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. 4, Summer 2002 Ethan A. Jolly, Editor


Features AHPS Update East TN, West NC, SW VA

From the HIC

The long, hot summer is here and there is a lull in forecast activities. This gives us time to review our hydrologic procedures and develop new techniques to provide products and services to our customers and partners. This will make us better prepared to meet the challenges that might occur during this hurricane season and the flood season of 2003.

Much of our efforts will be focused on the Advanced Hydrologic Prediction Services (AHPS). AHPS will be our next generation forecast system that combines better science with improved technology to expand our products and forecast services. The NWS is hoping for significant funding over the next 10 years starting in 2003 to implement AHPS. Much of our efforts over the summer and fall will be to prepare to utilize these funds to improve

and enhance our current hydrologic models and parameters. Much of this issue is devoted to AHPS and some of the products and services expected to be implemented over the next few years.

To help in understanding the hydrology and forecast locations, we will begin a routine article featuring each of our major river basins. This issue will focus on eastern Tennessee drainages since they will be the first to be implemented in AHPS.

Please let us know if we need to provide additional products and services. We always enjoy hearing from our cooperators and partners and meeting your needs.

- Dave Reed


Advanced Hydrologic Prediction Service (AHPS) Update

During the next few years the Lower Mississippi River Forecast Center along with the rest of the National Weather Service will begin implementing its Advanced Hydrologic Prediction Service (AHPS). This program will greatly enhance the hydrologic services the National Weather Service provides.

Figure 1: 90 Day Avereage Weekly Volume Probabililities
Stage III Precipitation Estimates from 11-26-2001 through 11-30-2001
Click for larger image

Each Year, floods kill more people than any other form of severe weather, and cause damages in excess of $3.5 billion. Three-quarters of all presidentially declared disasters are the result of flooding. National implementation of AHPS will save lives and an estimated $240 million per year in flood losses, and will contribute an additional $520 million per year in economic benefits to water resource users.

Figure 2 : 90 Day Mean Daily Flow Exceedence Probabilities
Stage III Precipitation Estimates from 12-13-2001 through 12-15-2001
Click for larger image

Some of the new services include:
More accurate and comprehensive predictions of river height and flood potential.
A main component of this more accurate forecast will be the

 

upgrade of the NWS River Forecast System (NWSRFS) to reduce the time required to produce forecasts and make it easier to implement new models. The models will be calibrated for each basin to ensure the best possible simulation and forecast accuracy using the latest calibration techniques and historical records. Also, sophisticated distributed hydrologic modeling capability will be implemented which accounts for the variability of precipitation and rainfall-runoff characteristics within a basin. This modeling capability will allow short-term forecasts to more realistically reflect the streamflow response to precipitation.

Probabilistic forecasts, providing graphical forecasts from days to months into the future.
These probabilistic products will provide information to local officials and water managers on long term forecasts on items such as how long a drought will last, how to price and manage water in an effective manner, and whether to release water from a reservoir or hold it for anticipated needs.

Flood inundation maps, providing a graphical display of forecasted flood areas in addition to implementation of hydraulic modeling on key rivers.
Due to the increased processing and storage demands of AHPS, the current LMRFC AWIPS system processing and storage needs will also be increased. These demands arise from the computational intensive nature of probabilistic forecasts and hydraulic modeling processes as well as the requirement of large volumes of historic and geographic data.

For more information on AHPS, go the National Weather Service AHPS website at:
www.nws.noaa.gov/oh/ahps

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

- Eric Jones


Know Your River Systems: East TN, West NC, & Southwest VA

The mountain/valley landscape of East TN, West NC, and Southwest VA provide the Lower Mississippi River Forecast Center (LMRFC) with fast response rivers and streams. In fact, the majority of LMRFC rivers that crest within 24 hours are located in these areas. This area is the headwaters of the Tennessee River which starts at Ft. Loudoun Dam.

Figure 1 shows the major rivers and streams in the region as well as topography. The French Broad River which originates in the mountains of western NC and the Holston River which has its origins in southwest VA both flow into the reservoir above the dam. This area has many lakes, most man made and most operated and controlled by the Tennessee Valley Authority (TVA) headquartered in Knoxville, TN. The primary purpose of most of the dams is hydroelectric power, but most are multipurpose providing flood control and recreation.

Figure 1: 90 Day Avereage Weekly Volume Probabililities
Stage III Precipitation Estimates from 11-26-2001 through 11-30-2001
Click for larger image

Hydro-Climatology of the Area

Due to the mountains the region, precipitation varies with topography ranging from 65 inches annually in the mountains of

Western North Carolina to less 35 inches annually in some of the valleys of Eastern Tennessee. Another function of the mountains is help create fast response rivers and streams. Most of the rivers and streams, especially those upstream of a dam or control structure, crest within the first 24 hours of a rainfall event. One function of the dams and river control structures is to hold back any wave moving down stream and help minimize any flooding that occurs by releasing water at a controlled pace at a later time.

History of the Region

Many rainfall events have occurred in the region; some localized affecting only a few locations and some widespread affecting the entire area. Many infamous flash floods have occurred in this region. In 1969, the remnants of Hurricane Camille caused flash flooding killing 153 in what is the worst natural disaster to hit the state of Virginia. 31 inches of rain fell in 6 hours in Nelson County, VA killing 121 in the county alone. Very little flooding occurred in the LMRFC area, however Camille showed us what can happen when a tropical system interacts with mountains.
In East TN, there has never (in written records) been an event such as the one in April 1977. Widespread 5 to 10 inches of rain fell during April 3-5, 1977 causing major flash and river flooding across almost all of the region. Of the 18 LMRFC forecast points in southwest VA, western NC, and east TN, 6 records still stand from this event with the 1977 flood ranked in the top 5 for 4 other locations. Along the Powell River at Arthur, TN the flood of 1977 surpassed the old record set in 1826 by 9.5 feet. The flood of record was surpassed by more than 2 feet along Blackwater Creek near Sneadville, TN where records date back to the mid 1800’s. Table 1 shows each of the rivers and LMRFC forecast points with flood stage, record flood, and most recent flood information given. One thing to note is the recent flood event of March 2002. A record was set along the North Fork Holston River at Saltville with many other locations experiencing moderate to major flooding.

- Keith Stellman
Table 1 - Record and Recent Flooding
River
Location Flood Stg (ft) Record Recent Flood (stage/date)
Emory
Oakdale 27 42.3 ft 3/1929 28.7 ft 1/24/02
Sequatchie
Whitwell 14 19.0 ft 3/1867 16.2 ft 3/18/02
Powell
Jonesville 18 44.3 ft 4/1977 32.4 ft 3/18/02

Arthur 17 39.0 ft 4/1977 29.0 ft 3/19/02
N. Fork Holston
Saltville 12 14.9 ft 3/2002 14.9 ft 3/18/02
  Gate City 12 19.8 ft 4/1977 19.5 ft 3/19/02
Clinch
Cleveland 14 26.4 ft 4/1977 20.7 ft 3/20/02
  Speers Ferry 18 36.7 ft 4/1977 32.3 ft 3/19/02
  Tazewell 12 29.3 ft 4/1977 21.3 ft 3/19/02
Nolichucky
Embreeville 12 21.5 ft 11/1977 11.6 ft 8/1994
Little Pigeon
Sevierville 11 18.1 ft 3/1965 11.2 ft 3/18/02
French Broad
Blantyre 17 25.5 ft 10/1964 none since 1996
  Asheville 8 23.1 ft 7/1916 none since 1996
  Marshall 8 22.0 ft 7/1916 none since 1996
  Hot Springs 13 22.4 ft 8/1940 none since 1996


Newport 10 19.3 ft 8/1940
(22.5 ft estimated in 7/1916)
11.5 ft 3/18/02
Swannanoa
Biltmore 12 19.0 ft 8/1940 none since 1996
Chickamauga ck Chattanooga 18 28.7 ft 2/1990 22.8 ft 4/5/00

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