SECTION 2. U.S. NAVY CLASSIFICATIONS OF OBJECTIVE FORECAST AIDS

Many objective aid forecasts are generated using initial conditions or forecasts supplied by a global or regional numerical model. Objective aids run at FNMOC use NOGAPS global fields while those run for the NHC use the NMC global and regional fields.

The U.S. Navy TC warning agencies (JTWC and the Alternate JTWC at Pearl Harbor) define six different classes of objective aids, while the National Hurricane Center (NHC) defines four classes ( Table 5.1). In this section, only the six classes used at JTWC are discussed. For a detailed discussion of individual objective aids, see Appendix C.

Past speed, direction and intensity trends are used as the initial guidance for short-term forecast decisions, especially during the first 12 to 24 hours. This aid is the best short-term guidance available. Only the current TC track is used as input for this model.

Climatology and analog aids are techniques that use historical storm records as a method of exploiting current and past motion and intensity trends to project future storm positions and intensities. Current and historical TC tracks are used as input for these models. Numerical model output is not required input for these models.

(a) Climatology

These techniques directly use the past motion of the current storm, and average motions of selected historical storms without application of any regression analysis to minimize the average forecast error for some dependent data set. Time and location windows relative to the current position of the storm determine which historical storms are used to compute the forecast guidance.

(b) Analog

JTWC's and NHC's analog aids use the same data base as the climatology aids except the analog techniques impose additional restrictions (e.g., TC speed and direction of motion) to select which storms are used to compute the forecast positions.

The common feature of these models is that regression analysis is used to minimize forecast error. Usually the 24, 48 and 72 hour forecast positions are determined using regression equations for the various types of measured quantities. These predictors may be any combination of parameters from the present storm, historical storms (climatology), synoptic analyses, and numerical prognoses. Many of these techniques require global numerical model analyses and forecasts.

Dynamic forecast methods are based on numerical integration of mathematical equations that approximate the physical behavior of the atmosphere. Dynamic forecasts are derived in a few different ways. Some actually track the movement of a tropical cyclone vortex, which is either explicitly resolved by or bogussed into a global or regional model. Regional NWP models use fine (i.e., finer than global NWP models) horizontal and vertical resolutions to compute storm motions. In addition, they can apply barotropic or baroclinic equations to create the forecast tracks resulting in a variety of solutions. The simplest approach in numerical forecasting is to use either global or regional numerical model wind fields to compute a steering flow that advects a point vortex. These techniques can also provide useful forecast information, particularly tropical cyclone speed forecasts.

These objective aids combine elements of two or more of the above categories. The elements are blended, and their weights are based on their past performance.

These subjective techniques are applied by the forecaster and the results are significantly influenced by the experience level of the forecaster.

Section 1 Section 3

Chapter 5

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