The 2009 Atlantic Hurricane Season will be remembered as a below-average season.  There were nine named storms, including three hurricanes.  Two of the hurricanes were major (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale).  Only two tropical storms made landfall in the U.S.:  Claudette in the Florida panhandle in August and Ida near Dauphin Island, Alabama in November.  No hurricanes made landfall in the U.S. in 2009, although Hurricane Bill caused widespread coastal flooding from surge and waves along much of the Atlantic coast.  Some coastal infrastructure damage resulted from the high waves at the coast.

The weak to moderate El Niño contributed, in part, to the reduced tropical cyclone activity through stronger-than-average vertical wind shear.  It should be noted that five of the tropical cyclones dissipated over the open Atlantic.  This is unusual and is a direct result of the increased shear.

Six of the tropical storms (Ana, Claudette, Danny, Erika, Grace and Henri) lasted only one to three days.  I have always been a fan of the Accumulated Cyclone Energy (ACE) Index which is a more robust measure of the tropical cyclone activity.  The ACE Index is defined as the sum of the square of a named storm’s maximum wind speed for each 6-hour period of its existence.  The ACE Index accounts for the number, intensity and duration of the tropical storms and hurricanes.  A long-lived major hurricane will have a much higher ACE vaue than several short-lived tropical storms.

Atlantic ACE_1995 to 2009

The above figure (kindly provided by Local 10’s Aliana Perez) shows the ACE values going back to 1995.  Data were provided by the National Hurriane Center (NHC).  The ACE value for the 2009 Atantic Hurricane Season is 52 (although this could change slightly after the NHC completes the best tracks).  Since 1995, only 1997 (also an El Niño year) had a lower ACE value than 2009.  To me, the ACE index is a much better parameter to use to characterize the overall activity of a hurricane season than simply referencing the number of storms or hurricanes.

While most folks, including me, celebrate November 30th as the end of the 2009 hurricane season, I like to remind myself that this date also marks the beginning of the six month period that we have to prepare for the 2010 hurricane season.

The graphic below shows the formation points of 26 tropical storms during the period November 15-30 from the National Hurricane Center’s historical data set extending from 1851 to 2008. 

November 15-30 Formation Points 1851 to 2008

The tropics remain quiet at the moment and the long-range models don’t show any tropical cyclone developments.  If we don’t have any more tropical storms or hurricanes, 2009 will go down as one of the least active years in recent memory at least in terms of Accumulated Cyclone Energy.

Tropical Storm Ida made landfall early this morning with the center moving inland over the Alabama coast.  The system has now lost all tropical characteristics and is what is called an “extratropical” cyclone.  That just means that the cyclone looks more like a winter-time low than it does a tropical cyclone.  The center is over the western Florida panhandle but the biggest concern for the next couple of days will be heavy rains over portions of the southeastern U.S.

Elsewhere, we are monitoring a non-tropical low pressure area over the central Atlantic located about 500 miles south-southeast of Bermuda.  According to the National Hurricane Center, there is a low chance, less than 30 percent, of this system becoming a tropical or subtropical cyclone during the next 48 hours.  Even if this system does develop, it will most likely turn northward and get closer to Bermuda than it does to South Florida. 

Here in South Florida, we are expecting a good chance of rain on Wednesday but will finally get some cooler and drier air into the area after that.  Let us hope that Ida was the end of the hurricane season.

For several days, the National Hurricane Center (NHC) has been talking about Ida becoming “extratropical.”  The definition of extratropical used by NHC is as follows:

“Extratropical:  A term used in advisories and tropical summaries that indicate that a cyclone has lost its “tropical” characteristics.  The term implies both poleward displacement of the cyclone and the conversion of the cyclone’s primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.”

The transition from tropical to extratropical is obviously not instantaneous.  Ida will gradually lose its tropical characteristics and is forecast to become “extratropical” within 36 hours.  But it is important to note that tropical storm force winds will still accompany Ida as the transition occurs.  And it is also important to understand that the strong winds and heavy rains are already spreading onshore along the Gulf coast, well in advance of the center approaching the coast, as the NHC has been saying.  The heavy rains will continue and spread from the central and eastern Gulf coast northward and northeastward into the eastern portions of the Tennessee Valley, southern Appalachians, and the southeastern U.S.

Although Ida has been downgraded from a hurricane to a tropical storm, the threat of strong winds, heavy rains, coastal flooding, and the possibility of isolated tornadoes remain.  And even after Ida transitions to an “extratropical” cyclone, these threats will continue.

Some have asked if Ida becoming a hurricane this morning was a surprise.  I can’t speak for the National Hurricane Center (NHC) forecasters, but I suspect they would say that it takes a lot to really surprise them.  The NHC Hurricane Specialists  issue forecasts on over 25 tropical cyclones each year if you combine the Atlantic and east Pacific basins for which they have responsibility.  These forecasters have seen a lot of erratic behavior when it comes to tropical cyclone tracks and a lot of rapid changes in intensities.  They understand very well the uncertainty in forecasting.

When the initial advisory was issued on Tropical Depression Eleven at 10:00 am EST on Wednesday, November 4th, there was a 10% probability of it becoming a hurricane within 24 hours based on the Intensity Probability Table issued with that advisory.  The probability of Tropical Storm Ida becoming a hurricane within 12 hours had increased to 17% based on the 10:00 pm EST Wednesday, November 4th advisory.   These probabilities were low, but certainly not zero.

Based on Air Force reconnaissance data received after the 4:00 pm EST advisory was issued yesterday, the maximum sustained winds were increased to 65 mph at 5:30 pm EST.  That is only 9 mph short of hurricane force winds.  The public advisories mentioned that Ida could approach hurricane intensity before making landfall although the forecast did not explicitly forecast a hurricane.  The Hurricane Watch for the coast of Nicaragua that was coordinated between NHC and the National Meteorological Service of Nicaragua and issued near 5:30 pm EST yesterday meant that there was a possibility of hurricane conditions within the watch area.

To me, the take-away lesson from Ida so far is that we have yet another reminder that the forecasts are not perfect.  The sooner we learn to accept that uncertainty is a fundamental characteristic of weather forecasting and that no forecast is complete without a description of that uncertainty, the better off we will be.

The first five months of the 2009 Atlantic Hurricane Season have produced well below average tropical cyclone activity.  November has had some memorable events, including last year’s Hurricane Paloma, a Category 4 hurricane that struck Cuba.  But normally, November doesn’t produce much activity.  During the satellite era starting in 1966, the Atlantic has averaged 0.6 tropical storms and 0.4 hurricanes during the month of November.

November Formation Points 1851 to 2008

The graphic above shows the formation points of 62 tropical storms during the month of November from the National Hurricane Center’s historical data set extending from 1851 to 2008.  Note that the developments are somewhat more concentrated over the western Caribbean.

Only time will tell whether the disturbance currently over the southwestern Caribbean Sea develops or not.  Given its structure in satellite imagery, it certainly bears watching.

I’m occasionally asked for a source on tropical cyclone forecasts in ocean basins other than the Atlantic.  There is a World Meteorological Organization (WMO) web site, called the Severe Weather Information Center or SWIC, that was originally intended for the media to access basic information on current tropical cyclones issued by WMO Regional Specialized Meteorological Centers (RSMCs) and Tropical Cyclone Warning Centers (TCWCs) as well as official warnings issued by National Meteorological and Hydrological Services for their respective countries or regions.  21 WMO members are participating in SWIC.

http://severe.worldweather.org/ is developed and maintained by the Hong Kong Observatory.  Current tropical cyclones are plotted on the map found on the SWIC home page.  Today’s map, for example, displays Typhoon Mirinae which is currently approaching the Philippines.  Clicking on Mirinae shows the official forecast from RSMC Tokyo.

Severe Weather Information Center map showing areas of responsibility (courtesy SWIC web site)

Given that different participating WMO members in some ocean basins may give different locations, intensities and even names for the same tropical cyclone, only the information supplied by the responsible RSMC or TCWC is displayed graphically on the maps to avoid confusion.

In my opinion, one of the most useful publications on tropical cyclones in the Atlantic basin is the so-called “Track Book.”   The publication has been recently updated and shows tracks of all the tropical/subtropical storms and hurricanes going back to 1851.  These tracks are shown by individual year, my month, and by 10-day intervals.

In addition, a wealth of information is included such as frequency of occurrence, beginning and ending dates of seasons, and U.S. landfalls.

The publication is officially called the Historical Climatology Series 6-2: Tropical Cyclones of the North Atlantic Ocean 1851-2006, and the sixth revision was prepared by the National Climatic Data Center (NCDC) in cooperation with the National Hurricane Center (NHC).  Current authors are NHC’s  Colin J. McAdie, Christopher W. Landsea, Charles J. Neumann (retired), Joan E. David, and Eric S. Blake as well as NCDC’s Gregory R. Hammer. 

The publication can be ordered as a soft cover book for a price from NCDC or can be downloaded for free.  See http://www.nhc.noaa.gov/abouttrackbooks.shtml.

NOAA's Tropical Cyclones of the North Atlantic Ocean 1851-2006

Several people have asked me if I think that the Atlantic hurricane season is over.   During the past ten years, we have had two seasons that ended in early October – in 2002 and in 2006.  But I assure you that no one can tell you with absolute certainty whether this season is already over or not.  I can, however, tell you that if you look at the historical records going back to 1870 that only about 25 to 30% of the seasons end by this date.

Beginning and Ending of Atlantic Hurricane Season (NCDC)

Although there are no tropical disturbances in the Atlantic basin that show a potential for tropical cyclone development at the moment, I would like to jog our memories a little with a reminder of some recent late season hurricanes that impacted South Florida.

In late October 2001, Michelle formed over the western Caribbean, struck Cuba as a Category 4 hurricane in early November, and brought tropical storm conditions to the Florida Keys and the southeast Florida coast.

Everyone in South Florida will remember Hurricane Wilma in October of 2005.  Wilma became a Category 5 hurricane over the northwest Caribbean with the all time lowest central pressure for an Atlantic basin hurricane, caused tremendous damage on the Yucatan peninsula, made landfall on the southwest Florida coast as a Category 3, and brought mostly Category 1 and 2 winds to the southeast Florida coast.  This resulted in over $20 billion in damages in Florida and made Wilma the 3^rd costliest hurricane in U.S. history.

Noel formed in late October and brought large loss of life over Hispaniola from heavy rains and mudslides.  It became a hurricane near the northwestern Bahamas. Noel missed South Florida but it was close enough for the National Hurricane Center to post tropical storm warnings for the southeast Florida coast.  Only a slight change in the steering currents could easily have brought that hurricane closer to South Florida.

October has been very inactive in the Atlantic so far.  But officially we still have 1 ½ months left in the hurricane season.  I’ll be surprised if this season doesn’t produce another tropical cyclone somewhere within the Atlantic basin.