Offsetting Weather Patterns

Meteorological fall is coming to an end on this final day of November.  This month will finish with near average temperatures in the Nation’s Capital, but with roughly a third less rainfall than average.  According to NOAA, this is the eighth consecutive drier than average November in Washington, D.C.

The warmest temperature this month was 78° on November 3.  Although not record setting, 78° is DC’s average high temperature on September 19.  To put the 78° in further perspective, one can look at the past two Novembers.  DC’s warmest temperatures in November 2015 and 2016 were 80° (11-6-2015) and 83° (11-3-2016), respectively, so having similar warmth this November is consistent with recent history.  

This month’s coldest temperature was 26° on November 11, which tied the record low for the date.  It was DC’s first instance of record cold during the month of November since November 9, 1976 (also 26°).  The high temperature on November 11 was only 39° and that made it the coldest November day in Washington, D.C. since November 25, 2013 (38°/24°).

However, that frigid day was the outlier this month.  November 2017 has featured a total of four days with high temperatures in the 70s, 10 days in the 60s, 11 days in the 50s, 3 in the 40s and only one day in the 30s.  Ironically, November’s average temperature (combining daily high and low temperatures) will finish remarkably close to average.  Through November 29, DC’s average temperature (measured at National Airport) is spot on average at 49.8°.  In contrast, 13 of the last 20 November’s have been warmer than average in Washington, D.C., including the last two.

Only 1.99” of rain fell this month, compared to the monthly average of 3.17”.  The bulk of that occurred on November 7 (1.07”), which was DC’s largest daily November rainfall total since November 26, 2013 (1.46”).  The DC Metro Area has had a dry fall (since September 1) which was more than 4.7” below average.  However, July and August were so wet that DC’s rainfall deficit since January 1 is only 1.49” (through November 29).  In fact, 14 of the last 20 November’s have been drier than average in Washington, D.C., including the last eight.

How Active was the 2017 Atlantic Hurricane Season ?

2017 Atlantic Hurricane Season  (Source: NOAA)

The 2017 Atlantic hurricane season was historic.  A season with 17 tropical storms and 10 hurricanes (including 6 major hurricanes) would, ordinarily, be impressive all by itself.  But, the fact that three major hurricanes made landfall in the United States and its territories magnifies that many times over.

While some recent years (2010 – 2012) have seen more named tropical storms in the Atlantic Ocean than 2017, what made this season significant was the number of major hurricanes.  The six major hurricanes in 2017 plus the four that developed last year produced a total of 10 major Atlantic hurricanes.  That’s the highest two-year total since 2004-2005 (13).  

A “major” hurricane is defined as a Category 3 or higher on the Saffir-Simpson Scale.  NOAA implemented a minor change to the Saffir-Simpson Scale prior to the 2012 Atlantic hurricane season.  The old criteria for a Category 3 hurricane was sustained winds from 111 mph to 130 mph, while a Category 4 was 131 mph to 155 mph.  However, the new criteria makes a Category 3’s winds 111 mph to 129 mph and a Category 4’s winds 130 mph to 156 mph.  That slight difference was enough to make both Harvey and Irma Category 4 storms at their respective times of landfall.  Both had sustained winds of 130 mph when they made landfall in the United States.  

Had this change not been made to the Saffir-Simpson Scale, then the 2017 season would have had only two landfalling Category 3 hurricanes and one Category 4 (Maria).  The fact remains, though, that 2017 not only saw an abrupt end to the 12-year streak without a major hurricane landfall in the United States, but also became the first season with three landfalling Category 4 hurricanes.

This season also produced two Category 5 hurricanes in the Atlantic Ocean: Irma and Maria.  Irma set a record for the strongest Atlantic hurricane to develop outside the Gulf of Mexico or Caribbean Sea with peak winds of 185 mph and a minimum central air pressure of 914 millibars (mb).  Maria’s minimum central air pressure was even lower (908 mb), which made it the tenth strongest hurricane overall on record in the Atlantic Ocean.  Meteorologists consider air pressure to be a more accurate measure of hurricane intensity than wind speed.  The lower the air pressure is, then the stronger the hurricane tends to be.

2017 Hurricane Season Wrap

Hurricane Maria on September 19, 2017  (Source: NOAA)

The 2017 Atlantic hurricane season comes to a close this Thursday, November 30.  It has been one of the busiest and most destructive hurricane seasons on record.  A total of 17 tropical storms, of which 10 became hurricanes – including 6 major hurricanes developed in the Atlantic Ocean Basin.  That’s compared to an average of 12 tropical storms and 6 hurricanes, including 3 major ones.  A “major” hurricane is defined as a Category 3 or higher on the Saffir-Simpson scale with sustained winds of 111/+ mph.

Both NOAA and Colorado State University called for a slightly busier than average 2017 Atlantic hurricane season when it began on June 1.  That forecast in the spring and early summer took into account the possible development of an episode of El Nino.  Even a weak El Nino event contributes to less favorable conditions for tropical storms and hurricanes in the Atlantic Ocean, such as increased wind shear and cooler sea surface temperatures.  

When it became apparent that El Nino wouldn’t be a factor, both NOAA and CSU scientists revised their forecast to call for an above average season.  Tropical Storm Arlene developed in April, while June and July saw a total of three tropical storms and a fourth tropical storm, Emily, developed on July 31.  The season’s first hurricane, Franklin, developed on August 9. 

Later in August, Harvey became the season’s first major hurricane.  It became a Category 4 hurricane with sustained winds of 130 mph before making landfall in Texas.  Harvey was the first major hurricane to make landfall in the United States since Wilma in 2005.  It was also the first Category 4 hurricane to make landfall in Texas since Carla in 1961.  But that was only the beginning as Harvey turned out to be the first of three devastating U.S. hurricanes this year.

2017 was the first season in a decade (and only the sixth overall) to feature two Category 5 hurricanes: Irma and Maria.  Fortunately, Irma weakened to Category 4 intensity before making landfall in the Florida Keys.  But, still caused widespread damage across Florida and Georgia.  Maria also weakened to just under Category 5 intensity before it devastated Puerto Rico.  

The final costs of Harvey, Irma and Maria won’t be known for some time.  All three could surpass Hurricane Katrina as the costliest natural disaster in U.S. history.  Congratulations are in order for the meteorologists with the National Hurricane Center and the others in private companies who worked tirelessly to inform their local communities in harm’s way.