Verification of National Weather Service tropical cyclone intensity probabilities and future plans

In 2008 the National Hurricane Center (NHC) and Central Pacific Hurricane Center (CPHC) began issuing a new intensity probability product. These intensity probabilities are computed using a set of 1,000 realizations, or alternate tracks and intensities, that vary around the official forecast based on a Monte Carlo sampling of historical errors in the NHC and CPHC track and intensity forecasts. The Monte Carlo technique also accounts for land interaction and decay. Verification of the Monte Carlo-based intensity probabilities issued by NHC and CPHC with each advisory in the central North Pacific, eastern North Pacific, and Atlantic basins during the 2008, 2009, and 2010 hurricane seasons will be provided. The probabilities will be verified using the NHC and CPHC best tracks. Preliminary results from 2008 and 2009 indicate that in the Atlantic, the forecasts for tropical storms have greater reliability than in the East Pacific. For hurricanes, 12-48 h forecast probabilities in the Atlantic were too high compared to the observed frequency at the low probability thresholds and too low at the high probability thresholds. The opposite was true in the East Pacific. These results suggest that in the technique has more difficulty discerning which cyclones will be hurricanes in the first 48 h of the forecast period in the East Pacific than in the Atlantic. At forecast lead times of 3-5 days, the hurricane forecast probabilities in the Atlantic show much greater reliability than those in the East Pacific, although the sample size for the East Pacific forecasts was small at the higher probability thresholds.

Future enhancements to the intensity probabilities, including the computation of intensity probabilities valid at landfall, will also be presented. To obtain accurate landfall intensity probabilities, conditional probabilities will be computed using the tracks and intensities of only those realizations that cross the coast between pre-defined points. The intensity of each realization when it crosses the coast between the pre-defined points can be used to create landfall intensity probabilities. This study will provide a summary of the methodology used to produce the landfall intensity probabilities, and will show examples from recent storms of how these probabilities could be used at NHC and CPHC, as well as potential uses for decision makers such as emergency managers.

Beginning in 2010, the method for generating the track realizations was generalized to include information from the spread of an ensemble of track forecast models. This change will influence the intensity probabilities for storms near the coast. The sensitivity of the intensity probabilities to this modification will be investigated by running cases near landfall with and without the ensemble spread adjustment.