By Daniel Brown, Dennis Feltgen and James Franklin, National Hurricane Center, Miami, FL
The NWS National Hurricane Center (NHC) will provide greater lead time for tropical cyclone watches and warnings beginning with the 2010 hurricane season.
With the ever-growing population along the U.S. coastline,emergency managers need more lead time to prepare their communities for tropical cyclones. In response, NHC will issue tropical storm and hurricane watches and warnings for threatened coastal areas 12 hours earlier than in previous years.
Tropical storm watches will be issued when tropical storm conditions are possible along the coast within 48 hours. Tropical storm warnings will be issued when those conditions are expected within 36 hours. Similar increases in lead time will apply to hurricane watches and warnings. The hurricane watches and warnings will generally be timed to provide 48 and 36 hour notice, respectively, before the onset of tropical-storm-force winds, a threshold that typically forces a suspension of many hurricane preparedness activities. Detailed below are the new tropical storm and hurricane watch/warning definitions.
New Definitions of Tropical Storms and Hurricane Watches and Warnings:
Tropical Storm Watch means that tropical storm conditions are possible somewhere within the watch area within the next 48 hours. Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within the next 36 hours.
Hurricane Watch means that hurricane conditions are possible somewhere within the watch area. A watch is typically issued 48 hours before the anticipated first occurrence of tropical-storm-force winds. Such winds make preparing outside areas difficult or dangerous.
Hurricane Warning means that hurricane conditions are expected somewhere within the warning area. A warning is typically issued 36 hours before the anticipated first occurrence of tropical-storm-force winds. Such winds make preparing outside areas difficult or dangerous.
Advances in observational capabilities, numerical weather prediction, and forecaster tools over the past 2 decades have enabled NHC staff to make more accurate track forecasts, thus allowing the extension of watch and warning lead times. Over the past 15 years, the average NHC forecast track error has been cut in half (see Figure 1). The average 2-day (48 hour) forecast error rate has been reduced from about 200 nautical miles in 1990 to well below 100 nautical miles in 2009.
NHC Director Bill Read commented, “The change in lead time in the watches and warnings brings us more in line with the lead time needed by emergency managers.” The NWS Central Pacific Hurricane Center in Honolulu instituted the change in 2009.
Glenn M. Cox KE4BMY