NOAA

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NOAAs Atlantic Hurricane Season
OutlooksByDr. Gerry BellLead Seasonal
Hurricane ForecasterClimate Prediction Center/
NOAA/NWSHurricanes Science and Society13
March 2012
Outlooks made in Collaboration with National
Hurricane Center/NOAA/NWSHurricane Research
Division/NOAA
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Outline

1. Features of NOAAs seasonal hurricane outlooks
2. Science behind the outlooks
3. Tools used to make the outlooks
4. Summary

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Features of NOAAs Seasonal Outlook
Outlooks indicate the expected overall seasonal
activity.

• Began in August 1998
• Issued late May and early August
• Outlooks are probabilistic, include
• Probabilities of season type,
• Likely (70 chance) ranges of activity
• Extensive discussion

www.cpc.ncep.noaa.gov/ products/hurricane/
CAUTION NOAAs outlooks are not a seasonal
hurricane landfall prediction. All persons in
hurricane-prone regions should prepare each and
every hurricane season regardless of the outlook.
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Measuring Hurricane Season Strength

• ACE index measures overall season strength by
  accounting for the combined number, intensity and
  duration of tropical storms and hurricanes.
• ACESum of squares of maximum sustained surface
  wind speed (measured 6-hourly) for all named
  storms while at least tropical storm strength.

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Science Behind NOAAs Seasonal Outlooks

• Hurricane season strength is not random.
• Seasonal activity reflects an inter-related set
  of atmospheric and oceanic conditions -- linked
  to three main tropical climate factors
• El Niño and La Niña Reflect large year-to-year
  changes in tropical Pacific Ocean temperatures.
• Year-to-year changes in Atlantic sea-surface
  temperatures
• Tropical multi-decadal (25-40 year) signal
  Reflects fluctuations in West African monsoon
  strength and Atlantic sea surface temperatures.
• NOAAs seasonal outlooks are based largely on
  these climate factors.

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Typical Conditions Associated withActive
Atlantic Hurricane Seasons
This inter-related set of conditions has strong
links to tropical climate factors. CPC has an
extensive monitoring program to assess and
predict these conditions and the associated
climate factors.
www.cpc.ncep.noaa.gov/products/hurricane/
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El Niño and La Niña
El Niño and La Niña reflect large year-to-year
changes in tropical Pacific Ocean temperatures.
El Niño
El Niño suppresses hurricane activity by
increasing the vertical wind shear. Strong wind
shear can prevent hurricanes from forming, and
can also kill an existing hurricane.
www.cpc.ncep.noaa.gov/products/analysis_monitoring
/enso_advisory/index.shtml
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Tropical Multi-Decadal Signal Associated with
Atlantic High-Activity Era
This climate pattern lasts 25-40 years, and
produces key ingredients of a high-activity era
for Atlantic hurricanes warmer waters, reduced
wind shear, and favorable winds that strengthen
cloud systems coming from Africa.
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Tools used to MakeNOAAs Seasonal Hurricane
Outlook

• Outlook is subjective blend of statistical and
  dynamical model forecasts.
• Two Step Process
• Step 1 Predict the key climate factors using
  observations and a suite of different statistical
  and dynamical models.
• Step 2 Predict activity likely to result from
  predicted climate conditions
• Statistical regression and analogue techniques,
• New NCEP dynamical model that produces direct
  hurricane counts.

Plot showing model seasonal forecast of tropical
storm tracks
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Example Showing One Way to Combine Predicted
Climate Factors to Make a Hurricane Outlook
Example Model Sea-Surface Temperature
(oF) Seasonal Forecast Made Before the Season
Predicted Average SST Departure is 0.5oF
Colder
Warmer
-6 -4 -2 -1 -0.4 0.4 1 2 4
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Summary

• NOAAs Atlantic hurricane season outlooks
  indicate expected overall seasonal activity .
  They are not a landfall forecast.
• There is a strong scientific basis for making
  these seasonal outlooks.
• Tropical climate patterns controlling seasonal
  hurricane activity are often predictable.
  Therefore, seasonal activity is predictable.
• Extensive ongoing monitoring, assessment, model
  development efforts.
• The current Atlantic high-activity era began in
  1995. Historically these last 25-40 years.
• Hurricane preparedness is the most important
  thing one can do to minimize impacts if a
  tropical storm or hurricane strikes. All persons
  in hurricane-prone regions should prepare each
  and every hurricane season regardless of the
  outlook.