The hurricane forecast process

One of the topics covered as part of North Carolina's Hurricane Preparedness Week is the overall process used to detect tropical cyclones and predict their behavior, and to convey that information to the public. It's a process that has become well-established through the years, but also continues to evolve and hopefully improve, due to gradual enhancements in technology, computer power, instrumentation, procedures and the scientific understanding of storm behavior.

In our country, the system is centered around the National Hurricane Center (NHC), a subset of NOAA's Tropical Prediction Center that is responsible for operational surveillance and detection of newly forming tropical cyclones, prediction of the path and intensity of these storms as they move along, and the issuance of forecasts, watches and warnings for use by the public as well as emergency management and other government officials.

An extensive network of sensors, from undersea current, temperature and salinity probes to surface buoys, land and ship-based weather instruments, land and ship-based radiosonde balloons, weather radar sites, reconnaissance aircraft and satellites, provides data and imagery that NHC uses to monitor and analyze the tropical atmosphere. In addition, forecasters at NHC utilize a variety of individual dynamical and statistical computer models, and systems of multiple models called ensembles, to predict the movement and intensity of tropical storms and hurricanes once they form.

Based on these models, together with the forecasters' training and experience, they produce the official forecasts that specify the expected location of the circulation center at  12, 24, 36, 48, 72, 96 and 120 hours in the future, together with a sustained maximum wind speed and size of the wind field for each of those times. These forecasts are updated every six hours, around 5 am and pm, and 11 am and pm for our time zone. They are then disseminated through a series of text and graphical products available on the NHC web site and in various ways translated into suitable forms for those of us in broadcast meteorology to present on television.

One of the products you're probably most familiar with is the track forecast that includes a "fan" or "cone" of uncertainty that accounts for potential errors in the forecast over time. It's interesting to note that these errors have been reduced substantially through recent decades, including a reduction by one-half in just the past 15 years. This improvement has allowed the "fan" to be narrowed considerably and still have about a 60-70% chance that the entire forecast track remains within the shaded area, and has also prompted extending the period of the forecast out to five days from an earlier limit of three.

When appropriate, they NHC forecasters also make the call as to which coastal areas should be placed under a tropical storm or hurricane watch or warning, a heavy responsibility due to the local emergency preparedness plans that may be implemented, up to and including mass evacuations, major highway reversals, activation and deployment of National Guard resources and the like. The track forecast improvements I mentioned above have also impacted confidence in this process, and beginning with this 2010 hurricane season, watches and warnings will be issued up to 12 hours earlier than in the past. Now, watches (meaning tropical storm or hurricane conditions are possible) will be issued up to 48 hours before those conditions are anticipated, while warnings (meaning conditions are likely) will be issued up to 36 hours out.

Finally, the guidance provided by NHC is also interpreted and locally disseminated by National Weather Service Forecast Offices such as those in our state in Wilmington, Newport and right down the street from here in Raleigh. At these offices, the broad outlines of storm behavior in the track/intensity projections are translated into much more specific forecasts, advisories and warnings regarding the potential for heavy rain, storm surge, inland flooding, wind damage and tornadoes that a hurricane can bring.

Be sure to check back here tomorrow for Kim Deaner's post on being prepared for a hurricane's arrival.