Platform

GOES - Geostationary Operational
Environmental Satellite

NCEP - National Center of Environmental Prediction

NEXRAD - Next Generation Weather
Radar

NOAA - National Oceanic and Atmospheric Administration

NWS - National Weather Service

NWSRFS - NWS River Forecast System

RFC - River Forecast Center

RPG - Radar Product Generator

SBN - Satellite Broadcast System

SHEF - Standard Hydrometeorological
Exchange Format

WFO - Weather Forecast Office

WHFS - WFO Hydrometeorological
Forecasting System

WSR - 88D - Weather Surveillance Radar 1988 Doppler

 


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