An Unofficial End to Summer ?

Meteorologists consider the end of August the unofficial end of summer since “meteorological summer” is defined as the three-month period of June, July and August.  By coincidence, August 31 coincides with the Labor Day weekend this year, another benchmark people cite as an unofficial end to summer.  August 2019 will finish as a warmer and drier than average month in the Nation’s Capital.

A total of only 1.99” of rain occurred this month in Washington, DC.  That’s well below the monthly average of 2.93”.  August 2019 will finish as DC’s driest month, overall, since March 2018 (1.92”).  DC’s wettest day this month occurred on August 7 with 0.9”.  By comparison, August 2018 was significantly wetter than average.  DC had two days last August with more than an inch of rain.

This month’s was dominated by a semi-permanent weather feature common this time of year known as the “Bermuda High.”  That’s a sprawling area of high pressure that tends to suppress cloud development and rainfall.  Unlike past Augusts, no tropical systems impacted the Mid-Atlantic this year.  For example, the remnants of Hurricane Irene brought a prolific amount of rainfall to the DC Metro Area in 2011.

Twenty of the first 22 days of August were warmer than average in the Nation’s Capital – with 15 days of 90° heat.  That’s 50% more than DC’s August average of 10 days of 90° heat.  DC’s hottest temperature this month was 98° on August 19, while the coolest temperature of 64° occurred twice (August 24, 26).  Temperatures have actually felt more like fall in the Nation’s Capital since August 23.  Today was actually the first day of 90° heat since August 22 and there have even been two days with highs in the 70s over the last eight days.

Late August can be quite hot in the Nation’s Capital.  For example, 90° heat occurred on nine of the final 10 days of August 1998.  Five consecutive days of 90° heat also occurred from August 27 – 31, 2018.  Those stretches featured predominantly hot and dry weather.  On the other hand, August 2017 was a cooler than average month in the Nation’s Capital and had a chilly and wet conclusion.  NOAA’s outlook for September is for an equal chance of above or below average temperatures in the Mid-Atlantic Region with the potential for above average rainfall.

Perspective on Hurricane Dorian

Dorian has become the Atlantic’s first major hurricane of 2019.  A “major” hurricane is defined as a Category 3 or greater on the Saffir-Simpson Hurricane Wind Scale with sustained winds of at least 111 mph.  Dorian ended a relatively quiet streak in the Atlantic Ocean Basin that had seen only three relatively weak storms prior to its development.  Although Hurricane Dorian largely spared Puerto Rico, the southeast coast of the United States isn’t expected to be as fortunate.

The latest National Hurricane Center forecast has Dorian is poised to make landfall as a Category 3 or 4 hurricane on the east coast of Florida early next week.  Once it nears the Florida coast, there is some uncertainty with its track inland as steering currents are expected to weaken.  That’s bad news as it could allow Dorian to potentially linger over Florida after coming ashore or stalling near the coast.  While hurricanes weaken after making landfall or in close proximity to a given land mass, high winds and storm surge would be long-term issues.

For slow moving tropical systems, long-lasting heavy rainfall becomes the primary danger after the high winds and storm surge diminish.  Hurricane Harvey made landfall in Texas two years ago and lingered for days after making landfall, causing widespread and devastating flooding.  Many Florida cities such as Tampa, Sarasota and Orlando have already had above average rainfall this summer, so the copious rainfall that a slow-moving tropical system would bring is bad news.  That’s especially true given the flat topography of Florida combined with the sandy soil that doesn’t have the capacity to absorb large amounts of rainfall.

Since 2000, three hurricanes have made landfall on the east coast of Florida.  Hurricanes Frances and Jeanne were Category 2 and 3 storms, respectively, and made landfall just three weeks apart in September 2004.  Hurricane Katrina also made landfall in eastern Florida as a Category 1 hurricane in 2005.  

Following Hurricane Wilma in October 2005, the state of Florida wouldn’t see another hurricane until Hermine in 2016.  Although Hurricane Matthew tracked along the east coast of Florida later in 2016, it never made landfall in Florida.  Hurricanes Irma (2017) and Michael (2018) also impacted Florida, but didn’t make landfall on the east coast.  If Dorian makes landfall on Florida’s east coast as a Category 4, it would be the strongest hurricane to make landfall there since Category 5 Hurricane Andrew in 1992.

There is still a fair amount of uncertainty as to what the exact track and intensity of Hurricane Dorian will be as it nears the Florida coast.  That’s because the computer models meteorologists look at when forecasting are not yet in agreement about what will happen.  Making preparations now and heeding local evacuation orders is vital to protect life and property.  Stay tuned to your favorite meteorologists and local authorities for instructions.

Where Are the Hurricanes ?

Barry was the first Atlantic hurricane of 2019  (Source: NOAA)

While five named storms have developed in the Atlantic Ocean Basin so far in 2019, none have developed into major hurricanes.  This season’s strongest tropical system to date remains Category 1 Hurricane Barry.  Dorian is currently situated in the northeastern Caribbean Sea and was just upgraded to hurricane status at 2 PM.  It's currently a minimal hurricane and the second hurricane of the 2019 season.  Dorian could become a major hurricane in the next few days as it approaches the Bahamas.

Meteorologists define a “major” hurricane as a Category 3 or greater on the Saffir-Simpson Hurricane Wind Scale with sustained winds of at least 111 mph.  The last major hurricane to develop in the Atlantic was Category 5 Hurricane Michael last October.  This June and July were predictably quiet months in the tropical Atlantic this season.  Until the last few days, August had continued that relatively quiet trend.  Tropical Storm Erin was named yesterday and was the fifth named storm of 2019.

No Atlantic hurricanes developed in August 2018 yet last season ultimately went on to be busier than average with eight hurricanes compared to the seasonal average of six.  That helps illustrate that any given hurricane season can become busier than average despite a slow start.  The month of September is commonly the most active month of the Atlantic hurricane season when atmospheric and environmental conditions are the most favorable.  Scientists and weather enthusiasts, alike, look for an uptick in tropical activity in August.

Not since the 2001-2002 seasons have there been consecutive Augusts without any hurricanes in the Atlantic Ocean Basin (that includes the Gulf of Mexico and Caribbean Sea).  That’s fairly unusual since NOAA statistics indicate that August is the second busiest month of the Atlantic hurricane season with 245 hurricanes between 1851 and 2018.  That’s second only to September (404). 

Having no hurricanes in August doesn’t in any way suggest the rest of the season will be quiet.  Hurricanes Florence and Michael both had major impacts in the United States last September and October, respectively.  Similarly, no Atlantic hurricanes developed in August 2002, yet Hurricane Lili made landfall on the U.S. Gulf Coast that October and caused $1.1 billion in damages (unadjusted for inflation).  All three hurricane names were subsequently retired by the World Meteorological Organization for being destructive and deadly.

All tropical storms and hurricanes bear watching regardless of intensity.  Earlier this month, NOAA updated its hurricane forecast for the remainder of the 2019 season that runs through November.  It is expecting 2019 to finish as a near average season.  There have been five named storms (including Dorian and Erin) so far in 2019.  An average season features 12 tropical storms of which six become hurricanes and two become “major.”  The next tropical storm in the Atlantic Ocean will be given the name “Fernand.”

Weather Quiz

Michael was the last major Atlantic hurricane to develop (October 2018) - Source: NOAA

True or False.  If no Atlantic hurricanes develop this August, then this would be the first time that none have developed in consecutive years for the first time in nearly two decades.

2019 Rainfall Year-To-Date

NOAA's Precipitation Outlook as August gives way to September

Many DC Area residents know last year was the wettest on record.  The Nation’s Capital had 66.28” of rain in 2018, more than 150% of the annual average of 39.74”.  It also smashed DC’s previous record for wettest year of 61.33” from 1889.  The wetter than average conditions in Washington, D.C. have continued well into 2019.  

Six of the first seven months of 2019 have been wetter than average in the Nation’s Capital.  That’s a higher number of wetter than average months than at the same point last year (five).  However, DC had more episodes of heavier rainfall last year than so far this year.  That makes sense since Washingtonians also experienced a record number of days in 2018 with an inch or more of rainfall (24).

A good example of this occurred in July 2018, which got off to a hot and dry start with no measurable rainfall until July 17.  Ten of the first 16 days of last July also featured 90° heat.  Despite having no measurable rainfall over the first half of July 2018, the month finished with 9.73” of rain (6” above average).  After the dry start to July 2018, it finished with measurable rainfall on seven of the final 10 days of the month.  That frequent rainfall contributed to there being only two days of 90°/+ temperatures during the second half of July 2018.

By comparison, July 2019 finished nearly 3” wetter than average in the Nation’s Capital but had 22 days of 90° heat.  The key difference between July 2018 and July 2019 is that with the exception of July 8, 2019 when 3.44” of rain occurred, there were lots of days with relatively small rainfall totals last month.  In fact, Washingtonians saw rain on 15 of 31 days last month.  

Washingtonians have had below average rainfall so far this month with only 1.97”.  But there will be ample opportunity to add to that total.  The Nation’s Capital averages 2.93” of rain in August.  Weather measurements for DC are made at Nation’s Airport.  Dulles Airport in Sterling, Virginia has had two days with more than an inch of rain since August 1.  That has helped this month become wetter than average at Dulles, but not yet at National Airport.  

Three of the previous four months have been drier than average at Dulles, but only April has been drier than average at National so far this year.  That helps illustrate how spotty late-spring and summer rainfall can be in the DC Metro Area.  Absent tropical systems, rainfall from scattered showers and thunderstorms is often isolated and not all areas see the same amount of rainfall.  Nevertheless, both National and Dulles Airports have rainfall surpluses since January 1.  They just aren’t as large as they were at this point in 2018.

Major August Hurricanes

Minimal Tropical Storm Chantal lurks in the north Atlantic  (Source: NOAA)

The Atlantic hurricane season typically gets busier in August as peak season approaches in September, when atmospheric and environmental conditions are most favorable.  Consequently, a number of major August hurricanes have made landfall in the United States over the years.  Several stand out that have had their names retired for being destructive and deadly.

Charley (2004):  A relatively compact hurricane, Charley was the second major hurricane of the 2004 season.  It made landfall south of Tampa, near Punta Gorda, on August 13.  Charley went through a quick intensification phase from Category 2 intensity to a Category 4 storm prior to landfall.  It made landfall with sustained winds of 150 mph and was the strongest hurricane to make landfall in the United States since Andrew in 1992.  Hurricane Charley did $16 billion in damage (unadjusted for inflation), according to NOAA.

Andrew (1992):  The first named storm of a below average hurricane season was the only major Atlantic hurricane of 1992.  Andrew caused catastrophic damage in southern Florida south of Miami.  Originally ranked as a Category 4 hurricane, scientists later determined it was a Category 5 storm with sustained winds of 165 mph at landfall on August 24.  Andrew was the costliest natural disaster in US history having caused $27 billion in damage (in 1992 dollars).  That’s a distinction Andrew would hold until Katrina in 2005.  

Allen (1980):  Not only did Allen reach Category 5 intensity, it still holds the record for highest sustained winds of any Atlantic hurricane at 190 mph.  That’s in conjunction with a minimal central air pressure of 899 millibars (mb).  At the time, that made Allen the Atlantic’s second strongest hurricane on account of air pressure behind only the 1935 Florida Keys Labor Day hurricane (892 mb).  Allen currently ranks as the fifth strongest hurricane based on air pressure.  Typically, the lower the air pressure the stronger the hurricane is with higher sustained winds.  By comparison, standard sea level air pressure is approximately 1013 millibars.  Fortunately, it weakened to Category 3 status before its south Texas landfall on August 10. 

Camille (1969): Like Andrew in 1992 and Allen in 1980, Camille was the first major hurricane of the season.  A “major” hurricane on the Saffir-Simpson Hurricane Wind Scale is a Category 3 hurricane or greater with sustained winds of at least 111 mph.  Camille became a Category 5 storm in the Gulf of Mexico with sustained winds of 175 mph and a minimum central air pressure of 900 mb.  It made landfall in Mississippi early on August 18 and caused widespread damage.  Camille remains one of only four Category 5 hurricanes on record to make landfall in the United States along with the unnamed Florida Keys hurricane of 1935, Andrew in 1992 and Michael just last October.  

Weather Quiz

True or False.

August is the hottest month of the year in the Nation’s Capital.

Late August Heat

For the first time since April 30, DC residents will have a sunset this evening before 8:00 PM.  Despite the decreasing amount of daylight in Washington, D.C., blistering heat is a frequent occurrence during the second half of August and much of September.  In fact, today will be the second consecutive day of 90° heat with a third on tap tomorrow that will make the current stretch of hot weather an official “heat wave.”

DC’s average high/low temperatures today are 87°/70° according to the National Weather Service.  That’s down from DC’s hottest average daily temperatures of 89°/71° from July 7 – 22.  However, there isn’t much tangible difference between those subtle differences in average high/low temperatures. 

Last August, the Nation’s Capital experienced a five-day heat wave from August 27 – 31 and again from September 3 – 7.  DC’s most significant recent August heat wave occurred in 2016 with 12 consecutive days of high temperatures of at least 90° from August 10 – 21.  During that span was also the last time triple-digit heat occurred in the Nation’s Capital.  DC’s hottest and second hottest temperatures on record, 106° and 105°, respectively, have occurred in both July and August.  August is second only to July in terms of being the hottest month of the year in Washington, D.C.

DC’s hottest temperature on record, 106°, happened twice (August 6, 1918 and July 20, 1930).  Both instances occurred before National Airport opened in the early 1940s, where weather measurements have been ever since.  Prior to that, DC weather observations were made downtown away from the moderating influence of the Potomac River.  The Nation’s Capital was also much less developed than it is now, so the urban heat island effect wasn’t the issue it is nowadays.  

DC’s second hottest temperature has occurred three times, twice in July (July 7, 2012 and July 10, 1936) and once in August (August 17, 1997).  The latest in the year 100° heat was recorded in Washington, D.C. was on September 8, 1939 (100°).  An honorable mention goes to September 24, 2010, that featured a record high of 99°.  That was the 67^th and final 90° temperature of 2010 that tied it with 1980 for DC’s highest annual total of 90° days on record.

Triple-digit heat is a relatively rare occurrence in Washington, D.C., typically only happening every three to four years.  However, August temperatures of less than 60° have been even rarer in the Nation’s Capital in recent decades.  There have been only nine days with low temperatures in the 50s during August in the Nation’s Capital, dating back to the early 1990s.  By comparison, there have been 10 days of triple-digit heat in Washington, D.C. since 1993.  Although those totals are close, the key difference is the last time triple-digit heat occurred in DC during August was in 2016, while there have not been any <60° August temperatures since 2004.

August 19 Record Highs / Record High Lows (Source: NOAA)

Washington, D.C. (National Airport – DCA): 98° (2002) / 78° (2002)
Dulles Airport (IAD), Sterling, VA: 95° (2002) / 74° (2002)
Baltimore, MD (BWI Airport): 97° (1914) / 78° (1937)

August 20

DCA: 101° (1983) / 78° (2002)
IAD: 104° (1983 – hottest August temperature on record) / 74° (2002)
BWI: 105° (1983 – tied for hottest August temperature) / 78° (1914)