NOAAs Atlantic Hurricane Season Outlooks By Dr. Gerry Bell Lead Seasonal Hurricane Forecaster Climate Prediction Center/ NOAA/NWS Hurricanes: Science and Society 13 March 2012 Outlooks made in Collaboration with: National Hurricane Center/NOAA/NWS Hurricane Research Division/NOAA

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

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

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

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: 1.El Niño and La Niña; Reflect large year-to-year changes in tropical Pacific Ocean temperatures. 2.Year-to-year changes in Atlantic sea-surface temperatures 3.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.

Typical Conditions Associated with Active 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.

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. El Niño and La Niña El Niño More Shear Increased Wind Shear, Fewer Hurricanes Warm, Wet La Niña favors more hurricane activity by decreasing the vertical wind shear. La Niña Less Shear Decreased Wind Shear, More Hurricanes Cool, Dry El Niño and La Niña reflect large year-to-year changes in tropical Pacific Ocean temperatures.

This climate pattern lasts 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. Drier Warmer Stronger West African Monsoon Tropical Multi-Decadal Signal Associated with Atlantic High-Activity Era

Tools used to Make NOAAs 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

Example Showing One Way to Combine Predicted Climate Factors to Make a Hurricane Outlook ColderWarmer Example Model Sea-Surface Temperature ( o F) Seasonal Forecast Made Before the Season Predicted Average SST Departure is +0.5 o F Strength Atlantic SST Departure ( o F) Table shows seasonal hurricanes as a function of El Niño/La Niña and Atlantic sea surface temperatures. Based on statistical regression equation developed for Atlantic high-activity era. La Nina El Nino Hypothetical Prediction of El Niño/ La Niña La Nina El Nino This prediction calls for a La Nina to develop Predicted Strength

Summary 1.NOAAs Atlantic hurricane season outlooks indicate expected overall seasonal activity. They are not a landfall forecast. 2.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. 3.Extensive ongoing monitoring, assessment, model development efforts. 4.The current Atlantic high-activity era began in Historically these last years. 5.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.