September Wrap: A Tale of Two Climates

Shenandoah National Park, Virginia  (Courtesy: Mallory Hughes)

September has come to a close on a fall-like note with temperatures cooler than average and highs only near 70° across the DC Metro Area.  This month will finish as the fifth consecutive drier than average and eighth consecutive warmer than average September in Washington, D.C.

This month’s warmest temperature in Washington, D.C. was 92° on September 24.  That was also significant since it was DC’s first 90°/+ temperature since August 22.  The coolest temperature experienced this month was on September 10 (53°).  This month’s weather can actually be broken up in two distinct halves in the Nation’s Capital.

The first half of the month was cooler and wetter than average for much of the Mid-Atlantic Region.  Twelve of the first thirteen days of September featured cooler than average high temperatures.  There was also a significant amount of rainfall over the first week of the month, with 1.39” at National Airport through September 6.  More than half fell on September 2 (0.77”), when the high temperature was only 65°.  That was the first time since September 6, 2011 that a daily high temperature during the first half of September remained below 70°.

The Mid-Atlantic Region has had a very warm and dry second half of September.  Since 1993, Washington, D.C. has averaged nearly four days of 90° heat and one day at 95°/+ in September.  Washingtonians were near average in that respect this year with three days of 90°/+ this month, which didn’t occur until this past week.  That puts this month in a tie for the second highest amount of 90° heat during the final week of September, behind only September 1881 (6).  According to NOAA, DC’s average monthly temperature (combining daily high and low temperatures) for September finished 1.5° above average.

Washingtonians have had no measurable rain during the second half of September and that will ensure a monthly total of 1.43” (less than half the monthly average of 3.72”).  The National Weather Service said this is the first time since 1941 that no measureable rainfall occurred during the second half of September in Washington, D.C.  NOAA expects a warmer and drier than average start to October.  That means no significant changes should be expected in the short-term from the warm and dry conditions of late September in the DC Metro Area.

Weather Quiz

As a rare Category 5 hurricane  (Source: WUSA9)

True or False.  2017 is the first hurricane season on record that two or more Category 4 hurricanes have made landfall in the United States.

---

Answer to August 31 Weather Quiz question.

A.  Carla.  Harvey was the first Category 4 hurricane to make landfall in Texas since Carla in 1961.  Bret and Ike were both Category 4 hurricanes but weakened before making landfall.

Is it Too Early to Think about Winter?

A recent headline should have caught your attention if you’re looking forward to fall-like weather as well as the upcoming winter season.  NOAA recently issued a “La Nina Watch” for an increased chance that La Nina conditions will develop this winter.  The opposite of “El Nino,” La Nina features cooler than average ocean water in the tropical east Pacific (west of Mexico). 

Not all La Nina winters are alike as there can be varying intensities of La Nina.  More often than not, La Nina winters have had below average snowfall in the Nation’s Capital.  Some of the snowiest winters on record in the Nation’s Capital have occurred during the last 25 years, during both El Nino and La Nina.  For example, while the 1995-1996 winter season (46” of snow) occurred during a weak episode of La Nina, the 2002-2003 (40.4”) and 2009-2010 (56.1”) winters featured a moderate episode of El Nino.  All three of these winters were colder than and much snowier than average (15.4”) in Washington, D.C. and ranked among the Top 5 for snowiest inters on record (dating back to 1888). 

The presence of El Nino or La Nina is just one of several factors that scientists look at when issuing seasonal forecasts for the winter season.  Some other factors scientists consider are the North Atlantic Oscillation (NAO) and the Pacific-North American Pattern (PNA).

More recently, the 2015-2016 winter season is remembered for two things.  December 2015 was the warmest on record in the Nation’s Capital and the first snow of the 2015-2016 winter season didn’t occur until January 12, 2016.  That set a new record for latest first snowfall.  However, later that month, Washingtonians had a major winter storm that tied for fourth largest snowfall on record.   The 17.8” that occurred on January 22-23, 2016 made up more than 80% of the snow that fell during the entire winter.  Despite the warm start and late first snowfall, the 2015-2016 winter finished snowier than average (22.2”). 

There was a very strong El Nino event during the 2015-2016 winter season.  By comparison, 27.6” of snow fell in the Nation’s Capital during another very strong El Nino winter in 1982-1983.  Similar to the 2015-2016 winter that saw most of the seasonal snowfall in one storm, more than half the 1982-3 snowfall occurred on February 10-11, 1983 (DC’s 7^th largest snowfall with 16.6”).  This underscores the fact that storm track is a variable that remains independent of large scale seasonal factors like La Nina or El Nino.  The 1997-1998 winter also featured a very strong El Nino but Washingtonians only saw 0.1” of snow during that entire winter. 

While a winter season could be warmer than average, one significant snow event could also make it snowier than average.  That’s what happened in the 2015-2016 winter where the unusual December warmth caused the seasonal winter temperature to finish at 42° - significantly above the average of 38.2°.  We will see if La Nina develops, know it only takes one big storm to turn seasonal predictions upside down.  Whether you love or loathe the snow, most people know the right set of ingredients has to come together only once to create a memorable winter storm.