One Extreme to the Other

As the month of March comes to a close, it’s a good time to analyze an impressive weather trend in the Nation’s Capital.  Most area residents know 2018 finished as the wettest year on record in Washington, D.C.  Since weather records kept by NOAA date back to the 1870s, setting an all-time record is a significant achievement.  The wetter than average trend has continued during the first quarter of 2019.

Some may forget, however, that the DC Metro Area was experiencing a prolonged stretch of drier than average weather prior to last April.  The 31-month period from August 2015 through March 2018 included 24 drier than average months in Washington, D.C.  In that time, the Nation’s Capital accrued a rainfall deficit of 16.42”.  Washingtonians experienced its fourth driest January (2018) and February (2017), the third driest June (2017) and fifth driest December (2017).  Consequently, there were intermittent periods of “abnormally dry” to “moderate drought” conditions for parts of the DC Metro Area.

There has been a rather dramatic shift in the weather pattern for the Mid-Atlantic Region since April 1, 2018.  11 of the last 12 months have been wetter than average, including March 2019.  Washingtonians experienced the fifth wettest May (2018) and September (2018), fourth wettest July (2018) and wettest November on record (2018). 

The calendar year of 2018 also set a record for highest number of days (24) with at least one inch of rain.  The 4” of rain that occurred on July 21, 2018 was DC’s wettest day in nearly eight years, since September 30, 2010 (4.66”).  The 4” rainfall total was also more rain than occurred in all but six months during the drier than average period from August 2015 – March 2018.  Such a major shift from a long period with below average rainfall to being wetter than average over the last year can be attributed to an overall shift in the weather pattern.  That includes factors such as the position of the jet stream and ENSO status (El Nino vs. La Nina), as well as other oceanic factors such as the North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO).

The DC Metro Area just had its wettest March since 2015.  That’s contributed to making DC’s rainfall surplus an astounding 29.67” since April 1, 2018.  The Nation’s Capital averages 39.74” of rain in an entire year, so DC’s rainfall surplus over the last year is truly remarkable, especially considering the prolonged stretch of drier than average weather that preceded it.  There was a net-swing of 46.09” of rain between the 31-month period through March 2018 and the record-setting rain in the year that’s followed it.

Weather Quiz

True or False.   

While Washington, D.C. had its warmest March on record in 2012, it also had its snowiest March in more than 50 years in 2014.

Impressive Weather Trends

As March 2019 comes to a close, it will be remembered as a relatively benign weather month in the Nation’s Capital.  Average temperatures are well-positioned to finish within a degree of average for only the third time in the last 20 years.  When the National Weather Service measures a monthly average temperature for a given location, both daily high and low temperatures are used.

While the first 10 days of March were exceptionally chilly with temperatures at or below freezing on five consecutive days, above average temperatures during the middle of the month largely offset them.  That’s why it’s sometimes misleading to look solely at average temperatures.  While this month’s warmest temperature was 78° on March 15, DC’s coldest temperature was 22° on March 6.

According to NOAA, there have been a combined 14 record high / record high-low March temperatures in the Nation’s Capital over the last two decades.  No record cold has occurred in Washington, DC during the month of March since 1996.  It’s a different story, however, at Dulles Airport in more rural Sterling, Virginia, where a total of 36 record high / record high-low March temperatures have occurred since 1999.  But, Dulles did have a total of 21 record low / record low-high March temperatures over the same time period.  A record “high-low” temperature means the low temperature on a particular day remained much warmer than average, while a “low-high” temperature is the opposite.

Two primary factors can account for that large difference.  In the Nation’s Capital weather records date back to the 1870s, so it’s often more difficult to break weather records.  Meanwhile, weather records at Dulles Airport only date back to the 1960s when it opened.  To add to that, the amount of urbanization in and immediately adjacent to the Nation’s Capital contributes to a more pronounced urban heat island effect compared to the more rural Dulles Airport.

Washingtonians have experienced no shortage of cold and snowy weather during the month of March in recent years.  Washington, DC had measurable snowfall in seven consecutive March’s (2013 – 2019), the longest such streak on record.  Not only was March 2014 DC’s coldest since 1996, but it was also the snowiest since 1960 with 12.7” of snow.  March 2014 was also the coldest and snowiest (19.8”) on record at Dulles Airport.

Despite the wide variation in March weather in recent years, one thing has remained constant over the last year.  March 2019 will finish with above average rainfall in the Nation’s Capital.  Since April 1, 2018, the Nation’s Capital has had 69.41” of rain.  That would make the 12-month period from April 1, 2018 through March 31, 2019, DC’s wettest 12-month period on record.  2018 was also DC’s wettest calendar year on record.  Washington, D.C. averages 39.74” of annual rainfall.