IRI Forecasts note
The IRI experimental dynamical tropical cyclone seasonal forecasts are issued in a
probabilistic 3-category (tercile-based) format for the 3 or 4 months of peak tropical
cyclone activity. To comply with the WMO format for the forecasts, the original
probabilistic forecasts are converted to a deterministic format, and in some cases
adjusted to match the period required by the WMO. We use the methodology of Briggs
and Wilks (1996) to transform probabilistic to deterministic forecasts. Once the
deterministic value for the peak season is obtained, we add the climatological values of
the months that we do not include in our original forecast. For instance, in the case of
the North Atlantic, the IRI forecast is issued for the August to October (ASO) season.
The WMO-defined forecast season is for June to November (JJASON). Therefore, after
transforming our probabilistic ASO forecast to a deterministic ASO forecast, we then
account for the climatological values for the months of June, July and November, shifted
by the same proportion as done for the IRI’s shorter season, to obtain our final forecast
value.
The IRI experimental dynamical tropical cyclone seasonal forecast confidence interval is
a one standard deviation interval; i.e. we expect the forecast to be within that interval
approximately 68% of the time. For a Gaussian distribution, this confidence interval
defines the 16 and 84 percentiles of our forecast distribution.
The climatological standard deviation (σ_obs_peak) for the peak season (season for which
the IRI releases their forecasts) for number of tropical cyclones (NTC) or accumulated
cyclone energy (ACE) is calculated from the observations (e.g., the NTC for peak
season (ASO) in the North Atlantic basin). Then we multiply this observational standard
deviation by the ratio of the mean NTC in the full WMO-defined season (μ_full) to that of
the IRI’s shorter peak season (μ_peak). Finally, to incorporate the reduced uncertainty
associated with the expected forecast skill, we multiply the adjusted observed standard
deviation by the square root of 1 minus the correlation value (r) for the given variable
and season, using correlation skills computed for multidecadal simulations forced with
observed sea surface temperature (Camargo and Barnston, 2009). These operations
are formulated by:
σ = [σ_obs_peak + (μ_full / μ_peak)] * (1-r2 )1/2
References:
Camargo, S.J., and A.G. Barnston, 2009: Experimental dynamical seasonal forecasts of
tropical cyclone activity at IRI. Wea. Forecasting 24, 472-491.
Briggs, W.M., and D.S. Wilks, 1996: Estimating monthly and seasonal distributions of
temperature and precipitation using the new CPC long-range forecasts. J. Climate 9,
818-826.