Improved Weighted-Analog Intensity and Intensity Spread Predictions for Western North Pacific Tropical Cyclones in Pre-Formation and Ending Stages

Tsai and Elsberry (2014) developed a 5-day Weighted Analog Intensity technique for western North Pacific tropical cyclones (TCs) called WAIP. Analog techniques are developed by selecting from the best-track files those cases (analogs) that have similar characteristics (in this case, tracks and initial intensities) as the target storm. Tsai and Elsberry (2015) then extended the WAIP to 7 days, but independent testing revealed an increasingly large intensity over-forecast bias in the 5-7 day interval that they attributed to “ending storms” due to landfall, extratropical transition, or to delayed development. Thus, an “ending storm” version of the 7-day WAIP has been developed because of the almost 5000 forecasts in the 2000-2015 Joint Typhoon Warning Center (JTWC) training set for WAIP, approximately 3300 of these forecasts have an ending in the 3 day–7 day interval. The 7-day WAIP has been modified to separately forecast ending storms within the 7-day forecast interval by simply adding a constraint in the selection of the 10 best historical analogs that the intensity at the last matching point with the target TC track cannot exceed 50 kt. A separate intensity bias correction calculated for the ending storm training set reduces the mean biases to near zero values and thereby markedly improves the mean absolute errors in the 5- to 7-day forecast interval for the independent set. A separate calibration of the intensity spreads for the training set to ensure that 68% of the verifying intensities will be within the 12-h WAIP intensity spread values results in smaller spreads (or higher confidence) for ending storms in the 5- to 7-day forecast intervals.

The pre-formation stage improvement in the 7-day WAIP has been to only select those analogs that were also in the pre-formation stage until the predicted formation time which is defined here as 35 kt. Thus, when the initial intensity of a target storm is between 15 kt and 30 kt, the WAIP intensity forecast will be constrained to be less than 35 kt until the predicted formation time. Then the selection of analogs for the post-formation stage will also be improved since those analogs will have an initial intensity within ± 20 kt of the target storm. By constraining the analog selections in the pre-formation, post-formation, and ending storm situations, the WAIP provides improved intensity and intensity spread predictions to 7 days.