Published: 2013-09-10 08:03:00
Updated: 2013-09-10 12:32:27
Posted September 10, 2013
By Mike Moss
In several respects, this has been a rather tranquil hurricane season so far, especially considering that a range of pre-season forecast indicated well above normal activity, with most of the organizations that produce forecast suggesting a season total of 13-20 in the initial outlooks posted back in April and May.
Today is September 10th, and as seen on the graph posted here it represents just about the peak of historical tropical cyclone activity in the Atlantic basin. A close look at how the numbers build up to that peak and then decline indicates, however, that it is not unusual to have more storms during the period following the peak than leading up to it.
So far this season, we've had 8 named storms, which is a little above the average number this far into the season, but no hurricanes, which is below the typical value of three or so at this point in the season. Among the reasons for generally short-lived and relatively weak systems so far have been frequent intrusions of dry, dusty air aloft pushing off of northwest Africa and into the Atlantic (something known as the Saharan Air Layer), and tendency for storms to encounter more in the way of vertical wind shear than was expected before the season. Whether these factors will continue to play an inhibiting role through the remainder of the season remains to be seen, but it does appear we'll have our first hurricane in the form a strengthening Humberto later on today or tomorrow, and history suggests that at least in some respects the initial forecasts for numbers of named storms may yet work out, although some associated forecasts of total activity (based on number of storm days and intensity of the storms) seems pretty likely to come up short.
If we equal or exceed the number of new storms that we've already had during the rest of the season, as often happens, that would leave the number of named storms somewhere around 16-17, pretty near the middle of the range of the spring forecasts. A couple of fairly recent years also give us a sense of the possibilities in terms of hurricanes. In both 2001 and 2002, no hurricanes developed before September. In 2002, only 4 storms that strong ever formed, but in 2001 the rest of the season produced 9 hurricanes.
However it turns out in terms of overall seasonal numbers, it's always worth keeping in mind the old rule that "it only takes one," so we'll all want to stay vigilant through the next three months. Right now we continue to track Humberto, of course, but Gabrielle has also re-generated into tropical storm this morning to give us a second active system, and there is a third disturbance over the western Caribbean that has a decent chance of becoming a named storm in the next 3-4 days as it slips into the southern Gulf of Mexico.