September 2019 Has Been Exceptionally Warm and Dry

The unusually dry conditions that have dominated local weather headlines over the last two months will continue for the next few days.  That will help September 2019 finish was one of DC’s driest on record.  High temperatures reached the low to mid 90s in the DC Metro Area for the sixth time this month on September 23.

DC’s average September temperature (combining daily high and low temperatures) is significantly warmer than average at 76.8° through September 24.  All but five days so far this month have been warmer than average.  Record high temperatures were set at National Airport on September 4 and September 12.  A record high temperature was tied on September 12 at Dulles Airport and a new one was also set on September 23.

In addition to being a warm month, September has also been exceptionally dry.  That’s a far cry to last September that finished as DC’s fifth wettest on record with 9.73”.  By comparison, a paltry 0.11” of rain has occurred so far this month.  That would tie 2005 for DC’s driest on record if no more rain occurs.  Making this month’s dry weather even more remarkable is how 14 of the last 17 months have been wetter than average in the Nation’s Capital dating back to April 2018.

Making September’s dry weather even more troublesome for area gardens and allergy sufferers is that August 2019 was also quite dry in the Nation’s Capital.  Washingtonians haven’t experienced consecutive drier than average months since September 2017 – January 2018.  It wouldn’t take much, however, for the DC Metro Area to make up the rainfall deficit of 3.77” that’s developed since August 1.  It would only take one decaying tropical system to bring the Mid-Atlantic Region a few inches of rain.  For example, Washington, D.C. saw a two-day rainfall total of 4.69” from the remnants of Hurricane Sandy in October 2012.

Unfortunately, NOAA’s extended outlook is for continued warmer and drier than average weather conditions for much of the eastern United States from October 2 through October 8.  That’s not good for area gardeners and fall foliage enthusiasts.  Prolonged stretches of warmer and drier than average weather in the late-summer and early-fall can cause the leaves to dry out prematurely.  That diminishes the amount of colors that are possible during peak leaf viewing season.

DC’s Warmest September’s Based on Average Monthly Temperature (Source: National Weather Service - NWS)

1.  78.2° (1881)
2.  77.1° (1980)
3.  76.0° (2016, 1930)
5.  75.5° (2018, 2010)
6.  75.4° (1998)
8.  75.0° (2005, 1970)
10.  74.9° (2015)

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DC’s Driest September’s (Source: NWS)

1.  0.11” (2005)
2.  0.14” (1884)
3.  0.20” (1967)
4.  0.32” (1977)
5.  0.53” (1953)
6.  0.60” (2007, 1986, 1906)
9.  0.63” (1954)
10.  0.65” (1955)

Warm and Dry Start to Fall

Great Falls, Virginia

The first half of September has been remarkably warm and dry in the DC Metro Area.  All but three days this month have been warmer than average, including two days with record-setting heat.  That makes September 2019 DC’s first since 1991 to have at least two record highs.

Through September 16, DC’s average monthly temperature (combining daily high and low temperatures) is 4.5° warmer than average at 77.9°.  There have also been six days of 90° temperatures so far this month.  DC’s record high temperature of 98° last Wednesday, September 12, was not only the second hottest temperature of the year, but also DC’s hottest September day since 2016.

This month is well-positioned to continue a long-running streak in the Nation’s Capital.  There have been nine consecutive warmer than average September’s here.  DC’s last cooler than average September came in 2009.  That was also the last time Washingtonians didn’t experience any 90° September heat.

The fact September has gotten off to a dry start has been compounded by the fact August was also a very dry month.  With a total of only 2.1” of rain since August 1, DC has a rainfall deficit of 2.85” over the last 6+ weeks.  That’s quite remarkable considering how wet the previous 15 months have been in the Nation’s Capital.  Washington, D.C. had a rainfall surplus of an astounding 33.12” of rain for the 15-month period from April 2018 through July 2019.

Having an extended stretch of drier than average weather in late-summer and early-fall isn’t that uncommon in the Mid-Atlantic Region.  It would only take the remnants of one decaying tropical system to make up the rainfall deficit that’s developed over the last six weeks.  However, since January 1 DC’s annual rainfall surplus has fallen to 2.47”. 

Having a year-to-date rainfall surplus is impressive considering how dry the DC Metro Area has been since August 1.  But it’s nothing like last year.  Through September 16, 2018 the Nation’s Capital had 47.13” of rain and set a new record.  According to NOAA’s Climate Prediction Center, warmer and drier than average conditions are likely to continue through the rest of the month.

Peak of Hurricane Season

Humberto has become the third hurricane of the 2019 Atlantic hurricane season.  Fortunately, it won’t have any direct impacts on the United States as it tracks north and east away from the East Coast.  However, it could impact the island of Bermuda.  Aside from Humberto, there are several other tropical disturbances that the National Hurricane Center is monitoring for possible development.

Hurricane season in the Atlantic Ocean Basin is long: spanning half the year from June through November.  The middle of September is commonly the most active time, when atmospheric and environmental conditions are the most favorable.  That’s when sea surface temperatures are at the warmest levels across the largest geographic area in conjunction with the lowest levels of wind shear (winds of differing speeds at different levels of the atmosphere). 

Tropical storms and hurricanes rely on warm ocean water – typically at least 80° Fahrenheit to a depth of at least 200’ – as their primary energy source.  That’s why June and July aren’t as active as August and September.  The second half of October and November typically see diminished tropical activity as atmospheric conditions become less favorable with higher amounts of wind shear.  Wind shear disrupts the circulation of a tropical storm or hurricane and causes weakening.

Earlier in the year, meteorologists called for a near or below average Atlantic hurricane season due to a weakening episode of El Nino.  However, by early August El Nino had fully dissipated and NOAA updated its seasonal hurricane forecast to call for an average to slightly busier than average season.  There have already been eight tropical storms, three hurricanes (including Humberto) and one major hurricane (Dorian).  NOAA's current forecast is for a seasonal total of 10 – 17 tropical storms and 5 – 9 hurricanes, including 2 – 4 major hurricanes.  A hurricane becomes “major” when sustained winds around the center of the storm reach 111 mph or greater.  That makes it a Category 3 on the Saffir-Simpson Scale.

Despite being just past the season’s midway point, the current hurricane season has already broken records.  When Dorian became a Category 5 hurricane on September 1, this year became the fourth consecutive year that one developed in the Atlantic Ocean.  The previous record for most consecutive years with a Category 5 hurricane was three from 2003 – 2005.  Although more Category 5 hurricanes developed (6) during the three-year window through 2005 than during the past four years (5).  That earlier three-year period was skewed by the record-setting year of 2005, which saw four Category 5 hurricanes.  

Fortunately, except for Hurricane Michael last October, no Category 5 hurricane has made landfall in the United States since 1992.  That helps illustrate that as rare as Category 5 hurricanes are, landfalls by such storms are rarer events still. 

Weather Quiz

True or False.  September 2019 has become DC’s first since 1991 to have set at least two record high temperatures.

Warm and Dry Weather Continues in the Nation's Capital

NOAA's Temperature Outlook through September 22

The warm and dry weather that dominated local weather headlines for long stretches last month has continued into September.  This month has had warmer than average temperatures on six of the first eight days of the month.

NOAA’s temperature outlook through September 22 calls for above average temperatures to continue.  To add to that, high temperatures will reach the low 90s in the DC Metro Area tomorrow and Thursday.  Although not quite at record levels, such temperatures are significantly above DC’s average high temperature of 81° on September 11 and 12.  Average high temperatures in the Nation’s Capital actually fall quite a bit during the month of September from 84° on September 1 to 74° by the end of the month.

That should help illustrate how much warmer than average any 90° heat is for Washingtonians in September.  Dating back to the early 1990s, DC Area residents have averaged nearly four days of 90° September heat, with roughly one day when temperatures climb to between 95° and 99°.  Triple-digit heat has only occurred four times in Washington, D.C. since weather records began in 1871.  The most recent occurrence was on September 2, 1980 with a high temperature of 101°.  DC’s record high of 96° just last Wednesday, September 4 was hottest September temperature in the Nation’s Capital since September 9, 2016 (also 96°).

Contributing to the warm and dry conditions so far this month has been a persistent ridge of high pressure situated off the east coast.  That’s not only acted to pump warm and humid air northward from the Deep South but has also helped suppress widespread showers and thunderstorms.  Longtime Washingtonians might recall that September 2005 was a warm and exceptionally dry month in the Nation’s Capital.  In fact, DC set a new record driest September in 2005 with only 0.11” of rain for the entire month.  September 2005 also had seven days of 90° heat that helped the month finish 4° warmer than average.

However, there was a major shift in the weather pattern by the time October 2005 got underway.  A total of 7.3” of rain occurred in Washington, D.C. on October 7-8, 2005 that contributed to make it the wettest October on record.  That’s similar to how rainy it was in the Nation’s Capital on July 8, 2019 when a daily record 3.44” of rain fell.  A total of only 3.73” of rain has occurred in Washington, D.C. in the two months since then.  That helps underscore how warm and dry it’s been in the Nation’s Capital in recent weeks.  August 2019 had only 1.99” of rain that made it DC’s driest month since March 2018.

Record Highs (Source: National Weather Service)

September 11

Washington, D.C. (National Airport, DCA): 98° (1983)
Dulles Airport (IAD), Sterling, Virginia: 99° (1983)
BWI Airport, Baltimore, Maryland: 100° (1983)

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September 12

DCA: 96° (1931, 1895)
IAD: 95° (1998)
BWI: 96° (1931)

The Arrival of Fall

Whether its meteorological fall that gets underway on September 1 or astronomical fall that arrives on September 23, autumn officially arrives in September.  September is also the most active month of the Atlantic hurricane season because atmospheric and environmental conditions are the most favorable.  The Mid-Atlantic coastline had a glancing blow from Hurricane Dorian, which had a destructive trek through the Bahamas earlier in the week.

Other tropical systems over the last 20 years have had significant impacts in the Mid-Atlantic Region during September like Floyd in 1999 and Isabel in 2003.  However, Dorian wasn’t such a storm for the DC Metro Area.  According to the National Weather Service, the Nation’s Capital averages 3.72” of rain in September.  Ten of the last 20 September’s have been wetter than average.  Many DC Area residents should remember that September 2018 was DC’s fifth rainiest on record with 9.73”.

Average high/low temperatures in Washington, D.C. fall from 84°/67° on September 1 to 74°/57° on September 30.  September’s hottest temperature on record in the Nation’s Capital remains the blistering 104° high on September 7, 1881.  By comparison, DC’s coolest September temperature is 36° from September 23, 1904.  Early September can be quite hot, however, as Washingtonians experienced earlier this week with a record high temperature of 96° on September 4.  That was DC’s hottest September temperature in nearly three years.

DC residents have experienced at least one day of 90° heat every September since 2010.  Not surprisingly, Washingtonians endured three of the five warmest Septembers on record over the last decade (2010, 2016, 2018).  As the Nation’s Capital has gotten more developed and seen a commensurate increase in automobile traffic, overnight low temperatures are warmer than they used to be.  There have been 15 record high-low September temperatures in Washington, D.C. during the last two decades.  A “high-low” temperature means the daily low temperature on a particular day was unusually warm.  By comparison, the last time DC residents had a record low temperature in September was in 1988.

September 2019 has gotten off to a warmer and drier than average start in the Nation’s Capital.  NOAA’s outlook for the month is for near average temperatures with a small probability for a wetter than average month.  If September finishes as a wetter than average month in Washington, D.C., then it would be the first time since 2010 – 2012 that the Nation’s Capital has had consecutive wetter than average September’s.

Hurricane Dorian Sets Records

Dorian's Latest Forecast Track from the National Hurricane Center

Although Dorian has weakened considerably from its peak intensity as a Category 5 monster, it is still a powerful and dangerous storm.  Residents of the southeastern United States should be paying attention to local authorities for safety information.  It’s essential to rely on reputable sources of information like the National Weather Service and your local broadcast meteorologists for updates – such as my colleagues and I on the WUSA9 Weather Team.

When Dorian became a Category 5 hurricane on September 1, it became the fifth Category 5 hurricane to develop in the Atlantic Ocean Basin in the last four years.  That surpassed the previous record of three consecutive years (2003 – 2005) that Category 5 hurricanes developed in the Atlantic.  Hurricane Matthew developed in 2016, Irma and Maria in 2017 and Hurricane Michael just last October.  

Hurricane Dorian made landfall in the Bahamas with sustained winds of 185 mph.  That tied it with the 1935 Labor Day hurricane, which made landfall in the Florida Keys as the most intense landfalling hurricane in the Atlantic Ocean Basin.  It’s important to point out that it was the most intense based on wind speed and not air pressure.  Dorian’s minimum central air pressure of 910 millibars (mb) tied it with 2004’s Hurricane Ivan for ninth lowest on record in the Atlantic Ocean Basin.  Its maximum sustained winds of 185 mph also tied it with Hurricane Gilbert (1988) and Hurricane Wilma (2005) for second highest wind speeds in an Atlantic hurricane.  However, Gilbert and Wilma weakened slightly before making landfall.  Only Hurricane Allen (1980) had a higher sustained wind speed (190 mph) in an Atlantic storm.

Not only was Dorian the strongest hurricane on record to make landfall in the Bahamas, it continues to be a very slow moving storm.  According to Dr. Phillip Klotzbach of Colorado State University, “Dorian tracked only ~25 miles in 24 hours –the second shortest straight-line distance tracked by an Atlantic major hurricane in a 24-hour period since 1950…”  The slow moving hurricane prolonged the devastating effects of the storm and made forecasting its ultimate track more difficult.

One good thing about its slow movement has been the resulting upwelling.  Upwelling occurs when wind along the surface of the ocean displaces the water, causing cooler water from lower depths to rise to the surface.  That’s quite common in slow moving tropical systems and the cooler water eventually leads to the weakening of the hurricane.  

Although Hurricane Dorian is much weaker than at its Category 5 peak intensity, it’s still a large and dangerous storm.  The wind field has actually expanding from what it was earlier.  That means tropical storm force winds extend a greater distance from its center than they did earlier.