Seven-day Intensity and Intensity Spread Predictions for Atlantic Tropical Cyclones

In this study, a 7-day intensity and intensity spread prediction technique for the Atlantic called WAIA is developed using the same procedures as for our technique for the western North Pacific. The WAIA technique is based on rankings of the 10 best historical track analogs starting from 1945 to match the National Hurricane Center (NHC) best-track and current intensity. A key step is development of a bias correction to eliminate an over-forecast bias. The second key step is a calibration of the original intensity spread among the 10 analogs to achieve a probability of detection (POD) of about 68% at all forecast intervals. The advantages of the 7-day WAIA as an operational intensity forecast product for Atlantic tropical cyclones are described in terms of mean absolute errors (MAEs), mean biases, correlation coefficients, and geographic distributions of WAIA versus various 5-day guidance products available at NHC. Special attention is given to the four guidance products that are included in the intensity consensus (ICON) technique that is the most skillful of all the NHC intensity guidance products. The forecast skill at 7 days relative to the 5-day forecast interval is a central focus since in the western North Pacific there was no degradation in accuracy relative to the five-day forecast. A development sample from the 2005-2009 seasons and an independent sample from the 2010-2014 seasons are used to investigate the forecast skill. Preliminary results show that the MAEs and the correlation coefficients essentially remain the same skill level from five-day to seven-day forecast interval. The mean biases are in the range of -2 to 3 kt for the independent samples during the entire 7-day forecast period after a bias correction has been applied. Also, the uncertainty estimates about the WAIA intensity forecasts also remain ~68% of POD during the five-day to seven-day forecast intervals after calibration of the raw intensity spreads among the 10 historical analogs each 12 hours.