BEHIND THE SCENES: NOAA'S NORTH ATLANTIC HURRICANE SEASONAL OUTLOOK

Hurricane Katrina was the costliest natural disaster in U.S. history.

May 22, 2007 — From emergency managers and planners at all levels of government, to businesses and the financial markets to the general public, nearly everyone uses the NOAA Atlantic Hurricane Seasonal Outlook. The outlook is meant to give the public ample warning of what to expect for the upcoming season, in order to mitigate the potential loss of life and property. The outlook also serves as a reminder for annual hurricane preparedness efforts to begin in earnest. Time and time again, emergency aid workers find that by far, those individuals and communities that are prepared for the hurricane season fare much better during an actual event than those who are not. (Click NOAA image to the left for a larger view of image of Hurricane Katrina, the costliest natural disaster in U.S. history. Click here for high resolution version. Please credit “NOAA.”)

While meteorologists at the NOAA National Hurricane Center predict the track and strength of individual storms, the annual Hurricane Seasonal Outlook is an official forecast product of NOAA’s Climate Prediction Center. It’s the job of NOAA’s seasonal hurricane forecasters to predict tropical storm and hurricane activity over the entire six-month season.

The Atlantic hurricane season runs from June 1 through November 30. By far, the bulk of the hurricane activity occurs during August-October, which is the normal peak of the season.

Atlantic hurricanes mostly affect lives and property along the U.S. Gulf Coast, East Coast and Caribbean nations. However, as has been seen in past years, hurricanes and slow-moving tropical systems can track well inland and produce flooding, damage and deaths even far from where the storms initially make landfall. A 2003 NOAA/U.S. Census report noted that 53 percent of the U.S. population lives within 50 miles of the coast. That’s a lot of people who could potentially be in harms way of these dangerous storms. (Click NOAA image to the right for a larger view of Continental United States Hurricane Strikes 1950-2006. Please credit “NOAA.”)

Every year the Climate Prediction Center issues the Atlantic Hurricane Seasonal Outlook in May and updates the outlook in August, just prior to the historical peak of hurricane activity. Started in 1998, NOAA’s seasonal hurricane outlooks have proven to be exceptionally reliable. However, due to a rapidly developing El Niño, the 2006 hurricane seasonal outlook was the only year that did not pan out as predicted.

Gerry Bell, NOAA hurricane climate specialist and lead seasonal hurricane forecaster at the Climate Prediction Center, has been the lead scientist on this project since the very beginning. Bell has assembled a cast of expert meteorologists, scientists, and researchers from the Climate Prediction Center, National Hurricane Center, Hurricane Research Division, and Hydrometeorological Prediction Center. Each expert is called upon to provide an independent, unbiased scientific assessment from their vantage point. This is where the dynamics of the seasonal hurricane forecast process begins.

The Process
Using mathematics, physics, research, global observations, and numerical models run on state-of-the-art supercomputers, seasonal hurricane forecasters clue us in to nature’s signals allowing scientists to give the public:

1. the probabilities of whether the upcoming Atlantic hurricane season will have an above-normal, near-normal or below-normal level of activity;
2. the likely range of tropical storms, hurricanes, and major hurricanes that we can expect during the season, and
3. the scientists’ overall confidence level with the forecast.

In April of each year, the seasonal outlook team begins a series of conference calls with team members from NOAA’s Hurricane Research Division, National Hurricane Center, Hydrometeorological Prediction Center and the Climate Prediction Center. Because of the extensive research done by NOAA and its supporting institutions, the scientists now understand the dominant climate patterns influencing Atlantic hurricane activity better than ever before.

The conditions that determine active and inactive Atlantic hurricane seasons are largely controlled by recurring rainfall patterns along the equator. These patterns are linked to two dominant climate phenomena:

1. The El Niño/ Southern Oscillation (El Niño and La Niña) cycle.
2. The tropical multi-decadal signal, which is a major contributor to the observed 25-40 year alternating periods of active/inactive hurricane seasons. This multi-decadal signal accounts for the inter-related set of atmospheric and oceanic conditions known to produce active hurricane eras, and is strongly related to monsoon rainfall patterns over western Africa and the Amazon Basin, and to Atlantic Ocean temperatures.

NOAA’s seasonal hurricane outlooks result mainly from the analysis and prediction of these two climate phenomena, along with predictions of upcoming Atlantic Ocean temperatures.

"Long term climate trends are incorporated into NOAA's seasonal hurricane outlooks via Atlantic sea surface temperatures, which are only one of a set of conditions contributing to the current active era," said Bell. "An ongoing and unresolved issue is the extent to which global climate change is contributing to the current active Atlantic hurricane era. Many studies are being developed to better understand both the multi-decadal signal and global climate change, as well as possible relationships between these two important climate factors. The goal is to better understand both factors so that we can ultimately quantify how much each is contributing in a given season."

Once the bases of the outlook are understood, the team then goes through the following steps:

Step 1. Predict the likely strength of ENSO, Atlantic sea surface temperatures (SSTs), and the tropical multi-decadal signal. The strength of the tropical multi-decadal signal is currently determined based on its average value in the recent active era, combined with recent SSTs, circulation anomalies, and other factors in core regions of the signal. The ENSO prediction is based on forecast tools updated monthly by the Climate Prediction Center. Atlantic SST predictions are based on ongoing trends and on computer model forecasts from NCEP’s Climate Forecast System.

Step 2. Once the strength of ENSO, the multi-decadal signal, and Atlantic SST departures are predicted, these strengths are used to determine probabilities of season type and ACE (Accumulated Cyclone Energy — the measure of overall seasonal activity). Two complementary techniques are employed:

• Technique 1: Compares past season types based on comparable strengths of predicted ENSO, Atlantic SST departures, and multi-decadal signal. These “climate-based analogue seasons” are used to obtain the likely ACE range and probabilities of various season types.

• Technique 2: Uses equations, derived from historical relationships between these climate factors and seasonal activity, to calculate the circulation patterns and levels of activity historically associated with their predicted strengths.

Step 3: Once ACE and probabilities of season types are determined, the likely ranges of tropical storms, hurricanes and major hurricanes are determined using a similar climate-based analogue approach and regression technique.

Finally, NOAA scientists must quantify their level of confidence with the outlook. This is similar to the way meteorologists predict the percentage chance of precipitation or sky conditions for a given day. NOAA seasonal hurricane forecasters then put the season in context of the probability for each category of an above-, near-, and below-normal season.

The Outlook

The final product is a narrative forecast that is segmented into three parts:

• Summary: gives the public the forecast, probabilities and a glimpse at the climate features expected to affect the season.
  
• Discussion: provides greater details of the science behind the outlook and how seasonal forecasters arrived at the predicted ACE Index.
  
• Cautionary Notes: provide details of what is not intended or included in this forecast product.

The Future of Seasonal Hurricane Prediction
The future of seasonal hurricane prediction will focus on overcoming two major scientific hurdles. One such hurdle is to advance ENSO predictions during what is commonly known as the springtime forecast barrier, the period between March through July when forecasting ENSO can be extremely difficult because the atmosphere is in a state of transition. ENSO forecasts issued during this period are critical, because they are the only meaningful way to predict the likely strength of ENSO during the peak of the hurricane season. An uncertain or poor ENSO forecast can directly lead to less confidence in the Atlantic hurricane seasonal outlook.

The other major forecast hurdle is two-fold: to better understand and predict the seasonal factors that control hurricane landfall, and then to develop a procedure to make confident hurricane landfall forecasts on seasonal time scales. Through the Climate Testbed, the Climate Prediction Center is partnering with the academic community to accelerate the transfer of research in this area into operations. One such effort underway is to predict, on a seasonal time scale, where hurricanes will make landfall. For example, NOAA scientists and researchers at North Carolina State University College of Physical and Mathematical Sciences are in the process of analyzing ways to predict, on a seasonal timescale, how many hurricanes are likely to strike the East Coast and how many hurricanes are likely to strike the Gulf Coast with increased confidence. “This is cutting-edge science,” said Bell. “Currently, the science does not support NOAA using such methods in an operational forecast mode, but progress is being made on many fronts.”

The Climate Prediction Center and the National Hurricane Center are two of nine components of the National Centers for Environmental Prediction, providing first alerts to weather hazards. The Climate Prediction Center researches, assesses, and forecasts short-term climate variability, emphasizing enhanced risks of weather-related extreme events, for use in mitigating losses and maximizing economic gains. Its products are operational predictions of climate variability, real-time monitoring of climate and the required databases, and assessments of the origins of major climate anomalies. The products cover time scales from a week to seasons, extending into the future as far as technically feasible, and cover the land, ocean, and atmosphere extending into the stratosphere.

Relevant Web Sites
NOAA's Atlantic Hurricane Season Outlook & Summary Archive

NOAA's Background Information on the Atlantic Hurricane Season Outlook

NOAA Climate Prediction Center

NOAA’s Definition of ENSO

NOAA Hurricane Research Division

NOAA National Hurricane Center

NOAA National Weather Service Safety/Preparedness

NOAA National Centers for Environmental Protection

FAQ / State of the Science: Atlantic Hurricanes & Climate

NOAA Hurricane Fact Sheet

Media Contact:
Carmeyia Gillis, NOAA Climate Prediction Center, (301) 763-8000 ext. 7163 or NOAA National Weather Service Public Affairs, (301) 713-0622