Throughout the spring and into summer, I've pointed out repeatedly that there has been a consensus, among seasonal hurricane forecasters, that there will be an active season this year in the Atlantic. 17 named storms was Bill Gray's number; NOAA said 13-17. Methodologically, both Gray and NOAA use a "statistical" forecasting technique: In other words, they get these numbers based upon correlations between various climatological factors (and particularly El Nino) and the number of storms that appeared in past seasons. But there is another, newer type of forecasting: Dynamical seasonal prediction, which uses multiple climate model runs to determine the number of tropical storms that will appear (hurricane-like storms in the models are taken as a guide to real life storm activity even though they are only rough analogues at best). This type of work has been pioneered in Europe. And last year, the dynamical models clearly beat the statistical models (which were all predicting an active season). Instead, El Nino showed up and more or less shut down Atlantic hurricane activity after September.
Last summer at around this time, I visited the UK Met Office and the European Centre for Medium Range Weather Forecasts and talked to scientists there--but they were not willing to make their dynamical forecast for 2006 public. This year, however, it has just been released: They are expecting a less active season, 7-13 storms between July and November. Apparently this is not based upon the notion that El Nino will come back again, but rather, on the prediction that Atlantic sea surface temperatures will cool down some. And so, you might say that the bets are down. There's a clear gap between the two types of forecasts--statistical and dynamical--going into the busier part of this season. Only time will tell which is right....
P.S.: As I mentioned earlier, in a new paper also using a statistical technique, Thomas A. Sabbatelli and Michael E. Mann of Penn State University predicted 15±4 total Atlantic storms for 2007. This is in press at the Journal of Geophysical Research - Atmospheres. The Sabbatelli/Mann methodology is also based in significant part upon SSTs and the state of the El Nino-Southern Oscillation...so I have to wonder, why are the European models apparently picking up an SST cooling that no one else seemed to be expecting?
P.P.S.: Jeff Masters also blogs the new forecast and has a much more thorough discussion of the dynamical seasonal hurricane forecasting technique. Storm World also contains a bit on this. Masters thinks the wisest outlook on the now disparate forecasts is to take a consensus of them all, which would leave us expecting "13 more named storms this year, for a total of 15"......
