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.

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.”