DATASTREME ATMOSPHERE SUPPLEMENTAL SUMMARY

To complement the Daily Summary for Tuesday, 2 December 2008

SPECIFIC TROPICAL & MARINE STATEMENT TERMINOLOGY

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 of 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 to 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 -- 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 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. 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.
• 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.
• 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.

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.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.