Memorable September Hurricanes

 

Hurricane Florence, September 2018  (Source: NOAA)

September is typically the most active month of the Atlantic hurricane season.  Whenever a tropical storm or hurricane is particularly destructive or deadly, the World Meteorological Organization retires its name.  There are four September hurricanes that have impacted the United States during the last 20 years that have had their names retired.

Hurricane Florence (2018): A long lasting Cape Verde hurricane that lasted more than two weeks, Florence was a Category 4 at peak intensity over the open Atlantic.  It had sustained winds of 150 mph and a minimum central air pressure of 937 millibars (mb).  By comparison, standard sea level air pressure is approximately 1013 mb.  Fortunately, it weakened to Category 1 intensity before its landfall in North Carolina on September 14.  However, because the atmospheric steering currents were very weak, it meandered across the Carolinas and Mid-Atlantic Region for several days and caused devastating flooding.  According to NOAA, Florence did an estimated $25.7 billion in damages in the United States (adjusted for inflation to 2021 dollars).

Hurricane Irma (2017): Irma was another long-lasting Cape Verde hurricane that reached Category 5 intensity on two separate occasions.  At peak intensity, Irma had sustained winds of 180 mph and a minimum central air pressure of 914 millibars (mb).  Irma left a deadly trail of destruction across much of the Caribbean and ultimately made landfall in Florida as a Category 4 hurricane.  It was the first Category 4 hurricane to make landfall in Florida since Charley in 2004.  Ultimately, Irma killed dozens in the United States and did $54.5 billion in damages, making it the fifth costliest natural disaster in U.S. history.

Hurricane Ivan (2004): Ivan was the strongest hurricane of a busier-than-average hurricane season.  It was also a Category 5 storm that caused significant damage to Jamaica, the Dominican Republic and the Cayman Islands.  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.  Its highest sustained winds around the center of the storm reached 165 mph.  Fortunately, Ivan weakened to Category 3 intensity before it made landfall in Alabama on September 16.  It caused nearly $29.7 billion in U.S. damages according to the National Hurricane Center.

Hurricane Isabel (2003): Hurricane Isabel was the first Category 5 hurricane in the Atlantic Ocean since 1998.  Isabel reached peak intensity on September 11, with maximum sustained winds of 165 mph and a minimum central air pressure of 915 millibars.  Isabel weakened to Category 2 intensity before its landfall in North Carolina on September 18.  Despite weakening, Isabel caused major flooding and damage across much of the Mid-Atlantic Region and northeastern United States.  In the D.C. Metro Region, Isabel caused approximately one million power outages. 

Hurricane Season 2021

 

There is currently only one active tropical storm or hurricane in the Atlantic Ocean Basin.  Hurricane Larry is expected to curve northward into the central Atlantic Ocean and remain east of Bermuda.  Larry became the 12^th named storm of the 2021 Atlantic hurricane season on September 1.  There have been 12 tropical storms, 5 hurricanes including 3 major hurricanes so far this season.

Prior to this season, the Atlantic Ocean averaged 12 tropical storm of which 6 became hurricanes, with 3 major hurricanes.  That was based on statistics from 1981 – 2010.  However, starting this season NOAA updated its annual average number of storms to reflect the 30-year period of 1991 – 2020.  An “average” annual Atlantic hurricane season now features a total of 14 tropical storms, 7 hurricanes, including 3 major hurricanes.  A “major” hurricane is a Category 3 or greater on the Saffir-Simpson Wind Scale with sustained winds of at least 111 mph around the center of circulation.

Last season was the busiest hurricane season on record for the Atlantic Ocean with a seasonal total of 30 named tropical storms.  While 2021 has been busier than average, it hasn’t been as busy as last season.  The 2020 “L” storm (“Laura”) developed on August 20.  Meanwhile, Larry was named a tropical storm on September 1, 2021, more than 10 days later than Laura.  The 2020 Atlantic hurricane season had an unusually active final two months with five major hurricanes after October 1. 

This season is already ahead of where last season was on Labor Day Weekend with three major hurricanes, compared with only one (Hurricane Laura) last year.  That has little bearing, however, on how the 2021 Atlantic hurricane season will finish as a whole.

The overall number of tropical storms and hurricanes is less important than where they track.  There have been above average Atlantic hurricane seasons when the United States remained relatively unscathed without any significant impacts such as in 2010.  Other seasons have been quieter than average, but the United States had catastrophic impacts.  Such was the case in 1992 when there was only one major hurricane.  However, that was Hurricane Andrew that was the costliest natural disaster in U.S. history until Katrina in 2005. 

Although final tallies won’t be available for several weeks, if not months, Hurricane Ida has been both the costliest and deadliest hurricane so far this season.  While September is the busiest month of the Atlantic hurricane season, it continues through November 30, so a lot of time remains to be watching the tropics.  Whenever the next tropical storm develops, it will be named “Mindy.”

September Gets Underway

 

A Stellar Start to Labor Day Weekend

August 2021 finished as a warmer and considerably wetter than average month in the Nation’s Capital.  Measurable rainfall occurred on 17 days last month, including on nine consecutive days from August 13 – August 21.  The 9.07” of rain at National Airport made last month Washington, D.C.’s ninth wettest August ever and wettest since 1967 (9.17”).

August 2021 also finished with 19 days that were warmer than average, including 16 days in the 90s.  That helped the Nation’s Capital finish with its fourth consecutive warmer than average August, with a monthly average temperature 1.5° above average.  Although September got off to a very wet start with the remnants of Hurricane Ida across the D.C. Metro Area, the weather heading into Labor Day Weekend has been stellar. 

Average daily high/low temperatures fall in the Nation’s Capital from 84°/67° on September 1 to 74°/57° by September 30.  In a refreshing change of pace, no 90° heat is expected in the D.C. Metro Area during the first week of September.  That’s in contrast to recent September’s that have featured a number of 90° days.  It’s too early to say whether or not this September could become D.C.’s first without any 90° heat since 2009.

Washington, D.C.’s hottest September day on record was September 7, 1881, with a high of 104°.  By comparison, D.C.’s coolest September temperature was 36° on September 23, 1904.  Weather records in the Nation’s Capital date back to 1871.  However, since World War II, measurements for the Nation’s Capital have been made at National Airport compared to down prior to that.  The hottest observed September temperature at National Airport was 101° on September 2, 1980 (also the last time 100° Washingtonians have experienced triple-digit heat in September), while the coolest September temperature at National remains 38° on September 30, 1942.

The Nation’s Capital averages 3.93” of rain in September, with nine of the last 20 Septembers being wetter than average.  September rainfall can fluctuate significantly depending on whether or not a tropical system impacts the region.  For example, September 2003 (Isabel), 2011 (Lee) and 2018 (Florence) were all wetter than average with former tropical systems a key reason why.  By comparison, Washington, D.C.’s driest September on record occurred in 2005 (0.11”), while only 0.25” fell in September 2019.  NOAA expects this month to be wetter than average in the Nation’s Capital.  This month has already gotten off to a wet start with the significant, flooding rainfall from the remnants of Hurricane Ida on September 1.