A quick review of 2005-2009 hurricane season predictions
May 25, 2010
Corrections : I used the verification of the Colorado State Forecasts as my source for storm numbers, forgetting that the verifications are issued in early November (before the end of hurricane season). Because of that, observed numbers and 5 yr averages were off (by 1-2 storms) for some seasons. Everything’s correct now, I think
In front of NOAA’s release of their 2010 hurricane season outlook, thought it would be worth reviewing the forecasts for the last five seasons. As last season came to a close, I saw some comments to the effect of “‘They said it was going to be another extreme season and they were wrong” (where who “they” were wasn’t specified). Recently, I’ve seen a couple of comments in random places suggesting that the forecasters “always forecast monster seasons”.
Tackling the latter statement first, here are the predictions from the Colorado State, NC State, and Florida State Universities’ forecast teams along with those of NOAA and the Tropical Storm Risk Consortium (for the entities that issue multiple forecasts before and during the season, the forecast nearest to June 1 is used). Consensus is the average (rounded to the nearest whole number) of their forecasts, while the 5 season trailing average represents an average of what was observed over the past five seasons. The numbers are tropical storms/hurricanes/major hurricanes:
|5 season trailing average||15/7/4||17/8/4||17/8/4||17/8/4||17/9/5||16/7/4|
Note that in 2005, everyone massively underforecast the record-breaking activity. In absolute terms, no one has come close to overforecasting the number of hurricanes as badly as the 2005 season was underforecast. In 2006, the rapid onset of El Niño conditions caused most to overpredict the number of storms. 2007 forecasts were better but slightly overdone at the top end, while 2008 numbers were on the mark.
As for 2009, the consensus numbers were off for named storms and hurricanes, but on the mark for major hurricanes. No one was forecasting an extreme season, as among other measures, everyone’s forecast was below the 5 yr averages. Furthermore, this table excludes the forecast from the UK’s Meteorology Office, which was even lower than everyone’s presented here; they predicted a mere six tropical storms. The only extreme they were predicting was quiet.
With that said, it is clear from the past five years that the forecasts haven’t been very good on aggregate. El Niño seasons excluded, the forecast consensuses were not an improvement on what the 5 year average suggested** (because El Niño seasons during an active hurricane period are what stock market people would call a cyclical downturn in a secular bull market, that’s not a surprising outcome). Note that “Dr. Hansimian’s forecast” of 6-8 hurricanes for 2010 just happens to nicely straddle the five year average. Hmm.
NC State 2006 forecast (Wayback Machine)
NC State 2007 forecast (Wayback Machine)
** Not an original observation and it is one that has been true in the past as well. Many others have pointed this out, perhaps inspired by On the Skill of Seasonal Tropical Cyclone Forecasts in the North Atlantic, which measures 1992-2005 seasonal forecasts against the benchmark of a 5 year average.