When will hurricane 2 appear?
Posted July 21, 2010
We're past the middle of July now and over six weeks into hurricane season, a season that is forecast by pretty much all of the groups involved in long range outlooks to be very active, with the Colorado State group forecasting 10 hurricanes for the season (versus an average of 6), NOAA 8-14 hurricanes and NCSU 7-11 hurricanes. So far, we've seen one hurricane develop in the Atlantic basin (Alex, which formed on June 25th) and one tropical depression which did not reach the intensity required to receive a name. As I write this, there is a notable tropical wave in the Caribbean that is being monitored by the National Hurricane Center for possible development, and they are giving it a better than even chance (60% - see attached image) of becoming a tropical cyclone within the next day or two. Computer models indicate the system would likely move to southern Florida or so by Friday. If it closes off into a cyclone, it could become the second named storm of the season over the Bahamas, but it's still unknown whether it will become the next hurricane of the 2010 season.
So, is the season slow to get rolling into the context of a very active forecast? Not really, based on a quick scan of some of the most active ones in the past. I looked back to the year 1900, and found that in all those years only seven seasons have yielded 10 or more hurricanes. For those seven seasons, I took note of when the second hurricane of the season developed, and found that in only two of those years had the second hurricane formed during the first three weeks of July. In fact, three of those very active seasons didn't have a second hurricane until August 14th or later, the latest "bloomer" being 1998, when the second hurricane developed on August 24th, then was followed by eight more before the season was done. On the other hand, in 2005 we had the second hurricane develop on July 4th, earliest of the group, and that went on to be a monster season, with 15 hurricanes!
Since May, we've been in a transition from El Nino to near normal to below normal equatorial Pacific upper ocean temperatures, and recent readings have indicated La Nina is either already underway or soon to officially commence. Since La Nina correlates to less in the way of disruptive wind shear across the region where Atlantic tropical cyclones form, it's continuing development is a key to supporting an active season. In addition, while sea surface temperatures have backed off a bit to only a little above normal in the Gulf of Mexico recently, those out over the south-central North Atlantic, in the main development region for long-track hurricanes, remain very warm and about 2-4 degrees C higher than normal in some areas, as shown in the second image. Given those factors, those above-average forecasts still appear to have a reasonably good chance of being accurate.