Decade in Review

 

NOAA weather averages are based on 30-year periods that are updated every decade.  For example, Washington, D.C.’s current weather averages have relied on the 30-year period of 1981 through 2010.  However, starting next month that will be updated to reflect the period of 1991 – 2020.  There have been a number of memorable weather events and trends in the Nation’s Capital over the last decade.

Although weather records in the Nation’s Capital date back to the 1870s, six of DC’s warmest months have occurred in the last decade.  They include February (2017), March (2012), April (2017), May (2015), July (2011) and December (2015).  Those months ranged from 4.7° warmer than average in July 2011 to an astounding 11.5° warmer than average in December 2015.  Longtime Washingtonians may recall the 2011 summer that featured bountiful 100° heat and ranked among DC’s hottest on record.  December 2015 was also memorable in the Nation’s Capital with 14 days of high temperatures of at least 60°, including three days in the 70s.  In fact, temperatures only fell to 32° or colder twice during that month. 

There have been some memorable cold months over the last decade, such as February 2015 that was DC’s coldest since 1979.  March 2014 was DC’s coldest since 1996 and finished as DC’s snowiest March since 1960.  However, the trend has clearly been for more warmer than cooler than average weather over the last decade.  Given the increased amount of urbanization and automobile traffic in recent decades, it’s much easier to reach warm weather benchmarks than cold weather milestones due to more significant urban heat island effect.

There have also been alternating rainy vs. dry periods in the DC Metro Area over the last decade.  The 32-month period of August 2015 through March 2018 had 25 drier than average months.  During that period, DC accrued a rainfall deficit of 16.42”, or nearly half of DC’s annual average (39.74”).  The subsequent 32-month period from April 2018 through November 2020 was remarkably wet with a rainfall surplus of 46.48”.  That’s more than a year’s worth of extra rainfall in DC during that time.  It also includes DC’s wettest year on record, 2018.  At the same time, 2020 is already in seventh place for DC’s wettest years and could climb to sixth place with more rain in the forecast on New Year’s Eve.

Those are some of DC’s overall temperature and rainfall trends of the last decade.  There have also been several major stand-alone weather events in the Nation’s Capital such as the June 2012 Derecho, “Superstorm” Sandy, and the record-setting January 2016 blizzard.  It will be interesting to see NOAA’s updated weather averages for the Nation’s Capital in 2021, as well as what major weather events are in store for Washingtonians over the next decade.

Is a White Christmas Rare in DC ?

 

There are two ways you can have a White Christmas: fresh snowfall on Christmas Day or the remains of a previous snowfall.  The National Weather Service defines a “White Christmas” as having “…at least 1 inch of snow being on the ground on December 25.”  Areas farther north and west of the DC Metro Area have considerably higher odds of seeing a white Christmas.  However, longtime Washingtonians know snow is rare on Christmas Day.  That’s despite the fact that December as a whole averages 2.3” of snow.  

Since 1884, Washington, D.C. has had measurable snowfall on Christmas Day only nine times or roughly every 15 years.  The last time was on December 25, 2002 when there was 0.2”.  Meanwhile, the last time Washingtonians had snow on the ground on Christmas Day was in 2009.  That’s because the Nation’s Capital had its largest December snowfall on record just the week before.  A total of 16.4” was measured at National Airport on December 18-19, 2009.  Although more than half had melted by Christmas, 7” remained on the ground, creating DC’s last official white Christmas.

The average high and low temperatures in Washington, D.C. on December 25 are 44°/30°.  The coldest Christmas Day in Washington, D.C. occurred in 1983 with high/low temperatures of 14°/3°.  The last time temperatures remained below freezing on Christmas was in 2000 when the high temperature was only 28°.  By comparison, DC’s warmest Christmas on record was in 1964 (72°).  More recently, DC residents had a high temperature of 69° on December 25, 2015 that tied with 1965 for third warmest Christmas.  Such temperatures are more common in late April and early May.

Precipitation of any kind, including rainfall, is much more common on Christmas Day.  DC residents have had measurable precipitation on December 25 a total of 53 times, dating back to 1871.  The most recent rainy Christmas in the Nation’s Capital occurred in 2015.  Last Christmas was a dry day in the DC Metro Area with a high temperature of 49°.

Winter weather enthusiasts may wonder if a warmer than average December is a reliable indicator of what kind of weather will ensue in January.  The short answer is no.  For example, December 2015 finished as DC’s warmest on record but gave way to a colder and snowier than average January.  December 2013 was also warmer than average, but the 2013-2014 winter finished as both a colder and snowier than average winter.  On the other hand, longtime Washingtonians may recall that December 2002 was both colder and snowier than average as part of the epic 2002-2003 winter that finished tied for DC’s seventh snowiest on record.

 

Washington, D.C.’s Snowiest Christmas Day’s  (Source: NOAA)

1.  5.4”  (1962)
2.  4.5”  (1909)
3.  4.3”  (1969)
4.  1.0”  (1902)
5.  0.6”  (1935)

 

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“White” Christmas’ in D.C. during the Last 30 years 

1.  2009
2.  2002
3.  1993
4.  1989

How Wet Has 2020 Been ?

Glen Echo Park, Glen Echo, Maryland

As of December 20, there has been 55.82” of rain in the Nation’s Capital since January 1 giving Washingtonians a rainfall surplus of 17.07”.  If there were no more rain the rest of the year, then 2020 would finish as DC’s seventh wettest year on record.  It was only two years ago that 2018 became DC’s wettest year on record, so two of the last three years will have the distinction of ranking among DC’s rainiest.

Some interesting comparisons can be made between 2018 and 2020.  The Nation’s Capital had a record 24 days in 2018 with at least an inch of rain.  Of those 24, four days had at least 2” of rain and those four accounted for 16.1% of DC’s annual 2018 rainfall.  Of the 24 days, 19 had between 1” and 1.99” of rain.  One of DC’s wettest, overall, days on record occurred on July 21, 2018 when 4” occurred at National Airport.

By comparison, there were 15 days so far this year with at least 1” of rain.  That rainfall accounted for nearly half (47.6%) of DC’s 2020 rainfall total.  While eight days so far this year had between 1” and 1.99” of rain – less than half of DC’s 2018 total of 19 – this year surpassed 2018 on a different rainfall statistic.  There has been a record seven days so far this year with between 2” and 2.99” of rain, compared to only four such days two years ago.  Those seven days this year have accounted for 29.4% of DC’s year-to-date rainfall total of 55.82”.

As in 2018, there were drier than average periods of the year mixed in with the exceptionally rainy periods.  Ironically, 2018 got off to a drier than average start.  For the 32-month period through March 31, 2018, the Nation’s Capital had a combined rainfall deficit of 16.42”.  However, Washingtonians followed that up with a rainfall surplus of 27.8” for the nine-month period from April through December 2018.  Not only did that surplus eliminate the rainfall deficit, but it made 2018 DC’s wettest year on record.

In similar fashion, Washingtonians have a rainfall surplus of 15.22” for the five-month period from July 1 through November 30, 2020.  This surplus, in combination with DC’s wet spell last week, helped push 2020 into the Top 10 for DC’s rainiest years.  That surplus was also more than enough to offset the fact that four of the first six months of 2020 were drier than average.  DC would need another 1.72” of rain in 2020 to reach sixth place on the list of DC’s rainiest years.

Washington, D.C.’s Ten Wettest Years (Source: NOAA)

1.  66.28” (2018)
2.  61.33” (1889)

3.  60.83” (2003)
4.  60.09” (1878)
5.  58.17” (1886)
6.  57.54” (1948)
7.  55.82” (2020)
8.  54.49” (1937)
9.  52.95” (1891)
10.  52.92” (1906)