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

David Reed, Hydrologist In Charge Vol. 2 No. 2, Winter 1999 Ethan A. Jolly, Editor


Special La Niña Edition
Features La Niña and its Impacts La Niña Links School to Work Program


 

From the HIC

LMRFC experienced an exciting 1998 and eagerly looks forward to 1999. During 1998, we installed our new computer system AWIPS and are using it as our primary forecast tool for most activities. During this period, we also expanded our forecast services based on user's needs by adding forecast points and increasing some forecasts form 3-day to 5-day forecasts. We expect to continue this trend with additional forecast points planned for early 1999. We began an effort to calibrate the Sacramento Soil Moisture Accounting Model for our headwater basins to improve forecast accuracy. Our webpage is providing LMRFC an effective venue to disseminate information. The staff accomplished much of this during an active hurricane season and while continuing to provide a high level of support to our users. Congratulations to the staff for their accomplishments during this year!

As we prepare for the new millennium, we will continue to work in these areas. We expect to provide all forecast services from AWIPS and utilize more of the capabilities of the system this year. All our Weather Forecast Offices (WFO) will have AWIPS in April and we expect to make changes in forecast services when that milestone is reached. We will continue to work with cooperators to meet forecast needs. Our model simulations and forecasts should improve because we have calibrated more of our headwater basins.

1998 was an exciting year and we enter 1999 with the anticipation of another. The staff at LMRFC is up to the challenge. We look forward to serving all our cooperators and users during these exciting times.

- Dave Reed


 

La Niña and its Impacts

What is La Niña?

La Niña and El Niño can greatly impact the global weather patterns. La Niña is defined as cooler than normal sea-surface temperatures in the central and eastern tropical Pacific ocean while warmer than normal conditions exist during El Niño events. La Niña conditions occur every 3 to 5 years on average, but can vary from 2 to 7 years. Typically, a La Niña lasts 9 to 12 months, although they can last as long as two years . A La Niña episode doesn't always follow a El Niño. In fact, since 1975 there has been only half the number of La Niñas than El Niños. During the month of December 1998 cold episode conditions strengthened across the tropical Pacific. According to the National Climate Prediction Center, we have a moderate La Niña in the east-central Pacific during the present time.

Atmospheric Patterns During a Moderate to Strong La Niña

Large scale changes occur in the atmospheric winds across the tropical Pacific during La Niña years. Eastward moving atmospheric and oceanic waves help to bring the cold water to the surface through a complex series of events still being studied. The eastern trade winds strengthen, upwelling increases off of Peru and Ecuador, and sea-surface temperatures drop below normal. Figure 1 Figure 1shows the typical January-March weather anomalies and atmospheric circulations for a moderate La Niña. During a moderate to strong La Niña there is a high amplitude blocking ridge in the east Pacific which causes the majority of the storm systems to enter the United States through the Pacific northwest. The strength of the eastern Pacific jet stream is highly variable with the mean jet entering the northwest United States/southwest Canada. In response to a highly variable jet large portions of central North America experience considerable month to month variations in temperature, rainfall, and storminess during the winter and spring seasons.

Impacts of La Niña on the Lower Mississippi River Valley

During La Niña years, over a large part of the LMRFC area and Ohio Valley, increased storminess and increased precipitation occurs during the December through February period. The southern sections of the LMRFC area tend to experience fewer storms, less precipitation, and warmer temperatures. Average precipitation for the last 10 La Niña events, 1950 to 1995, for December through February (Figure 2) Figure 2

shows 1 to 3 inches above normal across the Ohio and Tennessee Valleys into much of AR and north MS. During the moderate to strong La Niña of 1988, 6 to 8 inches above normal fell across west KY, west TN, north MS, and southwest AR. Precipitation anomalies (Figure 3)Figure 3 of around normal to 1 inches above normal occurred across the upper Mississippi Valley, southwest VA, southwest NC, and north AL. Normal to 1 inches below occurred across northeast TX, south LA, and south MS except along the coastal sections of LA and MS where 2 to 3 inches below normal occurred.

Rainfall over much of the LMRFC area for the March through May period during the last 10 La Niña years (Figure 4)Figure 4 averages below normal. During La Niña years for these months, rainfall over much of LA, MS, AR, and northeast TX average 1.5 to 2" below the 30 year normal. Eastern TN and northwest AR recorded about 1" more than the normal during the period with precipitation near normal over the remainder of the LMRFC area. For the period March through May, temperature averaged near normal.

Extended Outlook

Current model output and analysis indicate that there is a moderately strong La Niña still in progress. Forecasts call for La Niña to maintain its strength through February. The February temperature outlook calls for above normal temperatures across the entire LMRFC area with precipitation running around climatological averages (Figure 5 & Figure 6). Forecasts for March, April, and May used climatology for both temperatures and precipitation (Figure 7). The Climate Prediction Center's coupled model prediction indicates that cold episode conditions will continue through June, while some other models indicate that cold episode conditions will continue through the remainder of the year.

According to Dr. William Grey's research, the chances for the United States and the Caribbean Islands to experience hurricane activity increases greatly during La Niña.

References

Figures and information are based on the Climate Prediction Center web site, Southern Region Climate Center web site, and the National Weather Service Southern Region web site.

 

Figure 5

Figure 6

Figure 7


 

La Niña Links


 

School-to-Work Program

The LMRFC and WFO New Orleans continue to reach out to the local community by establishing a partnership with Slidell High's School-To-Work program. This program is a St. Tammany Parish School System effort designed to assist young people in making a successful transition from school to the work place. It consists of school based learning in various academies and worked based learning consisting of internships during a students senior year. Slidell High and other schools in the local school system offer academies in health careers, teaching, law, ecology, and engineering.

The LMRFC and WFO New Orleans began participation in this program during the Fall semester of 1998. A student

intern, from either the ecology or engineering academies, completes a 9 week program consisting of a combination of work, training, and education. The student learns about hydrology, hydrometeorology, NWS computer systems, and forecast operations. During their internship, each student is required to complete an assigned project that can be applied in forecast operations. This project requires the student to use the knowledge he/she has gained through their hydrology training and support from RFC hydrologists.

The program is beneficial to both the student as well as the LMRFC and WFO New Orleans. The LMRFC looks forward to continued participation in the School-To-Work program during the remainder of the school year and in the future.

 

 


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