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

David Reed, Hydrologist In Charge Vol. 3No. 3, Spring 2000 Ethan A. Jolly, Editor

Features Heavy Rain & Flooding Event of April 1-4, 2000 AWIPS Commissioning

From the HIC

The winter and early spring has been a busy time at the LMRFC. We along with the other 3 Southern Region RFCs began a 9 month test of routine 24-hour operations and are open around the clock seven days a week until December. LMRFC has commissioned its next generation computer system, the Advanced Weather and Information Processing System (AWIPS), and has shut down the old AFOS system. We are also beginning our preparations for the upcoming hurricane season by reviewing hurricane procedures to ensure all are current.

 

Along with these efforts, the flood season has been relatively quiet compared to previous years but we did have some moderate to major flooding in early April. We have provided a detailed writeup on this flood event.

We continue to modernize all aspects of the LMRFC operations to improve products and services. This newsletter is no exception. This edition and all future ones will be in color. We will continue to review products and services and make improvements based on your needs.

- Dave Reed

Heavy Rain & Flooding Event of April 1-4, 2000 Over the Southcentral US
Introduction
This winter and spring have been extremely dry for most areas of the country and especially the southcentral U.S. Only a major event over a large area would cause flooding along the rivers and streams. During the final week of March, the National Weather Service numerical models were predicting a large scale heavy precipitation event which usually means significant flooding.

Meteorological Synopsis
The large scale pattern showed the polar jet retreated far to the north across the northern tier of the United States during the week of March 26-31. A strong upper low had cut off from the main flow and dug into the Southwest with a very slow forward progression. As the cutoff low progressed eastward, it tapped into a strong subtropical jetstream moving across the Mexican Plateau and steered it northward across south Texas and into the central Gulf Coast States. Several disturbances rode along the subtropical jet into a diffluent upper wind pattern and an old frontal boundary at the surface to trigger several mesoscale thunderstorm complexes. Each thunderstorm complex that developed was followed by another only 12-24hours apart with rainfall totals of 2-4 inches for each
Figure 1: Radar (Stage III) analysis of the rainfall over Louisiana, Mississippi, and Alabama during the event of April 1-April 4, 2000
Figure 1
Click for larger image
complex. Fortunately, there was enough forward progression of the cutoff low to move each complex slightly to the southeast and prevent the training of complexes. While the cutoff low was supporting each of the mesoscale complexes with moisture and lift, the polar jet was taking a dip southward over the northern plains with a jet streak at the 250 mb level of 195 knots. This dip in the polar jet would phase in with the cutoff low, produce a full latitude trough over the eastern half of the United States by April 4, and produce a squall line out ahead of a strong cold front. All of these factors combined to produce widespread 5-12 inch rains across north Louisiana, central and north Mississippi, north Alabama, north Georgia, and southeast Tennessee for the 3 day period.

Rainfall Analysis
The areas with the heaviest rainfall included a large band stretching from north Louisiana, southeast Arkansas, central and north Mississippi, north Alabama, central and southeast Tennessee, to north Georgia. Stage III (radar/rain gage composite) rainfall for this event is shown in figures 1 and 2 since it closely matches the ground truth (rain gages). Most locations in north Louisiana, central and north Mississippi, and north Alabama received 5-8 inches of rainfall with a few isolated areas of 9 inches plus. The hardest hit areas were along the upper Big Black River near West, Mississippi, the middle Yazoo River basin near Money, Mississippi, and the upper Pearl River near Philadelphia, Mississippi (Fig 3). Money located along the Tallahatchie River in north central Mississippi reported the highest 72-hour rainfall total of 11.48 inches.
Figure 2: Radar (Stage III) analysis of the rainfall over Mississippi, Alabama, Georgia, and Tennessee during the event of April 1-April 4, 2000
Figure 2
Click for larger image

Heavy rainfall also occurred in areas of north Georgia and southeast Tennessee during this event. Most areas reported between 4-7 inches of rainfall for the 72 hour period however a few locations reported some higher amounts. Columbus City located in northeast Alabama reported the highest 72 hour rainfall for this region of 8.06 inches.

Flood Analysis
Although soil moisture conditions across the area prior to the rain event were abnormally dry, heavy rainfall generated significant runoff across north Louisiana, southeast Arkansas, central and north Mississippi, north Alabama, north Georgia, and portions of central and southeast
Figure 3: Map of Louisiana and Mississippi showing the flood location, flood stage, and actual crest height in feet
Figure 3
Click for larger image

Tennessee. Most of the significant river flooding occurred in north central and central Mississippi. See figures 3 and 4 for rivers which flooded along with flood stage (fs) and crest height.

The most significant flooding occurred in central Mississippi. Moderate to major agricultural flooding occurred along the Big Black River. The Big Black at West crested at 25.74 ft, second highest stage during the period of record from September 1971 to present. The record crest is 26.08 ft measured on May 21, 1983. Minor to moderate flooding occurred along the Pearl River above Ross Barnett Dam. Peak inflow into the reservoir was over 30,000 cfs. Minor flooding occurred along the majority of the lower Pearl River from below Jackson, Mississippi to Pearl River, Louisiana. Minor to moderate flooding also occurred in Mississippi in the drainages of the Yazoo, Big Sunflower, and lower Chickasawhay Rivers. The main impact of the flooding was agricultural interest although some access roads near the rivers were flooded. Otherwise, widely scattered minor to moderate flooding occurred in the Tennessee Valley and across north LA.
Figure 4: Map of Alabama and Tennessee showing the flood location, flood stage, and actual crest height in feet
Figure 4
Click for larger image

Conclusion
Although this was a significant event for some locations, flooding in general was only moderate. This was largely due to the abnormally dry weather that occurred over this past winter and the resultant extremely low soil moisture in these areas. Had this been average or above average winter rainfall, it could have been a record event for some locations across the southcentral U.S. Even with the heavy rainfall that occurred during the 3 day period, most locations are at or well below normal rainfall for the year.

- Keith Stellman & John Kuhn

AWIPS Commissioning

The Lower Mississippi River Forecast Center made the Advanced Weather Interactive Processing System (AWIPS) its official operating and communication system on Friday April 21, 2000. With commissioning at the LMRFC, 58 out of the 196 AWIPS systems NWS-wide have been commissioned.

AWIPS is being operated with fourth build software. With the capabilities provided in Build 4, AWIPS has sufficient functionality to run forecast and warning operations at River Forecast Centers and Weather Forecast Offices, free of the antiquated legacy system for data display and communications, known as Automation of Field Operations and Services (AFOS).

The LMRFC has been running in a problem-free test operational mode on AWIPS since the middle of February 2000. Communication to all internal and external agencies have been uninterrupted during this period with no degradation of service. With the

commissioning of AWIPS, modernization at the LMRFC is complete, six years after it was first initiated. AWIPS at all NWS offices should be commissioned by the end of August 2000.

With commissioning complete, AWIPS development efforts now focus on system evolution. The next set of functionality will become available with Build 5. Similar to previous software released, Build 5 will be implemented in a series of releases, beginning in the latter half of 2000. The focus of Build 5 is to make productivity enhancement and decision assistance tools available to forecasters. More efficient methods of data transmission, storage, and retrieval on AWIPS will also be developed to address the ever-expanding quantity of data produced by NWS observing systems and numerical forecast models. A map of the status of NWS AWIPS commissioning can be found at:
http://www.awips.noaa.gov/awips.jpg

- Eric Jones

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