SECTION 4. WARNING AND FORECAST PROCEDURES

As the previous discussion indicates, application of objective aid guidance to TC forecasting is a tricky business. This section describes how the forecast centers apply objective aid guidance to their forecast procedures; and how a single station forecaster, or forecaster at sea, could apply what is written in the official forecast discussions to forecast recommendation preparations.

 

The JTWC prepares analyses of the current and forecast synoptic situations to determine the TC's synoptic environment and the TC's motion trends. Then the Typhoon Duty Officer (TDO) downloads and displays the current set of forecast aids, and evaluates the pattern produced by the set of forecasts according to the following two considerations:

 

(a). The degree to which the current situation is considered to be and will continue to be climatological is refined by comparing the forecasts of the climatology-based objective techniques, dynamically-based techniques, and past motion of the present storm. This assessment partially determines the relative weight given the different classes of objective techniques.

 

(b). The spread of the pattern determined by the set of objective forecasts is used to provide a measure of the predictability of subsequent motion, and the advisability of including a low or moderate probability alternate scenario in the prognostic reasoning message or warning. The spread of the objective aids is typically small before and large near recurvature or during a quasi-stationary or erratic movement phase.

 

The TDO constructs the official forecast giving consideration to (1) the extent that the synoptic situation is and is expected to remain climatological, (2) past statistical performance of the various objective aids on the current storm, and (3) known properties of individual objective techniques given the present synoptic situation. The following guidance for weighting the objective aids is applied:

 

(a). Weight persistence strongly in the first 12 to 24 hours of the forecast period.

 

(b). Give significant weight to the last JTWC forecast at all times, unless there is significant evidence to warrant a departure. (Also utilize latest forecasts from regional warning centers, if applicable).

 

(c). Give more weight to the techniques that have been performing well on the current TC.

 

(d). Stay within the "envelope" determined by the spread of objective aid forecasts unless there is a specific reason for not doing so (all 12-hour objective aid forecasts are at a significant angle relative to the current motion of the TC) (JTWC, 1991).

Before numerical aid forecasts are applied at NHC, an analysis of the available global, regional and area numerical model analysis and prognosis is performed. This allows the forecaster to formulate an expected long-term TC track prior to applying the numerical aids.

In addition, animated satellite imagery is analyzed for qualitative assessment of current flow pattern and it's changes. Special emphasis is placed upon animated water vapor imagery for regions of moist and dry flow.

The next step involves detailed analyses of the TC itself. These analyses involve all available satellite, reconnaissance aircraft, buoy, and radar data, and ship observations to determine present and past motion, wind and pressure field distributions, etc. This information is used as input data for the five to seven numerical forecast models run during each forecast cycle.

Following the above procedures, the NHC forecaster arrives at an independent forecast based primarily on the guidance received from the NMC model outputs.

Single station TC forecasters monitor TCs that threaten DOD assets and provide forecast recommendations to area and task force commanders. The commanders then set conditions of readiness to safeguard manpower and materials. The following is a suggested procedure to assess the TC warning and its effect on naval operations:

 

(a). Pre-deployment Step. Numerical forecast guidance accuracy is key to the accuracy of TC warnings. Therefore, it is a good idea to become familiar with the numerical forecast guidance used by forecasters within the deployment area. The quarterly model performance summary from FNMOC, the JTWC Annual TC Reports, and Diagnostic Reports of the National Hurricane Center provide information on model performance. Pertinent information from these reports should be provided to operations officers and commanding officers during the pre- deployment briefings and prior to any TC evasion evolutions.

 

(b). Global Model Analysis Step. When a TC develops or moves into the forecast area, plot the warning and analyze the current synoptic environment to determine key synoptic features affecting the current motion and intensification trend. This analysis should center on how well the model forecasts verify against the analyses. One suggested approach is to overlay the surface and 500-hPa 24, 36, 48 and 72 hour forecast charts on the verifying analyses, and mark the areas which indicate significant departures from the analyses. Next identify the key features within 30 degrees longitude and latitude of the TC that appear to affect the motion and intensity change. Check both the overall synoptic pattern and individual values of pressure and wind. By performing this step, quick assessment of the global model performance can be made.

 

(c). Numerical Guidance Verification Step. In this phase, the TC forecast numerical guidance discussion is verified for accuracy and significant departures from the previous forecast discussions. As a first step, plot the official motion and intensity forecasts on the verifying prognostic charts from the NWP model (e.g., the NOGAPS 500-hPa 48-hour forecast) and notes the location with respect to the major synoptic features identified during the previous step. An initial assessment of forecast confidence can now be made. Table 5.2 shows examples of confidence value assignments. If a low confidence scenario is identified during the numerical aid verification step, the forecaster must make every effort to understand why this discrepancy exists. A review of the synoptic situation could shed some light on the situation. If the forecast center is changing from one primary forecast scenario to another, the numerical guidance should be described in detail in the prognostic discussions.

 

(d). Forecast Recommendations Step. A forecaster on a ship at sea must remember that TC position forecast errors can be quite large. For example, the average forecast errors for JTWC in the western North Pacific are approximately 120 nm at 24 hours, 240 nm at 48 hours, and 360 nm at 72 hours; however, individual forecast errors can be greater than 1000 nm. Therefore, it is important that the on-scene forecasters assess the numerical guidance information received, determine a confidence value for the official forecast, and brief this confidence value to on-scene commanders.

 

Section 3 Appendix A

Chapter 5