Hurricane seasons ends; did all of this season's storms deserve names?

The Atlantic hurricane season of 2007 is over, and it was a strange one. For the second straight year, we had a near average season, despite pre-season predictions of a very active season. The U.S. got off lightly for the second straight year. Just two tropical storms and a Category 1 hurricane hit the country. Humberto, which did $500 million in damage to Texas and Louisiana, was our only hurricane. However, it was devastating year in the Caribbean. Two Category 5 hurricanes, Dean and Felix, barreled through, two weeks apart. Dean killed at least 27 along its trail of destruction through the Lesser Antilles Islands, Jamaica, and Mexico, while Felix was responsible for 235 people killed or missing in Nicaragua. The Caribbean also suffered deadly flooding rains from Hurricane Noel, which killed at least 150 people in the Dominican Republic and Haiti.

Records set in 2007
1) Hurricane Felix set the Atlantic record for fastest intensification from the first advisory to a Category 5 hurricane. It took Felix just 54 hours to accomplish the feat.

2) Hurricane Humberto set the Atlantic record for fastest intensification from first advisory issued to hurricane strength--18 hours. (Actually, Humberto did the feat in 14 1/4 hours, but this will get rounded off to 18 hours in the final data base, which stores points every six hours). There have been six storms that accomplished the feat in 24 hours.

3) Hurricane Lorenzo tied the Atlantic record for fastest intensification from a tropical depression to a Category 1 hurricane--twelve hours.

4) With the occurrence of Dean and Felix, there have now been eight Category 5 storms in the past five years--the highest total ever observed over such a short time span.

5) Dean and Felix both made landfall at Category 5 strength, the first time two storms have done that in a single year.



Figure 2. Atlantic 2007 hurricane season forecasts issued near June 1, compared to the actual and normal values.

CSU=Colorado State University (CSU) Phil Klotzbach/Dr. Bill Gray forecast (May 31)
NOAA=NOAA's forecast (May 22)
TSR=Tropical Storm Risk, Inc. (TSR) forecast (June 4)
UKMET=UK Met Office (June 19)
ACE=Accumulated Cyclone Energy, a measure of a named storm's potential for wind and storm surge destruction defined as the sum of the square of a named storm's maximum wind speed (in 10^4 knots^2) for each 6-hour period of its existence. The 1950-2000 average value of ACE is 96.

While 2007 was a fairly normal year in terms of number of hurricanes and intense hurricanes, it was above normal in terms of number of named storms (14, vs. an average of 11). However, this is a misleading measure of how active the hurricane season was. Seven of this year's storms were weak tropical storms that lasted a day and a half or less. Three of the hurricanes lasted only one advisory at hurricane strength (six hours). The total destructive energy (ACE, or Accumulated Cyclone Energy) for 2007 was only 71% of normal. Only 33.50 named storm days occurred in 2007, the lowest value of named storm days since the El Nino year of 1994, when 27.75 named storm days occurred.

Did all of the named storms this year deserve names?
While all of this year's named storms did reach tropical storm strength, based on the best data available, it is quite likely that two or more of them would not have gotten named 20 or more years ago. For example, Tropical Storm Melissa was a tropical storm in the far eastern Atlantic, east of 30W, for one day. It was named based on a new satellite classification technique that was not available 20 years ago called the Objective Dvorak Technique. Furthermore, prior to the late 1980's,the National Hurricane Center did not have official responsibility for the Atlantic Ocean east of 30W. Storms that formed and died in that part of the Atlantic may not have made it into the official HURDAT database of Atlantic storms. Several of this season's storms were named based on data from the QuikSCAT satellite and other new measurement techniques that were not available 20 years ago.

Furthermore, the definition of what should be a named storm is subjective, and has changed depending upon who is director of the National Hurricane Center. In the 1980s, director Bob Sheets declared that subtropical storms were not to be given names. It was not until 2002 that subtropical storms were deemed worthy of names. This year's Subtropical Storm Andrea would not have been named in the 1980s. A borderline tropical depression/tropical storm will not get named until it holds together for a while. How long is a while? Back in the 1970s and 1980s under NHC directory Neil Frank, a storm sometimes had to stay at tropical storm strength for a full day, according to an interview published yesterday in the Houston Chronicle. Since three of this year's storms lasted a day or less, they would not have been named under his tenure at NHC. A borderline storm is more likely to get named if it is close to land, but none of the three short-lived storms (Jerry, Chantal, and Melissa) threatened land.

All of these uncertainties in storm naming makes it very difficult to determine if climate change is causing an increase in the number of named storms in the Atlantic. A "best track" committee is working its way through all the Atlantic hurricane records to standardize the data, but this effort will take many years.

Why did the pre-season hurricane forecasts do so poorly?
In June, forecasters gave several reasons to expect a very active season in 2007:

1) A continuation of conditions since 1995 that have put us in an active hurricane period (in particular, the fact that sea surface temperatures in the Atlantic Main Development Region for hurricanes were about 0.6° C above normal.

2) The strong likelihood of either neutral or La Nina conditions in the tropical Pacific Ocean, leading to average to below average wind shear conditions.

Well, La Nina conditions did develop, and wind shear gradually declined during the season. Wind shear was slightly above average in August, near average in September, and below average in October over the main development region for hurricane formation. However, sea surface temperatures declined to near average levels by July and August, thanks to a major incursion of African dust. According to the excellent write up of this hurricane season's activity posted by Phil Klotzbach and Bill Gray, 2007 was the dustiest year over the tropical Atlantic since 1999. All this dust acted to block sunlight from reaching the ocean surface, and sea surface temperatures were not able to maintain their above average state. We don't have the ability to predict major dust outbreaks from Africa more than a few days in advance, and this inability will continue to confound efforts at seasonal hurricane prediction for years to come.

NPR interview cancelled
I was supposed to be featured on the National Public Radio show "Day to Day" today, but that interview got canceled at the last minute.

Jeff Masters