Graphical Tropical Weather Outlook

Updated 2 June 2015

4. Probabilistic Genesis Forecasts

As part of the experimental process, the NHC tested in-house probabilistic TC genesis forecasts during the 2007 and 2008 hurricane seasons. The purpose of this exercise was to determine if there was sufficient accuracy in forecasting tropical cyclogenesis and whether such a forecast could be used as the basis for categorical genesis forecasts in the GTWO.

NHC forecasters subjectively assigned a probability of genesis (0 to 100%, in 10% increments) to each area of disturbed weather mentioned in the text TWO. The assigned probabilities represented the forecaster's assessment of the chance that TC formation would occur during the ensuing 48-h period. These experimental probabilistic forecasts remained "in-house" and were not released to the public in 2007. The probabilistic forecasts were continued in-house in 2008, and these forecasts were used to determine the three-tiered color-coded categorical forecast that was assigned to each disturbance mention in the GTWO during the 2008 season. The 2007-08 probabilistic genesis forecasts were verified based on TC development from the final NHC best-track data. A more detailed description of the probabilistic genesis forecast and its verification is presented in Brown et al. (2008)*. The results of the probabilistic forecast can also be found in the annual verification report issued by the NHC each spring. The report can be found on the NHC website at www.nhc.noaa.gov/verification/verify3.shtml.

The results of the probabilistic genesis forecast verification suggest that NHC has skill at distinguishing between three broad categories of genesis likelihood in both the Atlantic and eastern North Pacific basins (Tables 1 and 2). For example, disturbances assessed as having a low probability of genesis became a tropical cyclone on average less than 30% of the time in both basins, while disturbances in the high category underwent genesis greater than 60% of the time on average in both basins. These results were used as the basis for construction of the three-tiered color-coded genesis probability forecast bins that will be used in the GTWO in 2009.

	Expected %	Verifying %	Number of Forecasts
<30% (Low)	8	6	916
30-50% (Med)	37	34	243
>60% (High)	70	62	109

Table 1. Binned Forecast Reliability Table for the Atlantic basin, 2007-08. The expected percentage is an average of all the forecast probabilities within the bin.

	Expected %	Verifying %	Number of Forecasts
<30% (Low)	11	19	540
30-50% (Med)	38	52	166
>60% (High)	69	79	63

Table 2. Binned Forecast Reliability Table for the eastern North Pacific basin, 2007-08. The expected percentage is an average of all the forecast probabilities within the bin.

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4. Probabilistic Genesis Forecasts