What happened to hurricane season? And why we should keep forecasting it?

Source: Wash Posthurricane2

by Brian McNoldy

As we wrap up September, there have been just two short-lived Category 1 hurricanes in the Atlantic. Yet seasonal forecasts predicted an extremely active season. What?s going on?

Before diving into the seasonal forecasts, let?s take inventory on where the season stands.

In an average season,  8 tropical storms, 4 hurricanes, and 1 major (category 3 or higher) hurricane form by this date. This year, we?ve experienced 10 tropical storms, 2 hurricanes, and no major hurricanes.

Though we?ve had close to the average number of total storms, most have been short-lived and/or weak. If you went out for a cup of coffee at any time this hurricane season, you would?ve missed many of them.

Aside from tropical storm Andrea?s modest impact in the Southeast, none of these storms has made landfall in the U.S.

Atlantic tropical cyclones so far in 2013. Advisories were only written during the portions of the tracks shown with solid lines (dashed and dotted lines correspond to times when it was a disturbance or a post-tropical cyclone).

Atlantic tropical cyclones so far in 2013. Advisories were only written during the portions of the tracks shown with solid lines (dashed and dotted lines correspond to times when it was a disturbance or a post-tropical cyclone).

Total storm energy *much* below normal

I have referenced ACE (Accumulated Cyclone Energy) as a common measure of seasonal tropical activity before, but as a refresher, it?s the sum of the squares of all of the storms? peak wind speeds at 6-hourly intervals.  It is proportional to the kinetic energy of a storm, based on its peak wind value (not the size of the storm or distribution of its winds).

(A storm can achieve high ACE values if it lives a long life and/or is intense.  For example, last year, long-lived but relatively weak Nadine meandered in the tropics for 21 days and racked up an ACE of 26.3, while Katia in 2011 was only around for 11.5 days but was very intense ? reaching category 4 intensity ? and ended up with an ACE of 27.)

This year?s ACE is 23.1, 28 percent of average for this date or around the average on August 27, prior to the peak of hurricane season, which has now passed. Of all storms this year, Humberto leads the pack contributing a lowly ACE of 8.3 to the total.

The low activity so far this year is not unprecedented, but unusual.

According to meteorologist Ryan Maue?s Web site, only four other years have had lower ACE totals as of this date (since 1950): 1962, 1977, 1983, and 1994.  The highest end-of-season ACE among those years is just 35.6. So it would be a tremendous accomplishment if the 2013 hurricane season finished up with even HALF (52) the ACE of an average season (104).

daily_ACE

Mysterious lack of activity

But just before the season began, every group making seasonal forecasts was calling for an above average or ?very active? season? so what happened?

Prior to the hurricane season through today, the factors favoring lots of storm activity have included low surface pressures, warm sea surface temperatures, a strong African easterly jet (which enhances disturbances that enter the Atlantic and can potentially grow into storms), and the lack of an El Niño (which can promote hostile westerly winds).

Given that long list of storm-enhancing factors, what does the list of suppressing factors look like?

One signal that jumps out across the heart of the tropical Atlantic is very dry air. While some previous active seasons have had drier-than-normal air present (2004 is a great example), it has not reached the extreme of this year.

The plot below shows the departure of mid-level relative humidity from average, and peak values are around 24 percent. This anomalously dry air has also existed at lower and higher altitudes.  Furthermore, the same areas that show up as dry here are also characterized by large-scale subsidence (sinking air) which acts to squash organized thunderstorm activity necessary for hurricane formation. That signal was not present in previous active years.

Anomalies of mid-level relative humidity, averaged over Aug 1 - Sep 27. (NOAA)

Anomalies of mid-level relative humidity, averaged over Aug 1 ? Sep 27. (NOAA)

The jury is still out as to WHY so much subsidence and dry air has dominated the basin the past couple months, but clearly, it wasn?t very predictable.

full story here