Forecasting Tropical Cyclones: NOAA’s Role

Forecasting Tropical Cyclones: NOAA’s Role

Updated August 10, 2022
Forecasting Tropical Cyclones: NOAA’s Role
Timely and accurate forecasts of tropical cyclones,
Operational Environmental Satellites and Joint Polar
including tropical depressions, tropical storms, and
Satellite System, provide data as TCs form and intensify
hurricanes (hereinafter TCs or storms), can provide life-
during their journey across ocean regions.
and property-saving warnings to parts of the U.S. coastline
vulnerable to a storm’s impacts. The National Hurricane
If a storm is judged to pose a threat to U.S. interests,
Center and Central Pacific Hurricane Center (NHC), part of
NOAA and U.S. Air Force aircraft (often referred to as
the National Weather Service (NWS) within the National
Hurricane Hunters) may fly into the storm to collect real-
Oceanic and Atmospheric Administration (NOAA), is
time data. Moored and drifting buoys may collect additional
responsible for forecasting TCs in the Northern Atlantic
meteorological and oceanographic information. If the storm
Ocean and the Central and Eastern Pacific Ocean. (Other
gets close enough to the coast, land-based radars provide
entities are responsible for these storms in other parts of the
NHC with precipitation and wind-speed data. Additional
world.)
measurements are provided by NWS’s ground-based
Automated Surface Observation Systems and NOS’s
Tropical Cyclone Designations
National Water Level Observing Network instruments
when the storm is close to shore or makes landfall.
Tropical cyclones are categorized based on their wind speeds.
Designations include
Analyzing the Data

Tropical depressions—maximum sustained winds of ≤ 38
NHC gathers all the observational data to generate
miles per hour (mph).
numerical weather prediction models. The models use the

data to determine the state of the atmosphere and the ocean

Tropical storms—maximum sustained winds of 39-73
mph. NOAA typically names a storm once it reaches this
and then use mathematical equations to produce TC track
strength.
and intensity forecasts. Not all numerical weather
prediction models are the same; they may differ in how they

Hurricanes—maximum sustained winds of 74-95 mph
process information, such as when observations are fed into
(category 1 on the Saffir-Simpson Hurricane Wind Scale)
the model, which equations are used, how the solutions to
and 96-110 mph (category 2). Hurricanes also may be
the equations are used to make forecasts, and other factors.
called typhoons or cyclones.
These differences explain why NHC forecasts may differ

Major hurricanes—maximum sustained winds of ≥111
from those of other countries or institutions that also
mph, corresponding to a category 3, 4, or 5 hurricane.
produce forecasts (e.g., the European Center for Medium-
Range Forecasts produces Atlantic TC forecasts).

A tropical cyclone forecast uses a broad array of resources
Forecasts, Watches, and Warnings
and capabilities within NOAA, which must be coordinated
Using model outputs and their understanding of the
and interpreted by NHC. NHC estimates a tropical
meteorological and oceanographic data, NHC forecasters
cyclone’s track (i.e., the storm’s path) and intensity (i.e., its
produce tropical weather text and graphical products for
wind speed), as well as the size and structure of the storm.
emergency managers and the public. These include
NHC works with other parts of NOAA, such as the
outlooks covering the next 120 hours (5 days), watches and
National Environmental Satellite, Data, and Information
warnings, discussions, and updates. These products note
Service (NESDIS), National Ocean Service (NOS), and
storm location, motion, pressure, and wind speeds; contain
Office of Marine and Aviation Operations, on these
information about hazards to land (i.e., storm surge, surf,
estimates and on predictions of associated storm surge,
wind, rainfall, tornadoes, and inland hazards); and indicate
rainfall, and tornadoes.
the next expected advisory. NHC updates the products on a
set schedule throughout the storm’s lifetime, and more
This In Focus describes NHC TC-related activities, from
frequently as appropriate.
storm formation through forecasts and warnings. It also sets
out ongoing TC forecasting challenges that Congress may
Tropical Cyclone-Related Watches and
consider via oversight or legislative actions.
Warnings
Storm Observations
NWS may issue tropical cyclone-related watches and
The forecasting process begins with observations: satellites,
warnings depending on a tropical cyclone’s characteristics.
aircraft, ships, buoys, radar, and other sources provide data

Watches for storm surge and hurricane or tropical
used to create storm-track and intensity predictions. NOAA
storm-force winds. For example, a hurricane watch is an
weather satellites primarily provide the remote-sensing
announcement that hurricane conditions (sustained wind
observations during the early stages of storm development.
speeds of 74 mph or higher) are possible within a
Several satellites, including NESDIS’s Geostationary
https://crsreports.congress.gov

link to page 2
Forecasting Tropical Cyclones: NOAA’s Role
In addition to working with local, state, and federal
specified area, issued 48 hours in advance of the onset of
stakeholders, NWS provides briefings on storms to the
tropical storm-force winds.
public and cooperates with meteorological services in other

Warnings for storm surge, hurricane or tropical storm-
countries (e.g., Mexico and other Central American and
form winds, and extreme wind. For example, a hurricane
Caribbean countries). Neither the Hurricane Liaison Team
warning is issued when hurricane conditions are expected
nor NWS issues evacuations; those decisions are left to
somewhere within the specified area, issued 36 hours in
state or local officials.
advance of tropical storm-force winds.
NWS may release other storm-related warnings and watches
Challenges
for hazards such as flooding and tornadoes.
NOAA has identified several ongoing challenges in
forecasting TC intensity and storm-associated rainfall and
in sharing that information with the public, among other
NHC forecasts, watches, and warnings are coordinated with
forecasting concerns. Congress addressed these topics in a
local NWS Weather Forecast Offices that may be affected
broad weather forecasting law, the Weather Research and
by the storm. These offices use the information for their
Forecasting Innovation Act of 2017 (P.L. 115-25).
own forecasts, which consider local conditions.
Congress may consider conducting further oversight on
NOAA’s implementation of the act or directing NOAA to
Storm Surge
address these issues in additional ways.
Forecasts also provide information about potential storm
surge in coastal areas. Storm surge is defined as an
Intensity
abnormal rise in sea level accompanying a TC or other
While storm track forecasts have improved steadily since
intense storm, above the predicted astronomical tide
the 1960s, improvements to intensity forecasts have not
(Figure 1). Storm surge is primarily caused by storm winds
kept pace. Additionally, understanding how and why a
pushing waters toward the shore. The amount of surge is
storm may experience rapid intensification, where its wind
determined by the storm’s intensity; size; the storm’s
speed increases at least 35 mph in 24 hours (e.g.,
forward speed and angle of approach to the coast; the shape
Hurricanes Dorian in 2019 and Laura in 2020), remains a
of the coastline; the slope of the ocean bottom; and the
challenge. Storms that undergo rapid intensification close to
incidence of local features, such as bays and rivers. NHC
shore may be especially dangerous and destructive if
issues storm surge watches or warnings along the U.S.
forecasts do not predict the possibility of increased storm
Atlantic and Caribbean coasts.
intensity in time for coastal residents to prepare. According
to NOAA, forecast models currently are limited by
Figure 1. Storm Surge
computing capability, which cannot accurately simulate the
inner core of a TC, a key part of a storm’s intensity. NOAA
continues to work with other stakeholders, such as the
Naval Research Laboratory, to improve wind models.
Rainfall
Forecasting storm precipitation continues to be a challenge.
According to NOAA, flooding resulting from extreme
precipitation during TCs is responsible for a significant
portion of fatalities caused by a storm. The amount of
rainfall produced by a storm, however, may not necessarily
be related to the intensity of the storm. Rainfall location and
Source: National Hurricane Center at http://www.nhc.noaa.gov/
amounts appear to be more closely tied to a storm’s track
surge/ssu.php.
and the underlying geographic terrain than to storm
intensity. Improving track forecasts, and using models at
Rainfall
varying time and space scales, may help mitigate flood
The Weather Prediction Center provides rainfall forecasts
impacts, according to NOAA.
to NHC when TC wind warnings are in effect and as
appropriate. These forecasts note geographic areas at
Communication
greatest risk, including inland areas, and expected area-
Storm-related fatalities continue to occur in the United
average amounts. Storm surge, especially when combined
States even as storm forecasts improve, in part, because of
with large amounts of rainfall from a TC, can create
how forecasts, watches, and warnings are communicated to
dangerous flood conditions for coastal communities.
the public. For example, a storm’s Saffir-Simpson scale
rating (e.g., Category 1) does not reflect the full suite of
Working with Other Stakeholders
potential hazards, such as expected storm surge or
NWS participates in the Hurricane Liaison Team, led by the
precipitation-caused flooding. Incorporating social and
Federal Emergency Management Agency (FEMA). The
behavioral science research may improve how storm risks
term coordinates federal, state, and local emergency
are communicated and acted upon, according to NOAA.
managers; FEMA personnel; and NHC and Weather
Prediction Center forecasters and hydrologists. FEMA
Eva Lipiec, Analyst in Natural Resources Policy
typically activates the team for the duration of the
hurricane season, which runs May 15 or June 1, depending
IF10719
on the ocean region, through November 30.
https://crsreports.congress.gov

Forecasting Tropical Cyclones: NOAA’s Role

Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff to
congressional committees and Members of Congress. It operates solely at the behest of and under the direction of Congress.
Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has
been provided by CRS to Members of Congress in connection with CRS’s institutional role. CRS Reports, as a work of the
United States Government, are not subject to copyright protection in the United States. Any CRS Report may be
reproduced and distributed in its entirety without permission from CRS. However, as a CRS Report may include
copyrighted images or material from a third party, you may need to obtain the permission of the copyright holder if you
wish to copy or otherwise use copyrighted material.

https://crsreports.congress.gov | IF10719 · VERSION 5 · UPDATED