The number of named storms goes from 17 to 15. Intense hurrricanes goes from 5 to 3.
Dr. Gray provides the following reason for this reduction.
"We have reduced our forecast from our earlier predictions issued in early December, early April and late May. There have been no large changes in any particular atmospheric and oceanic predictor that have caused us to do this. There has, however, been a combination of changes in the ocean/atmosphere system that indicate to us that this season is no longer likely to be as active as our earlier predictions indicated. Physical features which have become less favorable for an active hurricane season are as follows:
1) An increase in sea level pressure values in the tropical Atlantic. Higher sea level pressure values indicate increased stability in the tropical Atlantic which inhibits tropical cyclogenesis.
2) An increase in strength of the trade winds in the tropical Atlantic. Stronger trade winds drive increased evaporation and upwelling which cools Atlantic sea surface temperatures. In addition, stronger trades usually indicate increased vertical wind shear in the tropical Atlantic.
3) A decrease in tropical Atlantic sea surface temperatures. Cooler Atlantic SSTAs (sea surface temperature anomalies) provide less latent heat (i.e., less fuel) for developing tropical cyclones.
4) An increase in Pacific eastern equatorial SSTAs. Sea surface temperatures have still not reached El Niño levels; however, increased warming implies a shift in tropical convection towards the dateline. This eastward-shifted convection often increases vertical wind shear over the tropical Atlantic.
The fact that we have had only two tropical storms during June-July does not necessarily impact our forecast for the upcoming season. There have been many active hurricane seasons (e.g., 1950, 2004, etc.) that had no activity in June and July. Last year (2005) was an unusually active early season with seven named storms and two major hurricanes before August 1. Last year broke most existing single season hurricane records. "
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