August 29, 2017
Forecasting Hurricanes: Role of the National Hurricane Center
The National Hurricane Center (NHC), part of the National
Analyzing the Data
Weather Service (NWS) within the National Oceanic and
The NHC gathers all the observational data collected as a
Atmospheric Administration (NOAA), is responsible for
tropical cyclone approaches the U.S. coastline and uses the
forecasting tropical cyclones, including hurricanes in the
data to generate a series of hurricane forecast computer
Atlantic Ocean and the eastern Pacific Ocean. The NHC
models. The principal purpose of hurricane forecast models
provides estimates of the path of a tropical cyclone (i.e.,
typically is to predict the hurricane track and intensity over
hurricane track), the intensity, and the size and structure of
a period of three to five days. Essentially, the model uses
the storm, as well as predictions of storm surge, rainfall,
the observational data to understand the state of the
and even tornadoes. Depending on the status of the tropical
atmosphere and then uses mathematical equations to
cyclone, this information may be used to create a hurricane
produce forecasts. Not all hurricane forecast models are the
watch or a hurricane warning and public advisories, which
same; they may differ in how they process information,
are issued on an increasingly frequent basis if a storm
such as when observations are fed into the model, which
strengthens and approaches the U.S. coastline.
equations are used, how the solutions to the equations are
used to make forecasts, and other factors. These differences
How the Process Works
explain why NHC hurricane forecasts may differ from those
A hurricane forecast involves many components and uses a
of other countries or institutions that also produce hurricane
broad array of resources and capabilities within NOAA and
forecasts (e.g., the European Center for Medium-Range
the NWS, all of which must be coordinated and interpreted
Forecasts produces Atlantic hurricane forecasts, as do some
by the NHC. The process begins with observations:
research institutions within the United States, such as the
satellites, aircraft, ships, buoys, radar, and other sources
National Center for Atmospheric Research).
provide data used to create storm-track and intensity
predictions. Most Atlantic hurricanes, for example, begin to
Forecasts and Warnings
form just west of the African continent over the ocean.
Using the results from hurricane forecast models, different
NOAA weather satellites primarily provide the remote-
components within and outside the NHC contribute to the
sensing observations during the early stages of tropical
hurricane forecast process. These include the Tropical
storm development in the eastern Atlantic. NOAA’s
Analysis and Forecast Branch (TAFB), the Hurricane
Geostationary Operational Environmental Satellites
Specialist Unit (HSU), and the Hurricane Liaison Team
(GOES) are stationed over the same location spanning
(HLT). Of these, the HSU produces the final, official public
North America (two satellites make up the active GOES
forecast products, issued every six hours after a storm forms
constellation, often referred to as GOES East and GOES
and more frequently if a hurricane watch or a hurricane
West) and provide continuous data as the storms form and
warning is issued. (A hurricane watch is an announcement
intensify during their journey across the Atlantic from
that hurricane conditions are possible within a specified
Africa to North America.
coastal area, usually issued 48 hours in advance of the onset
of tropical-storm-force winds. A hurricane warning is
Approaching the Coast
issued when hurricane conditions—sustained winds 74
If an Atlantic hurricane is judged to pose a threat to the U.S.
miles per hour [mph] or greater—are expected somewhere
coastline, NOAA hurricane aircraft and U.S. Air Force
within the specified coastal area.) The HSU also provides
aircraft (often referred to as Hurricane Hunters) fly directly
briefings on tropical storms to emergency managers and the
into the storm to collect real-time data from inside the
public and cooperates with meteorological services in other
cyclone. Within the NHC, it is the responsibility of the
countries (e.g., Mexico, as well as Central American and
Chief, Aerial Reconnaissance Coordination, All Hurricanes
Caribbean countries). The TAFB supports the HSU by
(CARCAH) unit, to coordinate all tropical cyclone
providing tropical cyclone position and intensity estimates,
operation reconnaissance in accordance with the National
conducting media interviews, and assisting in tropical
Hurricane Operations Plan. Data collected from the
cyclone operations.
Hurricane Hunters and other aircraft (e.g., the NASA
Global Hawk) are checked at CARCAH and provided to
The HLT is sponsored through the Federal Emergency
NHC forecasters. If the storm gets closer, approximately
Management Agency (FEMA) and is comprised of federal,
280 miles from the coast, land-based radars begin to
state, and local emergency managers, FEMA personnel, and
provide the NHC with precipitation and wind-velocity data.
NWS forecasters and hydrologists. On or before the
Additional ground-based measurements are provided by the
beginning of hurricane season (for the Atlantic, June 1 to
Automated Surface Observation Systems instruments when
November 30), the NHC director requests that FEMA
the storm is close to shore or makes landfall.
activate the HLT, which remains active throughout
hurricane season. If a tropical cyclone in the Atlantic or
eastern Pacific basin threatens either the United States or its
territories, then the NHC can request that NWS
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Forecasting Hurricanes: Role of the National Hurricane Center
meteorologists or hydrologists be assigned to the HLT until
predicting landfall location. That increased accuracy
the storm threat has passed.
provides useful information to local, state, and federal
emergency managers faced with decisions about evacuating
The HLT serves as a liaison between the NHC forecasters
coastlines and with staging emergency equipment and
and local emergency managers facing a threat of tropical
supplies. Better instruments aboard weather satellites, such
cyclones. A main function of the HLT is to communicate
as the GOES series and the polar-orbiting weather satellites,
the level of the threat to federal, state, and local officials.
have contributed to those improvements, together with
Neither the HLT nor the NHC assumes any responsibility
more sophisticated hurricane forecast models and vastly
for issuing evacuations, however; those decisions are left to
improved computing capabilities.
state or local officials.
Flooding resulting from intense precipitation during
Forecasts and warnings generally are coordinated between
hurricanes could be responsible for the majority of fatalities
the NWS national centers and the local forecast offices.
caused by a storm. Some studies show, however, that the
When the NHC issues its forecast, local NWS Weather
amount of rainfall produced by a storm may not necessarily
Forecast Offices use the information for their own
be related to the intensity of the hurricane. Intense
forecasts, which take into consideration local conditions.
precipitation and flooding during Hurricane Harvey, for
example, continued even after the storm made landfall and
Storm Surge
was downgraded from a hurricane to a tropical storm.
In addition to the hurricane forecast models that predict
Improving the ability to accurately forecast the amount and
intensity, winds, and precipitation, other forecast models
location of intense precipitation would advance the value of
are designed specifically to forecast storm surge. Storm
hurricane forecasting significantly.
surge is defined as an abnormal rise in sea level
accompanying a hurricane or other intense storm. It
Perhaps the biggest challenge for the NHC is how to
generally refers to the difference between the measured
improve the ability to predict hurricane intensity, basically
level of the sea surface during the storm compared to what
how strong a hurricane will become and when and where it
the sea level would have been without a storm. (See Figure
will reach its maximum intensity. In addition, hurricane
1.) Storm surge, combined with high precipitation from a
intensity itself is generally measured as highest sustained
hurricane, can create dangerous flood conditions for coastal
wind speeds, referred to as the Saffir-Simpson wind scale
communities during tropical cyclones and other intense
(1-5), ranging from sustained winds of 74-95 mph
storms. (Superstorm Sandy, for example, produced a storm
(Category 1) to 157 mph or higher (Category 5). However,
surge that inundated the heavily populated coastline in New
that measurement does not take into consideration the storm
Jersey and New York to cause enormous damage, despite
surge or precipitation-caused flooding, both of which may
the storm itself being a relatively weak hurricane
be among the most dangerous elements of a hurricane. (For
immediately prior to landfall.)
instance, intense, concentrated, and multiday precipitation
and flooding in and around the Houston, TX, region caused
Figure 1. Storm Surge
by Hurricane Harvey has been the storm’s most destructive
and dangerous feature.) Despite not being part of the
hurricane-intensity scale, hurricane forecast information
typically includes precipitation estimates—as in the case of
Hurricane Harvey—and storm-surge estimates, so that
affected communities may better prepare for the impending
hazard.
Legislation
The 115th Congress recently enacted P.L. 115-25, the
Weather Research and Forecasting Act of 2017, which
provides direction to NOAA and NWS on a broad range of
Source: National Hurricane Center at http://www.nhc.noaa.gov/
weather-related activities. The primary aim of the
surge/ssu.php.
legislation is to improve NOAA’s weather forecasts and
warnings. Section 104 of Title 1 of P.L. 115-25 is
The Storm Surge Unit, part of the NHC, models and
specifically aimed at improving hurricane forecasting. The
predicts storm-surge vulnerability over the U.S. Atlantic
legislation directs NOAA to provide a plan to implement its
and Gulf coasts, Hawaii, Puerto Rico, and the U.S. Virgin
project to improve hurricane forecasting within one year of
Islands. The Storm Surge Unit typically commences its
enactment (by April 2018). For more detailed information
operations when a hurricane warning is issued and
about P.L. 115-25, see CRS Report R44838, The Weather
continues 24 hours per day if the hurricane threatens to
Research and Forecasting Innovation Act of 2017:
make landfall until the storm surge threat subsides.
Congressional Direction to NOAA in P.L. 115-25, by Peter
Challenges
Folger.
Hurricane forecasts have improved in recent decades. In
Peter Folger,
Specialist in Energy and Natural
1992, hurricane forecasts extended out only three days;
today, the forecasts typically extend for five days. Over that
Resources Policy
same time span, the ability to forecast hurricane tracks also
IF10719
has improved, increasing the likelihood of more accurately
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Forecasting Hurricanes: Role of the National Hurricane Center


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