A Wind Speed Probability–Based Error Swath

Abstract The Joint Typhoon Warning Center (JTWC) generates a tropical cyclone error swath, depicted on each official warning graphic, based on the forecast track, 5-yr mean forecast track errors, and the size of the largest forecast 34-kt (1 kt ≈ 0.51 m s −1 ) wind radii quadrant for each forecast lead time. Unlike error swaths from other agencies such as the National Hurricane Center or Central Pacific Hurricane Center, JTWC error swaths are intended to describe the uncertainty in the potential extent of 34-kt winds, forming what is commonly known as a ship avoidance area to military operators. In this effort, the performance of the JTWC “traditional error swath” is compared to one constructed from the operational wind speed probability product for the 2022 season. The 5% threshold of the operational cumulative 34-kt wind probabilities also provides a graphical swath, but with a 15% (960 000 n mi 2 ; 1 n mi = 1.852 km) smaller area and covers 3% less of the JTWC-analyzed 34-kt wind speed area, on average. Much of the difference is seen in landfall cases, where neither algorithm is designed to address interactions between tropical cyclone winds and complex terrain. Additional advantages of the 5% threshold of cumulative 34-kt wind probabilities are as follows: 1) the definition is easy to explain to operators, 2) forecast wind asymmetries from the official forecast are addressed, 3) track guidance uncertainty is included in the algorithm, and 4) concerted efforts to improve the wind speed probability algorithm continue. Significance Statement All Joint Typhoon Warning Center (JTWC) warnings include an error swath that is routinely used for ship routing and base preparedness planning and decision-making. As forecasts improve, these error swaths should also improve by becoming smaller. Transition from the JTWC traditional error swath based on forecast-track-year mean forecast errors and the largest 34-kt wind radii to the wind speed probability–based swath results in a 15% (960 000 n mi 2 ) mean reduction in avoidance area, with only a 3% reduction in JTWC-analyzed 34-kt wind speed area covered, mostly over the land. Transition to the wind speed probability–based swath provides more area for ship navigation and less overprediction of tropical cyclone wind threat at bases. Switching to a probability-based error swath supports the U.S. Navy 2009 Pacific command operations (PACOM/J3) letter’s goal to reduce the size of the area of uncertainty about the forecast position by 50% to increase the operating space for the United States and coalition forces near a cyclone and to decrease the number of unnecessary storm evasions to provide operating cost savings.