Hurricane Season Update

Hurricane Lorenzo near peak intensity  (Source: NOAA)

If the 2019 Atlantic hurricane season were to end today, then it would essentially finish with an average number of named storms.  There have already been 12 tropical storms of which 5 became hurricanes, including three major hurricanes.  Although June and July were predictably quiet months in the tropical Atlantic, the first named storm of the season (Andrea) developed before June 1 for a record fifth consecutive year.

NOAA was expecting a near average hurricane season when it began on June 1.  However, once the weak episode of El Nino dissipated early in the summer NOAA revised its forecast in early August.  El Nino conditions tend to correlate with less favorable conditions for tropical development in the Atlantic Ocean Basin.  So it wasn’t a surprise than NOAA’s updated forecast called for a slightly busier than average hurricane season.

The two most significant Atlantic storms of the season to this point have been Hurricanes Dorian and Lorenzo.  Both achieved Category 5 intensity and that made 2019 the fourth consecutive year one has developed in the Atlantic Ocean.  The previous record for most consecutive seasons with a Category 5 hurricane was three, from 2003 – 2005.  Although Dorian devastated parts of the Bahamas at peak intensity, it weakened considerably before impacting the southeastern U.S. coastline.  Nevertheless, Dorian still had significant impacts in the United States.

While not a threat to the United States, Lorenzo remains a noteworthy storm for becoming the easternmost point an Atlantic hurricane has reached Category 5 intensity (45°W).  It also put this season in relatively rare company as one of a total of only seven seasons with at least two Category 5 Atlantic hurricanes (1932, 1933, 1961, 2005, 2007 and 2017).  This is the fourth time “Lorenzo” has been a named storm in the Atlantic Ocean Basin and the 2019 version is easily the most intense.

Tropical storms and hurricanes only develop when atmospheric and environmental conditions are favorable.  September is the busiest month of the Atlantic hurricane season so it’s common to see hurricanes develop.  October is also an active month of the hurricane season that’s seen a number of significant storms such as Hurricanes Mitch, Wilma and Matthew.  While all three reached Category 5 status, none of them impacted the United States at that intensity.  It remains to be seen how busy the rest of the Atlantic hurricane season will be before the season ends November 30.

Weather Quiz

True or False. 

No September in the Nation’s Capital has had a Top 10 finish for both warmest and driest on record.  Weather records in Washington, D.C. date back to 1871.

Late Season 90° Heat

NOAA's Temperature Outlook for the Second Week of October

Temperatures in the Nation’s Capital have reached the 90s on eight days so far this month.  That’s the highest such total since September 2010 when Washingtonians sweat through 10 days of 90° heat.  Two record high temperatures have also been set at National Airport this September for the first time since 1991.

The frequency of the 90° heat has been spread out fairly evenly throughout September.  There haven’t been more than two consecutive days of 90° heat in the Nation’s Capital since September 1.  That means there haven’t been any “heat waves” which are defined as a minimum of three or more consecutive days of temperatures at or above 90°.  It is somewhat ironic that September 2019 has been so warm in the DC Metro Area without an official heat wave.

NOAA’s Climate Prediction Center expects these warm temperatures to continue into early-to -mid October for much of the DC Metro Area.  Another day or two of 90° heat is even possible during the upcoming work week.  Any 90° heat in the Nation’s Capital in October is a very rare occurrence.  That’s because the sun angle is much lower than during the summer months.  That combined with the decreasing amount of daylight makes it more difficult to get very warm.  The latest in the year Washingtonians have experienced 90° heat is October 11.

There have been only five Octobers dating back to 1994 that DC residents have had temperatures in the 90s.  The most recent occurrence was on October 4, 2018.  Over the same 25-year time period, DC’s last 90° day of the year occurred in August eight times, most recently in 2011.

September 2019 is well-positioned to finish as one of DC’s warmest on record.  That’s because all but five days since September 1 have been warmer than average.  This month will also finish as one of DC’s driest.  That will make this month the first time the Nation’s Capital has had a September that’s ranked among both the top five warmest and driest.

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.