SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

28 October-1 November 2019

MARINE & TROPICAL WEATHER STATEMENTS

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 to sea.

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 property.

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 U.S. Government.

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 region.

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 hours.
• 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 action.
• 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 conditions warrant:

• 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.

Marine weather information, including forecasts and warnings, are transmitted to mariners and other interested parties by a many 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.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2019, The American Meteorological Society.