The first week of June marks not only the start of the hurricane season in the Atlantic basin, but also the release of updated seasonal forecasts from NOAA, Colorado State, and other agencies. The pre-season outlooks issued in the previous December, and updated in April, often show little actual forecast skill. However, the outlooks released closer to the actual start of the season begin to do a little better, and are somewhat more useful and reliable.
One of the best known tropical seasonal outlooks comes from Colorado State University and the forecast team led by Dr. Phil Klotzbach and Dr. Bill Gray. This year's CSU forecast - updated June 2 - calls for 11 named storms, 5 hurricanes, and 2 major hurricanes (an average season has 10 named storms, 6 hurricanes, and 2 major hurricanes). In terms of Accumulated Cyclone Energy (ACE), which takes into account the strength and longevity of the tropical cyclones that do occur in addition to just the numbers of storms, the CSU forecast also predicts a near to below average season, calling for 2009's total ACE to be approximately 72% of average.
The seasonal outlook from NOAA is worded a bit more vaguely, but is fairly close to the CSU forecast. NOAA calls for a 70% chance of 9-14 named storms, with 4-7 hurricanes and 1-3 major hurricanes. NOAA also produced a range in their prediction for ACE, predicting the ACE for 2009 to be anywhere from 65-130% of normal.
Other forecasts from firms such as Tropical Storm Risk and AccuWeather - not to mention Frontier Weather's own outlook - are in line with the general expectation for a season that comes in near average for the numbers of named storms and hurricanes in the Atlantic basin.
Most of the current seasonal forecasts have trended downward with their predicted levels of activity. The earlier outlooks from CSU and Tropical Storm Risk forecast 2009 to be similar to other recent years with numbers similar to the 1995-present average of 15 named storms. Through the spring months, revisions to the outlooks have led the prognosticators to lower their expectations based on two primary factors: El Nino, and Atlantic sea surface temperatures.
The recent La Nina episode in the equatorial Pacific Ocean has recently come to an end, but a quick transition to El Nino is possible. Conditions are currently ENSO-neutral, lying in between the extremes of La Nina and El Nino. However, sea surface temperatures (SSTs) across the equatorial Pacific have been warming over the past few months. Although not warm enough to qualify as El Nino at this time, additional warming over the next couple of months would bring about the onset of an official El Nino. This scenario would be somewhat similar to the 2006 hurricane season in the Atlantic, which began with a bang under ENSO-neutral conditions, but finished with a whimper as a developing El Nino suppressed activity and effectively ended the season in October. The Climate Prediction Center, which monitors ENSO, issued the final La Nina advisory on May 24 and then has recently (June 4) initiated an El Nino watch.
The possibility of an El Nino developing has ramifications for later in the season, however the current state of Atlantic SSTs has implications beginning right now. We've already seen how cooler than normal SSTs have affected possible developments with Tropical Depression One failing to strengthen into a named storm owing to unsuitably cool waters. A large part of the tropical Atlantic has cooler than normal temperatures. This includes the areas immediately to the north of the Caribbean islands, as well as a large part of the Main Development Region, or MDR, which occupies the area between 10 and 20 degrees north latitude, and between the African coast and the Lesser Antilles. Cooler SSTs means less "fuel" is available, and the shortage of fuel translates into fewer storms.
Even though most of the outlooks for the 2009 Atlantic hurricane season have been revised downward, it still only takes one major storm hitting the U.S. to make a season. The classic example, which has been beaten to death but still maintains its relevance, is the 1992 season. That year only had 7 named storms with 4 hurricanes. Only one hurricane made landfall in the U.S. - but that one was Category 5 Hurricane Andrew.
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