Figure 1. Accuracy of long-range forecasts of Atlantic hurricane season activity performed by Bill Gray and Phil Klotzbach of Colorado State University (colored squares) and TSR/Tropical Storm Risk (colored lines). The skill is measured by the Mean Square Skill Score (MSSS), which looks at the error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TS=Tropical Storms, H=Hurricanes, IH=Intense Hurricanes, ACE=Accumulated Cyclone Energy, NTC=Net Tropical Cyclone Activity. Image credit: TSR.
How good are these December hurricane season forecasts? Unfortunately, they're pretty much worthless. The skill of the December forecasts issued by Dr. Gray and TSR (Figure 1) have averaged near zero since 1992. Not surprisingly, the forecasts get better the closer they get to hurricane season. The June and August forecasts show some modest skill, and are valuable tools for insurance companies and emergency planners to help estimate their risks. The problem with the December forecasts is that the current statistical computer models used to forecast El Nino are not skillful beyond about six months. For example, none of these models foresaw the current El Nino event that began in September--until April. Until we can forecast the evolution of El Nino more than six months in advance, these December forecasts are not worth paying much attention to. I think it's important for these groups to keep trying, though.
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