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. 1, Autumn 2000 Ethan A. Jolly, Editor


Features Drought Conditions Across the LMRFC Area QPF - Move to 12 Hour

From the HIC

Drought conditions persist over much of the LMRFC area. Rains have lessened the severity of the drought but we are still in a critical situation. As defined by the Drought Mitigation center, most of the LMRFC area is experiencing severe to extreme conditions. To provide more information, we will focus on the effects of the drought in this issue.

 

Our procedure development efforts continue at a fast pace. We are developing more graphical products to provide information to

our users in a more usable manner. Efforts are underway to provide information to the Weather Forecast Offices to support short-term backup, and we are preparing for the upcoming flood season by reviewing all forecast procedures. It has been a busy time for the office despite the drought conditions.

 

We will continue to improve our forecast techniques and work with our cooperators to provide better products and services. Any comments and ideas are greatly appreciated.

- Dave Reed


Drought Conditions Across the LMRFC Area

There has been much talk and discussion of the ongoing drought situation across much of the U.S. during the past two years. The Southeast U.S. and, hence, the LMRFC area, has seen significant impacts as a result of the ongoing dry conditions. After two to three years of less than normal rainfall over much of the area; low reservoir levels, record or near record low river stages and flows, and receding aquifers are some of the more noticeable, hydrologic impacts. Agriculture, wildlife, and commerce are also impacted by the drought conditions.

 

As shown in Figure 1 (U.S. Drought Monitor for Oct 31, 2000), severe to exceptional drought conditions exist over much of the LMRFC area; including, all of Mississippi and Alabama, most of Louisiana, the west one-half of Arkansas, the south one-half of Tennessee, western North and South Carolina, and north Georgia. Other areas within the LMRFC area are, at best, considered abnormally dry. Currently, the driest conditions exist over west central Arkansas, north and southeast Mississippi, and north Alabama.

Figure 1: US Drought Monitor for October 31, 2000
US Drought Monitor
Click for larger image

Precipitation has been below to much below normal over the area for the past two years. Figure 2 shows that for the 12 month period ending October 2000, most of the LMRFC area is experiencing a precipitation deficit. Approximately 75% of the area was 10 inches or more below normal for the period with south Louisiana, south Mississippi, and portions of south Alabama having deficits of 20 inches or greater. Station rainfall data (Table 1) from January 1 through October 7, 2000 show deficits of 4 inches to 25 inches over much of the same areas. The Palmer Drought Index (PDI - Figure 3) shows a pattern consistent with the rainfall deficit figures.

Figure 2 : Precipitation Deficit for 12 Months Ending October 2000
US Drought Monitor
Click for larger image

Table 1: Year to Date (10/7/2000) Precipitation For Selected Stations in the LMRFC Area
State Station Total (Inches) % of Normal Departure (Inches)
Alabama Mobile 27.6 53% -25
  Huntsville 29.4 67% -14
Arkansas Little Rock 26.9 72% -10
  Fort Smith 26.4 86% -4

Louisiana

New Orleans 24.5 50% -24
  Baton Rouge 24.6 51% -24
  Lake Charles 34.0 80% -8
  Shreveport 40.2 117% 6
Mississippi Jackson 30.4 73% -10
  Meridian 26.6 61% -17
  Tupelo 28.2 67% -14
Missouri Cape Girardeau 31.9 90% -4
  Springfield 26.8 80% -7
N Carolina Asheville 25.1 67% -12
S Carolina Greenville 28.0 70% -12
Tennessee Memphis 28.0 72% -11
 

Nashville

32.4 89% -4
  Chattanooga 37.4 91% -4
  Knoxville 36.7 109% 3
Texas Dallas/FTW 21.4 80% -5

Figure 3 : Palmer Drought Index
US Drought Monitor
Click for larger image

Impacts of the drought are seen and felt in numerous ways around the area. Water levels along the White River system are the lowest in many years. Most of the Corps of Engineers' (COE) reservoirs in Arkansas are at 75% or less of their seasonal pools and many stages along the Black and White Rivers are near record low levels (See Table 2). In Mississippi and Louisiana, although COE reservoirs in the Yazoo River system of north Mississippi are at 100 % of seasonal pool, observed flows along the Yazoo River are between 58% and 70% of normal (Table 3). Along the Red and Ouachita Rivers in Louisiana observed flows are running between 12 % and 60% of normal.

Table 2: Current and Record Stages as of 10/26/2000
Black River, Arkansas
Location Record Low Stage (ft) Date Current Stage (ft)

Black Rock

0.2 1954 0.7
Pocahontas -0.2 1956 0.4
Corning -0.5 1941 1.9
Lower White River, Arkansas
Location Record Low Stage (ft) Date Current Stage (ft)
Newport -1.5 1954 -1.05

Augusta

8.6 1954 9.4
Georgetown -3.0 1916 -2.6
  -2.6 1954 -2.6
Des Arc -1.9 1887 -1.9
Clarendon 4.1 1887 6.4

Table 3: Reservoir and River Levels as of 10/25/2000
Data from Corps of Engineers, Little Rock & Vicksburg
Reservior Levels
Reservoir Location River System % of Normal Conservation Pool

Arkabutla

Mississippi Yazoo 100%
Sardis Mississippi Yazoo 100%
Enid Mississippi Yazoo 100%
Grenada Mississippi Yazoo 100%
Blakely Arkansas Ouachita 94%
DeGray Arkansas Ouachita 97%
Narrows Arkansas Ouachita 89%
Beaver Arkansas White 72%
Table Rock Arkansas White 75%
Bull Shoals Arkansas White 52%
Norfolk Arkansas White 80%
Greers Ferry Arkansas White 71%
River Levels
River Gage Location River System % of Normal Flow
Swan Lake Mississippi Yazoo 70%
Whaley Mississippi Yazoo 58%
Greenwood Mississippi Yazoo 62%
Camden Arkansas Ouachita 60%
Monroe Louisiana Ouachita 12%
Fulton Arkansas Red 15%
Shreveport Louisiana Red 17%
Alexandria Louisiana Red 20%
Arkansas City Arkansas Mississippi 60%
Vicksburg Mississippi Mississippi 73%
Natchez Mississippi Mississippi 80%

On the lower Mississippi River between Arkansas City, AR and Natchez, MS the flow is 60% to 80% of normal. Of particular note is that the stage at Vicksburg, MS has been below normal for 245 of 293 days this water year and that the `spring crest' of 29.4 feet was the third lowest seasonal high on record. The low levels along the Mississippi River have caused restrictions in barge traffic since early Spring 2000.

Other interesting impacts in Louisiana include high prices and reduced availability of crawfish - a real problem for many connoisseurs - and a significant increase in the mosquito population. This latter result is counter intuitive but was explained by a naturalist at Tickfaw State Park near Springfield, LA. The mosquito requires very little water to breed - a relatively well known fact. However, other species which prey heavily on mosquito larvae need substantially more water depth to survive - less predation, more mosquitos!

 

Figure 4 shows soil moisture anomalies as of October 2000. The anomaly is the greatest over Mississippi, Alabama, east Arkansas, and all Louisiana - except the northwest corner. These extremely low soil moisture conditions pose modeling difficulties for the LMRFC and a challenge for the LMRFC forecasters. Soil moisture accounting models generally are less reliable during transitions from excessive dry periods to wet cycles.

Figure 4 : Soil Moisture Anomoly - October 2000
US Drought Monitor
Click for larger image

 

Figure 5 is an estimate of the rainfall needed, as of October 21, 2000, to bring the Palmer Drought Index(PDI) to near normal levels across the U.S. and indicates that more than 9 inches are needed over much of the LMRFC with some places needing over 12 inches! Long range, meteorological predictions show some relief over the next several months as slightly above average rainfall is expected over the area from November through January. The forecast excess, however, is likely to be insufficient to seriously impact the PDI, and low flow and low soil moisture conditions could prevail into the next dry season.

Figure 5 : Additional Rainfall Needed
US Drought Monitor
Click for larger image
- Jim Coe


SHEF Code Explained

Quantitative Precipitation Forecasts (QPF)s have been used daily by the LMRFC since the mid 90's. The QPF process changed during the winter/spring of 2000 giving the Hydrometeorological Analysis and Support (HAS) forecaster the responsibility of producing 24 hour QPF for direct use into our hydrologic models. The complexities and uncertainties with producing 24 hour QPF for sub-basins caused the LMRFC to reevaluate the QPF process.

 

Starting on October 16, LMRFC began routinely using 12 hours of QPF in river forecasts instead of 24 hours of QPF. LMRFC feels this will provide a better guidance to our customers and

remove the spatial and timing uncertainties associated with QPF more than 12 hours in the future. We will continue to process QPF for the next 24 hours and post the information to the LMRFC home page. The QPF will also be provided to the Weather Forecast Office (WFO) through the AWIPS system.

 

QPF will continue to be processed every 12 hours at 12z and 00z. We will update the QPF at 18z and 06z when significant rainfall is occurring or is expected. If significant changes are needed to the QPF, an update to the QPF can be modified anytime.

-Jeff Graschel


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