Seven-day Intensity and Intensity Spread Predictions for Western North Pacific Tropical Cyclones

Our Weighted-Analog Intensity technique for western North Pacific (now called WAIP) tropical cyclone intensity and intensity spread guidance has been extended from five days to seven days. A perfect-prog approach that utilizes the Joint Typhoon Warning Center (JTWC) best-tracks is adopted and the 10 best historical track analogs are selected from the 1945-2009 JTWC best-track file. A development sample from the 2000-2009 seasons is used to develop an intensity bias correction and an intensity spread calibration. Tests with an independent sample from the 2010-2014 seasons demonstrate that the intensity mean absolute errors and the correlation coefficients of the WAIP forecast intensities with the verifying intensities essentially remain constant in the five-day to seven-day forecast interval. After calibration of the raw intensity spreads among the 10 historical analogs each 12 hours, the uncertainty estimates about the WAIP intensity forecasts also do not increase during the five-day to seven-day forecast intervals. The conclusion is that the seven-day WAIP will provide intensity and intensity spread predictions of western North Pacific tropical cyclones with a similar performance as our five-day WAIP technique. Examples of the performance for this seven-day WAIP for westward-moving and northwestward-moving cyclones that make landfall, or for recurving storms that begin decay after recurvature over the ocean, demonstrate the value of constraining the intensities at the end of the WAIP forecast. Less satisfactory WAIP forecasts occur for rapid intensification, rapid decay, and for cyclones with extended periods of non-intensification. The advantages of this WAIP technique are that it requires only a historical data base, the official track forecast, and the initial intensity to make a skillful seven-day intensity and intensity spread prediction, and it can be produced in a few seconds on a desktop computer.