The La Niña Influence on The La Niña Influence on Central AlabamaCentral Alabama

Rainfall PatternsRainfall Patterns

La Niña in GeneralLa Niña in General

• Cooling of sea surface temperatures in the Cooling of sea surface temperatures in the Central and Eastern Pacific with an observed Central and Eastern Pacific with an observed SST of -0.5 below climatic averageSST of -0.5 below climatic average

• Using data since 1950, the observed Using data since 1950, the observed breakdown of the previous El Niño in 2006-breakdown of the previous El Niño in 2006-2007 shows the strongest SST cooling on 2007 shows the strongest SST cooling on record record

• La Niña conditions tend to develop in spring La Niña conditions tend to develop in spring to early summer with greatest impacts later to early summer with greatest impacts later in the year in the year

• La Niña conditions can tend to favor La Niña conditions can tend to favor hurricane development due to lack of shearhurricane development due to lack of shear

La Niña in GeneralLa Niña in General

• La Niña tends to develop every 3 to 5 La Niña tends to develop every 3 to 5 years but may vary from 2 to 7 yearsyears but may vary from 2 to 7 years

• La Niñas last for a period of 1 to 3 La Niñas last for a period of 1 to 3 yearsyears

• Drier and warmer than normal Drier and warmer than normal conditions are noted across much of conditions are noted across much of the south the south

La Niña SST RegionsLa Niña SST Regions

May 7 SSTsMay 7 SSTs

August 26 SSTsAugust 26 SSTs

September 1 SSTSeptember 1 SST

La Niña Atmospheric La Niña Atmospheric VariabilityVariability

• There are 3 prominent atmospheric There are 3 prominent atmospheric flow changes that can occur during flow changes that can occur during winter and springwinter and spring

• Increased blocking over high latitudes of the Increased blocking over high latitudes of the eastern North Pacificeastern North Pacific

• Increased meridional flow across the Increased meridional flow across the continent and eastern North Pacificcontinent and eastern North Pacific

•Highly variable strength of the jet stream Highly variable strength of the jet stream over the eastern North Pacific with a mean over the eastern North Pacific with a mean jet position over the Northwest U.S.jet position over the Northwest U.S.

La Niña PatternLa Niña Pattern

La Niña ExpectationsLa Niña Expectations

• Increased storminess & frequency of Increased storminess & frequency of significant cold-air outbreaks over significant cold-air outbreaks over Central North AmericaCentral North America

• Southern states are drierSouthern states are drier• Can be considerable month to month Can be considerable month to month

variations in temp and rainfall variations in temp and rainfall patterns, due to more variability in patterns, due to more variability in atmospheric circulation atmospheric circulation

La Niña DatabaseLa Niña Database

• La Niña episodes were determined La Niña episodes were determined from the CPC ENSO website with dates from the CPC ENSO website with dates beginning at 1950beginning at 1950

• Rainfall and temperature data were Rainfall and temperature data were acquired through the NCDC databaseacquired through the NCDC database

• There were a total of 66 seasons There were a total of 66 seasons sampled split up over 30 different sampled split up over 30 different yearsyears

• Seasons are DJF, MAM, JJA, SONSeasons are DJF, MAM, JJA, SON

La Niña DatabaseLa Niña Database

• Raw data comparison to climatic Raw data comparison to climatic averagesaverages

• No adjustments for climatic trendsNo adjustments for climatic trends

• Autumn 18 eventsAutumn 18 events

• Winter included 20 eventsWinter included 20 events

• Spring 15 eventsSpring 15 events

• Summer 13 eventsSummer 13 events

The Tropics During La NiñaThe Tropics During La Niña

• 1950 through 20011950 through 2001

• 14 tropical systems that impacted 14 tropical systems that impacted central Alabama during La Niñacentral Alabama during La Niña

• 3 Summer storms3 Summer storms•2 were late August into September; 1 in July2 were late August into September; 1 in July

• 11 Autumn storms11 Autumn storms

• More significant impact on MGM than More significant impact on MGM than BHMBHM

Birmingham Autumn Birmingham Autumn RainfallRainfall

Autumn Months SON

0

5

10

15

20

25

Rain Average Rain

18 events; 78% were at or below 18 events; 78% were at or below climatologyclimatology

Birmingham Autumn Birmingham Autumn RainfallRainfall

Autumn Hurricane Adjustment

0

5

10

15

20

25

Rain Adjusted Average Rain

Birmingham Winter RainfallBirmingham Winter RainfallWinter Months DJF

0

5

10

15

20

25

30

1950 1955 1957 1965 1971 1974 1976 1985 1996 2000

Rain Average Rain

20 events; 55% were at or above climatology20 events; 55% were at or above climatology

Birmingham Spring RainfallBirmingham Spring RainfallSpring Months MAM

0

5

10

15

20

25

1950 1954 1955 1956 1962 1964 1968 1971 1974 1975 1976 1985 1989 1999 2000

Rain Average Rain

15 events; 73% were at or below 15 events; 73% were at or below climatologyclimatology

Birmingham Summer Birmingham Summer RainfallRainfall

Summer Months JJA

05

10152025

Rain Average Rain

13 events; 69% were at or above 13 events; 69% were at or above climatologyclimatology

Birmingham SummaryBirmingham Summary

• Spring and Autumn are drySpring and Autumn are dry

• Winter and Summer are wetWinter and Summer are wet

• There is some variability in each There is some variability in each seasonseason

Montgomery Autumn Montgomery Autumn RainfallRainfall

Autumn Months SON

0

5

10

15

20

25

1950

1954

1955

1956

1961

1964

1967

1970

1971

1973

1974

1975

1983

1988

1995

1998

1999

2000

Rainfall Average Rainfall

18 events; 61% were at or below 18 events; 61% were at or below climatologyclimatology

Montgomery Autumn Montgomery Autumn RainfallRainfall

Autumn Hurricane Adjustment

0

5

10

15

20

25

1950

1954

1955

1956

1961

1964

1967

1970

1971

1973

1974

1975

1983

1988

1995

1998

1999

2000

Rainfall Adjusted Average Rainfall

78% were at or below climatology78% were at or below climatology

Montgomery Winter RainfallMontgomery Winter RainfallWinter Months DJF

0

5

10

15

20

Rainfall Average Rainfall

20 events; 65% were at or below 20 events; 65% were at or below climatologyclimatology

Montgomery Spring RainfallMontgomery Spring RainfallSpring Months MAM

0

5

10

15

20

25

Rainfall Average Rainfall

15 events; 60% were at or below 15 events; 60% were at or below climatologyclimatology

Montgomery Summer Montgomery Summer RainfallRainfall

Summer Months J JA

0

5

10

15

20

1950 1954 1955 1956 1964 1970 1971 1973 1974 1975 1988 1998 1999

Rainfall Average Rainfall

13 events; 62% were at or above 13 events; 62% were at or above climatologyclimatology

Montgomery SummaryMontgomery Summary

• Autumn, Winter and Spring are Autumn, Winter and Spring are typically drytypically dry

• Summer tends to be near average or Summer tends to be near average or slightly aboveslightly above

• There can be periods of variabilityThere can be periods of variability

Latest ForecastLatest Forecast

• The most recent SST anomalies show La The most recent SST anomalies show La Niña is beginningNiña is beginning

• SSTs in the 3.4 region are below SSTs in the 3.4 region are below defined criteriadefined criteria

• A large pool of cold sub-surface water is A large pool of cold sub-surface water is persisting in the central to eastern persisting in the central to eastern tropical Pacific Oceantropical Pacific Ocean

• Computer model ensembles forecast La Computer model ensembles forecast La Niña to last through at least next springNiña to last through at least next spring

SST Outlook: NCEP CFS SST Outlook: NCEP CFS Forecast Issued 27 August Forecast Issued 27 August 20072007

The CFS ensemble mean (heavy blue line) indicates a transition from neutral to La Niña conditions during August.

ConclusionsConclusions

• Late beginning of La Niña may delay Late beginning of La Niña may delay some of the Autumn impacts?some of the Autumn impacts?

• Stronger La Niñas have led to greater Stronger La Niñas have led to greater impacts (CPC study)impacts (CPC study)

• Odds of continued long term dry Odds of continued long term dry pattern are higher than averagepattern are higher than average

• Long term seasonal forecast are going Long term seasonal forecast are going to reflect this higher probability to reflect this higher probability