Day: October 31, 2013

Although the 2013 Atlantic hurricane season has not yet concluded and we still have November to contend with before the season will officially be over, all indicators are beginning to make it obvious that the end of the show is near. If you recall in my July hurricane season forecast , all signals were looking for an active season. All the criteria seemed to fit but for some unknown reason, a big piece of the puzzle has alluded forecasters as to why this years hurricane season was very, very below normal. Yes, the Atlantic basin did have a total of 13 named storms, so in one sense the season has been “active” but there were only 2 hurricanes and both were minimal category 1 hurricanes. However, 13 named storms does really cannot tell the whole story.

So, how do we get a better feel of the hurricane season activity for this year? One method called [tooltip title=”ACE” content=”Accumulated Cyclone Energy” type=”info” ]ACE[/tooltip] Index. ACE Index is using a sum of the energy accumulated with all the cyclones (tropical storms, sub-tropical storms and hurricanes) that happened to form within the hurricane season. Calculating ACE is done by using the square of the the wind speed every six hours during the storm’s lifetime. Remember, a tropical depression is below 35 knots and it is not added. Hence, a cyclone that is longer lived will have a higher ACE indices verses a shorter lived cyclone which will have a lower ACE indices.  ACE indices of a single cyclone is windspeed (35 knots or higher) every six hour intervals  (0000, 0600, 1200, 1800 [tooltip title=”UTC” content=”Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time.” type=”info” ]UTC[/tooltip]) or ACE = 10^4 kn².

Total ACE:

$latex \Sigma = \frac{Vmax^2}{10^4} \ &s=2\ $

* Vmax is estimated sustained wind speed in knots. *

An average seasonal ACE in the Atlantic is between 105 -115 or mean average of 110. Although technically we are in a Neutral state, this season “almost” seems more like a El Niño year. An El Niño tends to hinder tropical cyclone development in the Atlantic, due to higher amounts shear. But in a true El Niño year, you normally will have a very active season in the East Pacific (EPAC), with a few major hurricanes, and a subdued Atlantic.

As part of the post-season review, the low pressure system in December(AL152013) was classified as a subtropical cyclone and the ACE Index has been updated to reflect the ACE numbers during it’s short lived status as a subtropical cyclone.

Two notes of interest: One is that of the two hurricanes this year, Humberto was just hours from beating the record for the latest first Atlantic hurricane of the season. Secondly, this will be the 8th year in a row where the Atlantic hurricane season has not had a major hurricane (category 3 or higher). In fact, 10/24/2013 was the 8th anniversary when Hurricane Wilma made landfall on the southwestern side of Florida as a Category three hurricane and headed northeastward and the areas of the Miami-Dade, Broward, and Palm Beach counties were impacted with category two winds. If by at the end of this year hurricane season with no major hurricanes, we will tie the record.

So is there a last hurrah for one or two more storms or is it time to fold the tents and call the season over? Technically, no as the hurricane season does not oficially end until November 30. This does not mean a cyclone cannot develop. It is just that a cyclone developing after the official season is over, is a somewhat rare event, but it is not impossible. Usually any late season storms develop either in the Southwestern Atlantic or more likely in the Caribbean. The two late season cyclones late that come to mind were Sandy (2012) and Mitch (1998). As of yet, it appears that the rest of November will be quiet and although there is a chance development of in the Caribbean due to the [tooltip title=”MJO” content=”Madden Julian Oscillation – lift  or upward motion into the atmosphere allows easier development of a tropical cyclone increased convection/thunderstorms activity.” type=”info” ]MJO[/tooltip], I am somewhat skeptical on that. I won’t rule it out yet but for the moment I just am not convinced this will happen, IMHO.

As always, remember to always keep an eye to the sky and to always stick with official information, either your local [tooltip title=”WFO” content=”Weather Forecast Office” type=”info” ]WFO[/tooltip] or the NHC