Tuesday, 23 April 2019


A coastal flood occurs when low lying land areas bordering a large body of water are inundated. Coastal flooding may be caused by abnormal rise in water level associated with an approaching ocean storm or by large breaking waves on the shore from large ocean swells. The extent of coastal flooding depends upon the tide levels, the underwater and shoreline topography and the runoff from rivers and estuaries.

Coastal floods are often associated with high water from a storm surge, caused by winds generated by tropical (hurricanes) or extratropical cyclones. Coastal flooding can be produced also by a tsunami, a seismic sea wave produced by earthquakes, landslides or volcanic activity; these waves have been erroneously called tidal waves. Alaska, Hawaii and the Pacific coast are most vulnerable to a tsunami.

The greatest danger and damage associated with most tropical cyclones (hurricanes) is the storm surge. A storm surge is a dome of water, perhaps 50 miles wide that sweeps across the coastline ahead of a tropical cyclone landfall. The magnitude of a storm surge depends upon the strength of the storm, the ocean bottom conditions where the surge comes onshore, the storm's movement relative to the shoreline, and the astronomical tides at landfall time.

In addition, the destruction of the storm surge is often compounded by the hammering effect of the breaking waves. Because many tropical cyclones produce excessive rainfall rates and total amounts of precipitation even after landfall, they may also be responsible for floods and flash floods. The strong and gusty winds, coupled with the possibility of tornadoes, are added perils to residents of areas near the landfall of a hurricane.

Coastal floods and beach erosion can also be associated with extratropical cyclones, such as the "nor'easters", that may batter the East Coast from Cape Hatteras to Maine, especially during the winter season. Coastal flood watches and warnings are issued by certain designated National Weather Service Offices and pertain to the region extending from the ocean beaches inland, including waterways, estuaries and river mouths.

The National Weather Service issues various public statements for tropical weather systems as well as for other types of situations that can cause coastal floods. These statements follow the advisory, watch and warning format previously described, and they are intended to inform the coastal residents and commercial and recreational marine interests of a potentially hazardous weather situation.

The National Weather Service issues various marine-related advisories, watches and warnings to the public that pertain to a variety of severe weather conditions as well as unusual water, waves and current conditions that could affect life and property. The area of responsibility includes the coastal waters and the open waters of the Atlantic and Pacific Oceans and the Gulf of Mexico. Some of these statements cover tropical weather systems, including tropical storms and hurricanes. The Ocean Prediction Center (formerly called Marine Prediction Center or MPC), another component of the National Centers for Environmental Prediction (NCEP), issues marine warnings for situations not involving tropical weather systems. Their responsibility covers coastal and offshore waters as well as the high seas for latitudes poleward of 30 degrees north, while the Tropical Prediction Center is responsible for waters equatorward of this latitude. 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 National Hurricane Center (NHC) in Miami, FL, a part of the Tropical Prediction Center, is responsible for issuing statements concerning tropical weather systems for the North Atlantic Basin (including the Gulf of Mexico and the Caribbean Sea) and the eastern Pacific Basin (to 140 degrees West longitude). 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 following list includes terminology used by the National Weather Service for those public statements pertaining to tropical weather:

The following statements can be issued for several types of events, not limited to tropical weather systems:

The following marine weather statements can be issued:

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.

Additional information is available from "A Mariner's Guide to Marine Weather Services Coastal, Offshore and High Seas" from the National Weather Service at

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2019, The American Meteorological Society.