One of the best known seasonal hurricane forecasts comes out of Colorado State University courtesy of William Gray (although Gray's student Phil Klotzbach is now lead author of the forecast). Another one comes out of the University College-London Benfield Hazard Research Center courtesy of Mark Saunders and Adam Lea. Both groups update their forecasts monthly; the August Klotzbach-Gray forecast is now out and the August Saunders-Lea forecast will be out tomorrow (I think). So let's see what the first group says... Gray and Klotzbach are no longer calling for 17 tropical cyclones in the North Atlantic; now they're only predicting 15, or 13 after August 1, one of which (Chris) has already formed. So, if they're right, we will see 12 more total storms this season, of which 7 will be hurricanes and 3 will be major hurricanes (Cat 3, 4, or 5). These last two numbers also indicate a scaling back; originally Klotzbach and Gray were predicting 9 hurricanes and 5 majors. Why are they scaling back? Let's let them explain it:
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.
We're not out of the woods yet; it only takes one hurricane to cause incredible damage. But this seems like just one more sign that this hurricane season will not be at all like 2005, even if it's still projected to be an above average season by historical standards. Let's see what the British forecasters have to say tomorrow....
