SUPPLEMENTAL INFORMATION...IN GREATER
25-29 October 2021
MARINE & TROPICAL WEATHER
Weather systems moving across the ocean can produce
life-threatening situations not only for mariners at sea but also for
those living in coastal communities. Therefore, a National Weather
Service (NWS) program monitors the weather, prepares weather forecasts,
and issues warnings for marine and coastal interests. The NWS area of
responsibility includes the coastal and open waters of the Atlantic and
Pacific Oceans, the Gulf of Mexico, and the Great Lakes. Data used in
preparing these forecasts are obtained from a variety of sources,
including ships, buoys, and Earth-orbiting satellites.
Coastal or near shore forecasts are intended for those
mariners staying in coastal waters that are roughly within 20 nautical
mi of the coast. Offshore forecasts are for those mariners operating
farther offshore, typically a day or more from safe harbor, or between
60 and 250 nautical mi offshore. Open seas forecasts are mainly geared
for large ocean-going vessels operating more than 250 nautical mi out
In addition to the forecasts, various marine-related
advisories, watches and warnings are issued to the public. These
pertain to a variety of severe weather conditions as well as unusual
water, wave, and current conditions that could affect life and
THE FORECAST CENTERS
The Ocean Prediction Center (OPC), formerly known as the
Marine Prediction Center (MPC), is the component of the National
Centers for Environmental Prediction (NCEP) that issues marine
forecasts for coastal and offshore waters as well as the high seas of
the western North Atlantic and much of the North Pacific poleward of 30
degrees N. This center also issues marine warnings for situations not
involving tropical weather systems.
The Tropical Prediction Center (TPC), another component of the
National Centers for Environmental Prediction, is responsible for
marine forecasts equatorward of 30 degrees N. This responsibility
includes the tropical North Atlantic, the Caribbean Sea, the Gulf of
Mexico and eastern portions of both the North and South Pacific Oceans.
The Honolulu Forecast Office (HFO) has responsibility for central and
western portions of the North and South Pacific Oceans that include the
Hawaiian Islands and the Pacific Islands that are administered by the
The National Hurricane Center (NHC) in Miami, FL, part of the
Tropical Prediction Center, is responsible for issuing statements
covering tropical storms and hurricanes for the North Atlantic Basin
(including the Gulf of Mexico and the Caribbean Sea) and the eastern
Pacific Basin (to 140 degrees W). The Pacific Hurricane Center in
Honolulu, HI monitors the Central Pacific to the International Date
Line (at 180 degrees). The Hawaiian Islands are located within this
The Pacific Tsunami Warning Center (PTWC) in Honolulu, HI and
the West Coast/Alaska Tsunami Warning Center (WC/ATWC) in Palmer, AK
issue tsunami watches and warnings for Pacific basin.
The following terminology is used by NOAA's National Weather
Service for public statements pertaining to tropical weather:
- Tropical storm watch --
Issued by the National Hurricane Center to alert the public
in a coastal area when a threat of tropical storm conditions (sustained
winds within the range 34 to 63 knots or 39 to 73 mph) is predicted
within 36 hours.
- Tropical storm warning --
Issued to warn the public of imminent tropical storm
conditions (sustained winds within the range of 34-63 knots), which are
expected in a specified coastal area within 24 hours or less.
- Hurricane watch --
Issued by the National Hurricane Center for specific areas
that an incipient hurricane condition poses a possible threat to
coastal areas generally within 36 hours. The public should be prepared
to take precautionary action if a hurricane warning were issued.
- Hurricane warning --
Issued to warn the public that one-minute sustained surface
winds of 64 knots (74 mph) or higher associated with a hurricane are
expected in a specified coastal area within 24 hours or less. A
hurricane warning can remain in effect when dangerously high water and
exceptionally high waves continue although winds may be less than
hurricane force. The public should take immediate action. For mariners,
the hurricane warning signals for this condition are (a) two flags that
are square red with black center by day and (b) two red lanterns
surrounding a white lantern by night.
The National Hurricane Center (Tropical Prediction Center)
issues Tropical Weather Outlooks. The information
contained in these guidance products is used on television
weathercasts. These Outlooks include levels of risk.
The following statements can be issued for several types of
events, not limited to tropical weather systems:
- Coastal flood watch --
Issued to alert coastal residents of the possibility of the
inundation of land areas along the coast within the next 12 to 36
- Coastal flood warning --
Issued to warn residents of coastal areas that land areas
along the coast will be inundated by sea water above the typical tide
- Heavy surf advisory --
Issued to inform the public that high ocean surf may pose a
threat to life or property. The criteria for such advisories depend
upon the locale, but typically, these include minimum wave heights of
between 8 and 12 feet with periods on the order of 10 seconds. The
heavy surf is typically produced by large ocean swells associated with
a distant storm system over the ocean, supplemented at times by
astronomical high tides.
- Tsunami watch/warning
Issued by the National Weather Service to either alert or
warn residents in regions along the Pacific Ocean that an impending
tsunami may cause damage to low lying regions. The type of bulletin is
based on the magnitude and the location of the underwater geological
event. The content and format are similar to coastal flood watches and
warnings. Usually, the tsunami warning is issued for a limited area
around an earthquake epicenter, with the time of tsunami's arrival
within 2 to 3 hours.
The following marine weather statements can be issued if
- Small-craft advisory --
Issued to advise mariners of sustained (exceeding two
hours) weather and/or sea conditions, either present or forecast,
potentially hazardous to small boats. These conditions generally
include winds of 18-33 knots (21-38 mph) and/or dangerous wave
conditions. Small craft advisories may be issued also for hazardous sea
conditions or lower wind speeds that may affect small craft operations.
Advisories can be issued up to 12 hours prior to the onset of adverse
conditions. The small-craft advisory signals for this condition are (a)
one triangular red pennant by day, and (b) a red lantern over a white
lantern by night.
- Small-craft warning --
Issued as a warning, for marine interests, of impending
winds up to 28 knots (32 mph); used mostly in coastal or inland waters.
- Gale warning --
A storm warning, for marine interests, of impending winds
associated with extratropical low pressure systems with speeds ranging
from 34 to 47 knots (39 to 54 mph) within a 24-hour period. The storm
warning signals for this condition are (a) two triangular red pennants
by day, and (b) a white lantern over a red lantern by night.
- Storm warning
(also known as whole-gale warning) -- A warning, for marine interests, of impending winds
associated with an extratropical low that are greater than 48 knots (55
mph). The storm-warning signals for this condition are (a) one square
red flag with black center by day and (b) two red lanterns by night.
- Special marine warning --
A warning issued for marine interests of potentially
hazardous over-water events of relatively short duration, usually up to
2 hours. Typically, these warnings are issued for strong to severe
thunderstorms that may include strong winds, frequent lightning, heavy
rains or waterspouts.
DISSEMINATION OF MARINE WEATHER INFORMATION
Marine weather information, including forecasts and warnings,
are transmitted to mariners and other interested parties by several
methods. The U.S. Coast Guard transmits weather maps to ships at sea by
HF Radiofax and forecasts by either voice (HF, VHF or MF radio) or text
transmission (NAVTEX). Recently, marine weather information can be
obtained using "Internet-Ready" digital cellular phones and Personal
Data Assistants (PDAs). Coastal and near shore forecasts (typically
within about 25 mi of shore) can be obtained from the NOAA Weather
Radio network. Graphics and text are also available on the Internet
through the National Weather Service.
Return to Ocean Studies Maps & Links Page
Prepared by Edward J. Hopkins,
© Copyright, 2021, The American Meteorological Society.