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The La Niña Influence on Central Alabama Rainfall Patterns.

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Presentation on theme: "The La Niña Influence on Central Alabama Rainfall Patterns."— Presentation transcript:

1 The La Niña Influence on Central Alabama Rainfall Patterns

2 La Niña in General Cooling of sea surface temperatures in the Central and Eastern Pacific with an observed SST of -0.5 below climatic average Cooling of sea surface temperatures in the Central and Eastern Pacific with an observed SST of -0.5 below climatic average Using data since 1950, the observed breakdown of the previous El Niño in 2006-2007 shows the strongest SST cooling on record Using data since 1950, the observed breakdown of the previous El Niño in 2006-2007 shows the strongest SST cooling on record La Niña conditions tend to develop in spring to early summer with greatest impacts later in the year La Niña conditions tend to develop in spring to early summer with greatest impacts later in the year La Niña conditions can tend to favor hurricane development due to lack of shear La Niña conditions can tend to favor hurricane development due to lack of shear

3 La Niña in General La Niña tends to develop every 3 to 5 years but may vary from 2 to 7 years La Niña tends to develop every 3 to 5 years but may vary from 2 to 7 years La Niñas last for a period of 1 to 3 years La Niñas last for a period of 1 to 3 years Drier and warmer than normal conditions are noted across much of the south Drier and warmer than normal conditions are noted across much of the south

4 La Niña SST Regions

5 May 7 SSTs

6 August 26 SSTs

7 September 1 SST

8 La Niña Atmospheric Variability There are 3 prominent atmospheric flow changes that can occur during winter and spring There are 3 prominent atmospheric flow changes that can occur during winter and spring Increased blocking over high latitudes of the eastern North Pacific Increased blocking over high latitudes of the eastern North Pacific Increased meridional flow across the continent and eastern North Pacific Increased meridional flow across the continent and eastern North Pacific Highly variable strength of the jet stream over the eastern North Pacific with a mean jet position over the Northwest U.S. Highly variable strength of the jet stream over the eastern North Pacific with a mean jet position over the Northwest U.S.

9 La Niña Pattern

10 La Niña Expectations Increased storminess & frequency of significant cold-air outbreaks over Central North America Increased storminess & frequency of significant cold-air outbreaks over Central North America Southern states are drier Southern states are drier Can be considerable month to month variations in temp and rainfall patterns, due to more variability in atmospheric circulation Can be considerable month to month variations in temp and rainfall patterns, due to more variability in atmospheric circulation

11 La Niña Database La Niña episodes were determined from the CPC ENSO website with dates beginning at 1950 La Niña episodes were determined from the CPC ENSO website with dates beginning at 1950 Rainfall and temperature data were acquired through the NCDC database Rainfall and temperature data were acquired through the NCDC database There were a total of 66 seasons sampled split up over 30 different years There were a total of 66 seasons sampled split up over 30 different years Seasons are DJF, MAM, JJA, SON Seasons are DJF, MAM, JJA, SON

12 La Niña Database Raw data comparison to climatic averages Raw data comparison to climatic averages No adjustments for climatic trends No adjustments for climatic trends Autumn 18 events Autumn 18 events Winter included 20 events Winter included 20 events Spring 15 events Spring 15 events Summer 13 events Summer 13 events

13 The Tropics During La Niña 1950 through 2001 1950 through 2001 14 tropical systems that impacted central Alabama during La Niña 14 tropical systems that impacted central Alabama during La Niña 3 Summer storms 3 Summer storms 2 were late August into September; 1 in July 2 were late August into September; 1 in July 11 Autumn storms 11 Autumn storms More significant impact on MGM than BHM More significant impact on MGM than BHM

14 Birmingham Autumn Rainfall 18 events; 78% were at or below climatology

15 Birmingham Autumn Rainfall

16 Birmingham Winter Rainfall 20 events; 55% were at or above climatology

17 Birmingham Spring Rainfall 15 events; 73% were at or below climatology

18 Birmingham Summer Rainfall 13 events; 69% were at or above climatology

19 Birmingham Summary Spring and Autumn are dry Spring and Autumn are dry Winter and Summer are wet Winter and Summer are wet There is some variability in each season There is some variability in each season

20 Montgomery Autumn Rainfall 18 events; 61% were at or below climatology

21 Montgomery Autumn Rainfall 78% were at or below climatology

22 Montgomery Winter Rainfall 20 events; 65% were at or below climatology

23 Montgomery Spring Rainfall 15 events; 60% were at or below climatology

24 Montgomery Summer Rainfall 13 events; 62% were at or above climatology

25 Montgomery Summary Autumn, Winter and Spring are typically dry Autumn, Winter and Spring are typically dry Summer tends to be near average or slightly above Summer tends to be near average or slightly above There can be periods of variability There can be periods of variability

26 Latest Forecast The most recent SST anomalies show La Niña is beginning The most recent SST anomalies show La Niña is beginning SSTs in the 3.4 region are below defined criteria SSTs in the 3.4 region are below defined criteria A large pool of cold sub-surface water is persisting in the central to eastern tropical Pacific Ocean A large pool of cold sub-surface water is persisting in the central to eastern tropical Pacific Ocean Computer model ensembles forecast La Niña to last through at least next spring Computer model ensembles forecast La Niña to last through at least next spring

27 SST Outlook: NCEP CFS Forecast Issued 27 August 2007 The CFS ensemble mean (heavy blue line) indicates a transition from neutral to La Niña conditions during August.

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30 Conclusions Late beginning of La Niña may delay some of the Autumn impacts? Late beginning of La Niña may delay some of the Autumn impacts? Stronger La Niñas have led to greater impacts (CPC study) Stronger La Niñas have led to greater impacts (CPC study) Odds of continued long term dry pattern are higher than average Odds of continued long term dry pattern are higher than average Long term seasonal forecast are going to reflect this higher probability Long term seasonal forecast are going to reflect this higher probability


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