‘Quiet’ Hurricane’ Season for the Gulf?

Sep 2, 2010 - We're into September -- the busiest of the hurricane months.  With our recent flurry of tropical activity in the open Atlantic, the questions come up almost daily: are we done this season with storms in the Gulf? After all the talk about conditions pointing to a very active hurricane season, is 2010 going to be 'quiet' for Louisiana?

Of course, 'quiet' can be a deceptive term. Let's not forget that many in south and central Louisiana and southwestern Mississippi have already seen enough of the current season, as remnants of Tropical Depression #5 reminded us that a tropical system doesn't need a 'name' to make its mark. TD #5 threatened us twice, with the second pass delivering widespread rains of 3" to 6", with storm totals of 10" to 15" for some parts of the state.

There is no denying that this season started more slowly than we had expected.  A review of a dozen different hurricane outlooks from groups -- ranging from the National Weather Service, to university experts, to several private forecasting companies -- all called for an above-average hurricane season this year, with the majority of these groups proposing storm counts that would be in the 15-20 range for 2010.

After a seemingly slow start, the basin appears to be heating up!  As of mid August, there had only been three named storms (Alex, Bonnie and Colin), with only Alex achieving hurricane strength.  Three storms:  far from active.  But not inactive either -- in fact, the storm count was exactly "average" according to statistics compiled by the National Hurricane Center, with three named storms and one reaching hurricane strength by mid-August.

Over the past two weeks, it seems as though the tropics have livened up, and we open September with Gaston, the season's 7th named storm -- we are now ahead of the climatological average number of storms for this time of year. The abrupt increase in activity over the last couple of weeks should not come as a total surprise:several hurricane forecast groups anticipated this uptick in activity as most of the conditions that were expected to support an 'active season' have remained in place during the early-season lull. For example, ocean sea-surface temperatures (SSTs) remain above-normal for this time of year, and that usually means lower atmospheric pressures as well:  two ingredients known to enhance development.

Yet there have also been some unexpected "inhibitors" at work over recent weeks.  Upper-level shear has been a big reason that several tropical waves, T.D. #5, and even T.S. Bonnie all failed to intensify as many had anticipated. Remember that pre-season commentaries often noted that given the absence of an El Niño, shear over the Atlantic Basin would be expected to be reduced.  Indeed, the central Pacific is now classified as being in a La Niña pattern, and this is usually associated with minimal mid/upper-level shear over the Atlantic's tropical region. There is a lesson here:  regional shear and the phase of ENSO (El Niño vs. 'neutral' or La Niña conditions) may indeed show a solid seasonal association over time, but the relationship between the shear and ENSO phase is not 'a sure thing' throughout any given season.

As we begin the second half of the 2010 Hurricane Season, we must remember that September and October are very busy months for the basin. The climatological peak of tropical activity comes in the early part of September (on or about September 10th), but the basin storm frequencies are slightly skewed to dates following the peak. In other words, history tells us that more storms have come after the start of September than before.

Don't forget that we've been experiencing a trend of "above average" seasons since the mid-1990s. Looking back to 1995 (15 seasons), we find that the basin has averaged about nine 'named' storms (five as hurricanes) from September 1 until season's end.  Even just half that number will yield an "above average" hurricane season for 2010, and just over half would still take the 2010 storm count into the 'teens.

By the way, the Gulf has averaged two 'named' storms during the second half of the season over that 15-year period, so don't put the generator into storage just yet.