Ed Dunham
Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017)
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Posts: 2565
Loc: Melbourne, FL
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So far this season there has been six Tropical Storms - which certainly indicates an above average level of activity - but the basin is again quiet at a time when the climatological peak of the season is less than two weeks away. In an average season with 11 named storms, by September 1st there would have been 5 named tropical cyclones and 2 of them would have reached hurricane strength. Many seasonal forecasts for 2013 pointed to another year of high tropical cyclone activity in the Atlantic, but is there enough time left in the season for these forecasts to verify?
As an example, the latest (August 2nd) seasonal forecast from Colorado State University still anticipated 18 named storms this year. With 6 so far, the rest of the season needs 12 more for the forecast to verify, or an average of one named storm per week from September 1st through November 23rd (with 8 of the 12 becoming hurricanes since their forecast also expected 8 hurricanes). Checking the past 60 years (1953-2012), has this frequency ever happened before? The answer is 'Yes', although not very often. In 1969, 13 named storms formed in that time frame; 2005 had 14 and 2010 had 16. Other years that came close include 1953 - 10, 1961 - 10, 1984 - 11, 1995 - 11, 1998 - 10, 2000 - 10, 2001 - 11 and 2012 - 11. In a couple of those years a named storm or two actually developed after November 23rd.
Based on the past 60 seasons, the probability of 12 or more named storms forming in the next 12 weeks (through November 23rd) is 5%.
As noted above, how often is the Atlantic basin this quiet prior to the peak of the season? Once again I looked at the past 60 years to find seasons with no named storms from August 27th through September 2nd - with the following results: 1956, 1959, 1968, 1970, 1987, 1994 and 1997. The probability of occurrence for a quiet week during that timeframe is 11.7% - a little more common than I would have guessed. An Invest area with a low pressure center is about to exit the west African coast and could become a named tropical storm by September 2nd, but it will be close as to whether the system develops by then or a day or two later.
Easterly windshear over tropical central Africa remains uncommonly strong and most of the easterly waves over the continent this season have been disorganized under this constant shear. A band of westerly shear from roughly 15N-20N remains strong in the central Atlantic from 70W eastward to 30W. So far this season, atmospheric conditions are simply not yet supportive for significant tropical cyclone development in the central Atlantic primary genesis area - but that could change and a busy late season is still possible.
ED
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