NHC Track and Intensity Models
Updated 28 February 2017
The term "forecast model" refers to any objective tool used to generate a prediction of a future event, such as the state of the atmosphere. The National Hurricane Center (NHC) uses many models as guidance in the preparation of official track and intensity forecasts. The most commonly used models at NHC are summarized in the tables below.
Forecast models vary tremendously in structure and complexity. They can be simple enough to run in a few seconds on an ordinary computer, or complex enough to require a number of hours on a supercomputer. Dynamical models, also known as numerical models, are the most complex and use high-speed computers to solve the physical equations of motion governing the atmosphere. Statistical models, in contrast, do not explicitly consider the physics of the atmosphere but instead are based on historical relationships between storm behavior and storm-specific details such as location and date. Statistical-dynamical models blend both dynamical and statistical techniques by making a forecast based on established historical relationships between storm behavior and atmospheric variables provided by dynamical models. Trajectory models move a tropical cyclone (TC) along based on the prevailing flow obtained from a separate dynamical model. Finally, ensemble or consensus models are created by combining the forecasts from a collection of other models.
Table 1. Summary of global and regional dynamical models for track, intensity, and wind radii.
Table 2. Summary of ensembles and consensus aids for track and intensity.
Table 3. Summary of statistical models for track, intensity, and wind radii.
Early versus Late Models
Numerous objective forecast aids (guidance models) are available to help the NHC Hurricane Specialists in the preparation of their official track and intensity forecasts. Guidance models are characterized as either early or late, depending on whether or not they are available to the Hurricane Specialist during the forecast cycle. For example, consider the 1200 UTC (12Z) forecast cycle, which begins with the 12Z synoptic time and ends with the release of the official forecast at 15Z. The 12Z run of the NWS/Global Forecast System (GFS) model is not complete and available to the forecaster until about 16Z, or about an hour after the forecast is released – thus the 12Z GFS would be considered a late model since it could not be used to prepare the 12Z official forecast.
Multi-layer dynamical models are generally, if not always, late models. Fortunately, a technique can be used to take the latest available run of a late model and adjust its forecast to apply to the current synoptic time and initial conditions. In the example above, forecast data for hours 6-126 from the previous (06Z) run of the GFS would be adjusted, or shifted, so that the 6-h forecast (valid at 12Z) would exactly match the observed 12Z position and intensity of the tropical cyclone. The adjustment process creates an "early" version of the GFS model for the 12Z forecast cycle that is based on the most current available guidance. The adjusted versions of the late models are known, for historical reasons, as interpolated models.
Interpreting Forecast Models
NHC provides detailed information on the verification of its past forecasts with a yearly verification report (http://www.nhc.noaa.gov/verification/verify3.shtml). On average, NHC official forecasts usually have smaller errors than any of the individual models. An NHC forecast reflects consideration of all available model guidance as well as forecaster experience. Therefore, users should consult the official forecast products issued by NHC and local National Weather Service Forecast Offices rather than simply looking at output from the forecast models themselves. Users should also be aware that uncertainty exists in every forecast, and proper interpretation of the NHC forecast must incorporate this uncertainty. NHC forecasters typically discuss forecast uncertainty in the Tropical Cyclone Discussion (TCD) product. NHC also prepares probabilistic forecasts that incorporate forecast uncertainty information (http://www.nhc.noaa.gov/aboutnhcprobs.shtml).
The National Weather Service produces some of the models used by the National Huricane Center. These models are run by NOAA/NWS National Centers for Environmental Prediction (NCEP) Central Operations (NCO). Output images from the NOAA/NWS models can be found through NCEP's Model Analyses and Guidance (MAG) interface. Raw data from the models can be found through the NOAA Operational Model Archive and Distribution System (NOMADS).
Other model background information
Read about the Inland Wind Model and the Maximum Envelope Of Winds