Tropical Cyclone Wind Data and Report

Questions & Answers: 

1. How should wind estimates provided by Early Alert be interpreted?

Tropical Cyclone (TC) wind estimates for sites provided by Early Alert represent the “worst case” estimates for a given tropical cyclone. Due to the uncertainty and chaotic nature of tropical cyclone movement and intensity, it is recommended that this wind table report and data be used for planning purposes only and not verbatim or a precise expectation.

2. Why are “worst case” estimates used?

They are used to assist decision makers to account for the typical Saffir-Simpson scale intensity forecast error of one category. Hence, decision makers are able to plan for the “worst case” storm should there be a forecast error of one category too low.

3. What is the “worst case” winds based upon?

Various assumptions are used in the “worst case” wind computations which are designed to err on the high side. Specifically, they are based upon the National Hurricane Center (NHC) forecast of maximum winds exceeding tropical storm and hurricane force wind thresholds (e.g., 74 mph) and how far the winds extend from the tropical cyclone’s center in the storm’s four quadrants (i.e., northeast, southeast, southwest, and northwest).

4. If the NHC forecast of tropical cyclone intensity is accurate with respect to the Saffir-Simpson scale,, how would the “worst case” wind estimates compare to observed wind? 

In short, the “worst case wind” estimates would be too high. This is so for two principal reasons. The maximum wind is the highest speed observed at a given point within a given forecast area or the highest wind observed at given point over time. Hence, the observation of a maximum wind is relatively small in scale and brief in time. Also, our computations are based upon NHC maximum wind estimates. While NHC does lower wind speeds for a tropical cyclone crossing land, their estimates do not fully account for the reduction in wind speeds due to the relative degree of friction or roughness of the terrain at various locations and the influence of vegetation or buildings.

5. Tropical Cyclone Decision Making for Protective Actions.

All tropical-storm-force wind time frames, displayed within the wind tables, are based on the anticipated onset of tropical-storm-force winds, set at a sustained 45 mph, potentially affecting the location and typically in concert with Civil authorities (Emergency Management, Law Enforcement, Fire Rescue, etc.). It is recommended that all protective actions are complete prior to and in advance of the anticipated onset of sustain tropical-storm-force winds.

Note: Due to the uncertainty and chaotic nature of tropical cyclone movement and intensity, it is recommended that this windfield table report be used for planning purposes only and not verbatim or a precise expectation.

6. Tropical Cyclone Wind Arrival and Departure Periods

Due to the uncertainty and chaotic nature of tropical cyclone movement and intensity, the tropical cyclone wind report indicating “Arrival and Departure Periods” is set by increments of 3 hours, instead of a precise expectation of time. Therefore, it is recommended that the windfield table report be used for planning purposes only.