Sep 1, 2013

Let's Hope it stays this way

The peak of the Atlantic hurricane season is approaching, the date is September 10th and so far we have not had ONE hurricane yet in the Atlantic basin. The last time there were no hurricanes in the Atlantic through this point in the season was 2002. Now the quietness of the season thus far is no guarantee that it will remain so thus we should not let our guard down but it is interesting nonetheless.

Why so inactive?

Thus far we have had five named tropical systems all of tropical storm intensity; this number is about normal for the number of named storms for this point in time in a hurricane season. One storm,  Andrea, made landfall on the U.S. mainland bringing beneficial rains to a drought  parched Southeast U.S.

A number of factors have come into play making for an inactive season so far:

First the Bermuda high a large area of high pressure that spans the Atlantic Ocean has averaged out to be a bit stronger than normal thus far this summer. Higher than normal pressure across the Atlantic is an unfavorable factor for tropical cyclone development.. (However I will point out that over the past week or so the overall strength of the high is weakening a bit). Another note about this high is that it is a bit farther south  of its "normal" climatological position,

Second, at times across the tropical latitudes of the central and east-central Atlantic a Tropical Upper Tropspheric Trough or TUTT has persisted; depending on the configuration of the trough, strong SW or W winds aloft have occurred over this part of the Atlantic. Both TUTTS and strong winds (shear) are unfavorable factors for tropical cyclone genesis or intensification.

Third blame it on SAL. SAL or Saharan Air Layer is a cloud of dust or better yet SAND from the Sahara Desert that moves westward off of the continent of Africa and then out over the Atlantic Ocean. There have been at least three occasions this summer where SALs have migrated all the way across the ocean to Texas! SAL and all that it carries with it - the warm, dry (stable) and sandy air are negative factors for tropical cyclone development.

Currently yet another SAL is moving off the west coast of Africa

So far through from June 1st the start of the Atlantic Hurricane Season through August 27 of this year, largely because of the SALs that have moved off of Africa, the average relative humidity over the Atlantic has been way below normal. (Keep in mind tropical cyclones "like" warm and moist air in and around their circulations NOT warm and dry air). Here is a map of the departure from normal of the mean relative humidity (from NOAA):

For more on SAL checkout: What is SAL .   So for at least the next week tropical cyclone activity should remain on the quiet side.

What areas are favorable for tropical storm formation?

Right now little shear and moist air is confined to a relatively small area of the Atlantic basin, namely the Western Caribbean Sea and the extreme south portions of the Gulf of Mexico(Bay of Campeche). If any tropical systems can move or form over these locations over the next few days conditions are "marginally" favorable for some possible development.

Some other interesting points

The least number of storms that has occurred in the Atlantic in any given Hurricane season is 4 (1983).

It has been been 7 years and 10 months since a major (category 3 or greater) hurricane has struck the U.S. This is the longest streak on record. The last time the U.S had a major hurricane make landfall was Wilma in 2005.

Between 1962 and 2012 (50 years) an average of 5.6 major hurricanes per decade made landfall on the U.S. For the preceding 50 years (1911 and 1961) an average of  8.4 major hurricanes struck the U.S.

Global hurricane activity has actually been in decline over the past 40 years (in spite of some active years in the Atlantic:

Let's hope it stays this way! ;-)