Fall Takes Over

Glen Echo Park, Glen Echo, Maryland

It was hard to imagine a few short weeks ago that the cooler, fall-like weather would be here by this point in September.  That’s because this month got off to an exceptionally hot start with seven of the first nine days in the 90s.  However, by comparison, today’s average high is only 75 degrees at National Airport – where weather measurements are made for Washington, D.C.

Although the weather has gotten much more fall-like since the hot start to September, it has remained relatively dry for Washingtonians.  This month has had only 0.5” of rain through yesterday.  Since August 1, the rainfall deficit in the Nation’s Capital has grown to more than 4.5”.  The dry conditions have contributed to many brown lawns and dry leaves falling off trees across much of the DC Metro Region.

Unlike past Septembers, the Mid-Atlantic Region hasn’t seen any rainfall from decaying tropical systems.  Over the last twenty years there have been some very wet Septembers due to the remnants of former hurricanes and tropical storms.  The remnants of Fran came in September 1996, while Hurricane Floyd brought appreciable rainfall to the Nation’s Capital in September 1999.  Most will remember Hurricane Isabel in September 2003, which caused widespread power outages due to the high wind associated with the weakening tropical system. 

According to the National Weather Service, 2011 was the last September when the Mid-Atlantic Region had any impacts from a decaying tropical system.  The remnants of Tropical Storm Lee helped make September 2011 one of the wettest on record in the Nation’s Capital.  That goes to show that a tropical system doesn’t have to be especially strong to cause major impacts.
 

Although this month has been significantly drier than average, it isn’t near the record.  September 2005 was the driest on record in the Nation’s Capital when only 0.11” of rain fell (compared to the September average of 3.72”).  However, that was followed by the wettest October on record in Washington, D.C.

This month has had a total of seven 90 degree days in the Nation’s Capital, which is the highest monthly total since September 2010 when there were ten such days.  The hottest temperature this month was 96 degrees on September 3, while the coolest temperature in the Nation’s Capital was 55 degrees on September 14.  The warm start to the month has put this September in a good position to finish among the warmest on record in the Nation’s Capital.  Fortunately for those ready for autumn, no extreme heat is in the forecast locally.  My colleagues and I on the WUSA9 weather team will keep you apprised of the latest weather forecasts.

Weather Quiz

Category 5 Hurricane Felix in 2007  (Source: NOAA)

True or False.

Although September is traditionally the busiest month of the hurricane season in the Atlantic Ocean, this season has been relatively quiet and no Category 5 hurricane has developed anywhere in the Atlantic since 2007.

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Answer to Weather Quiz question from September 10.

True.  With seven to date, this September has indeed featured the highest number of 90 degree days in the Nation’s Capital since 2010 when September had ten such days.

How Rare are Category 5 Hurricanes?

View of Category 5 Hurricane Ivan from Space Station in 2004  (Source: NASA)

Hurricane season runs a full six months from June 1 through November 30 in the Atlantic Ocean, with September being the busiest month.  That’s because atmospheric conditions are the most favorable in September with ample warm ocean water and little or no wind shear (winds that change directions with altitude) in the tropics.  A tropical storm becomes a hurricane when the sustained winds around the center of the storm reach 74 mph.

The intensity of a hurricane is measured on the “Saffir-Simpson Scale.”  A Category 1 hurricane is considered “minimal,” while a Category 5 is as strong as a hurricane gets – capable of “catastrophic” damage.  Category 5 hurricanes only occur under the rarest of circumstances when atmospheric and environmental conditions are their most favorable.

The past decade from 2001 to 2010 saw the highest number of Category 5 hurricanes on record.  According to the National Hurricane Center, a total of eight Category 5 hurricanes developed in the Atlantic Ocean Basin from 2001 to 2010.  Prior to that, the previous highest total of Category 5 hurricanes measured by decade was six, which occurred twice (in the 1930s and the 1960s).  Since 1920, the Atlantic Ocean has averaged approximately four Category 5 hurricanes per decade.

Although the last decade saw the highest number of Category 5 hurricanes on record, none have developed in the Atlantic Ocean since 2007.  The current streak of eight years since the last Atlantic Category 5 hurricane has tied the second longest streak for length of time in between these rare storms.  In 1988, Hurricane Gilbert became the first Category 5 in the Atlantic since Hurricane Allen in 1980.  The longest streak in between Category 5 hurricanes in the Atlantic Ocean is 15 years – in between the 1938 hurricane that struck New England and Hurricane Carol in 1953.  

There are two important distinctions to consider, though, when I mention this 15 year streak.  Weather satellites weren’t operational until the 1960s.  Prior to the advent of weather satellites, the only way to tell if there was a hurricane was if it made landfall or if there happened to be a ship in its path.  The U.S. Air Force Reserve began flying hurricane “reconnaissance” flights in 1944.  Aside from a landfall, a hurricane reconnaissance flight is the most accurate way to measure the intensity of a hurricane.  There may have been more Category 5 hurricanes that developed in between the 1938 storm (prior to when tropical storms and hurricanes began receiving names) and 1953, but there was no way to tell they existed if they developed over the open ocean.  So given this uncertainty, this 15 year streak should be taken with a grain of salt.

The current eight year streak without a Category 5 hurricane in the Atlantic Ocean remains active.  The United States has been relatively fortunate, too, without a landfall from a major hurricane (Category 3 or higher) since 2005.  Although the United States has had some very destructive hurricanes since then (such as Ike in 2008 and Irene in 2011), they weren’t major storms.

September’s Strongest and Most Destructive Hurricanes

Hurricane Hugo headed for South Carolina (9/21/1989, Source: NOAA)

September is traditionally the busiest month of the Atlantic hurricane season and has seen some significant storms over the course of history.  Whenever a hurricane causes a lot of fatalities or is particularly damaging, the World Meteorological Organization retires its name so it can never be used again.  For example, there will never been another “Katrina,” “Sandy” or “Andrew.”  Here are four September hurricanes that have all had their names retired:

2007: Hurricane Felix was a Category 5 hurricane that formed in the Atlantic Ocean Basin during the 2007 season.  Felix’s highest sustained winds reached 175 mph and its lowest air pressure fell to 929 millibars.  Hurricane Felix tracked relatively far south through the Caribbean Sea and made landfall near the border of Nicaragua and Honduras on September 4.  Tragically, Felix caused widespread damage and multiple fatalities.  Hurricane Felix is also significant because it was the last Category 5 hurricane to form in the Atlantic Ocean. 

2004: Hurricane Ivan was the strongest hurricane of a busier-than-average hurricane season.  It was also a Category 5 storm that caused significant damage to the island nations of Jamaica, the Dominican Republic and the Cayman Islands.  It remains in the top ten for strongest hurricanes to form in the Atlantic Ocean.  Air pressure is considered a more accurate measure of hurricane intensity than wind speed, and at peak intensity Hurricane Ivan’s lowest air pressure was 910 millibars and highest sustained winds around the center of the storm were 165 mph.  By comparison, standard sea level air pressure is approximately 1013 millibars.  Fortunately, Ivan weakened to Category 3 intensity before it made landfall in Alabama on September 16.  Nevertheless, it caused nearly $19 billion in damage according to the National Hurricane Center, making it the 6^th costliest U.S. hurricane on record.

1998:  The most damaging storm to affect the United States in 1998 was Hurricane Georges.  At peak intensity, Georges was a borderline Category 4/Category 5 hurricane with sustained winds of 155 mph and a minimum air pressure of 937 millibars.  Georges had a long and destructive track across many island nations in the Caribbean where it caused widespread damage and multiple fatalities.  Hurricane Georges’ final landfall came along the U.S. Gulf Coast on September 28 near Biloxi, Mississippi.

1989:  Hurricane Hugo was one of the most powerful hurricanes to ever make landfall in South Carolina when it came ashore as a Category 4 hurricane on September 22.  At one point, Hugo was a rare Category 5 hurricane with sustained winds of 160 mph.  Fortunately, Hugo didn’t make landfall at that intensity, but did cause widespread damage and dozens of fatalities in the Caribbean.  Hugo also had the unfortunate distinction of being the costliest natural disaster in U.S. history up until that time having caused $10 billion worth of damage.