SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Tuesday, 27 November 2018

COASTAL FLOODING, MARINE WEATHER
SPECIFIC TROPICAL & MARINE STATEMENT TERMINOLOGY

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:

• 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 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 (to include winds greater than 63 knots) 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) a vertical arrangement of two red lanterns surrounding a white lantern by night.

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 to 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 (seismic sea wave) may cause damage to low lying regions. The type of bulletin is based on the magnitude and the location of the source 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 the earthquake epicenter, with the time of tsunami's arrival within 2 to 3 hours.

The following marine weather statements can be issued:

• 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 of anticipated conditions. 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 (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.

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 http://www.nws.noaa.gov/om/brochures/marinersguide_coastal.htm.