A Break from the Heat?

Keeping Cool on Labor Day, Rockville, MD

While Hermine has dominated weather headlines in recent days, people may not realize that temperatures have been relatively comfortable in Washington, D.C. since September 1.  That’s in stark contrast to last September when seven of the first nine days were 90 degrees or hotter.  In fact, every September since 2012 has featured several days at or above 90 degrees in the Nation’s Capital – significantly warmer than September’s highest daily average temperature of 84 degrees.

Although longtime Washingtonians have grown accustomed to occasional hot days in September, those are still the exception rather than the rule.  The Nation’s Capital will begin another heat wave today following highs in the 80s over the first five days of September.  A “heat wave” is defined as a stretch of three or more consecutive days with temperatures at or above 90 degrees.  Washington, D.C. has already had 51 days at or above 90 degrees in 2016, compared to an annual average of 36.  

Some record highs may be set at Dulles Airport this week, but record highs downtown should be safe.  It’s easier to break records at Dulles since the pool of data is much less than for downtown Washington, D.C. because records began in 1963 at Dulles vs. 1871 downtown.  The impact of the urban heat island effect is also less significant at Dulles Airport than at National Airport.  For example, only 2 record highs have either been tied or set in Washington, D.C. over the last decade during the month of September compared to six at Dulles Airport.  Also, in the last decade there have been a total of 11 record high low temperatures tied or set at Dulles compared to seven downtown.  That illustrates how increased urbanization has helped keep downtown areas warmer at night than they used to be.

Eight of the last 10 Septembers have been warmer than average in the Nation’s Capital – and include two of the top 10 warmest overall.  Since 1996, there have been 10 wetter than average and 10 drier than average Septembers.  That trend can largely be attributed to the years where the Mid-Atlantic Region saw the remnants of tropical systems such as Fran in 1996, Floyd in 1999 and Isabel in 2003.  Fortunately, there are no new tropical systems on the horizon.

Record Highs

September 7:

Washington, D.C. (National Airport – DCA): 104 (1881) --> monthly record
Dulles Airport, Sterling, VA (IAD): 95 (1985)
Baltimore, MD (BWI Airport): 101 (1881) --> monthly record

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September 8:

Washington, D.C. (National Airport – DCA) 100 (1939)
Dulles Airport, Sterling, VA (IAD): 93 (2015)
Baltimore, MD (BWI Airport): 100 (1939)

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September 9:

Washington, D.C. (National Airport – DCA): 98 (1872)
Dulles Airport, Sterling, VA (IAD): 95 (1985)
Baltimore, MD (BWI Airport): 94 (2015)

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September 10:

Washington, D.C. (National Airport – DCA): 98 (1983)
Dulles Airport, Sterling, VA (IAD): 99 (1983) --> monthly record (also occurred 9/2/1980, 9/11/1983)
Baltimore, MD (BWI Airport): 98 (1983)

Hermine and other September Storms

Hurricane Hermine Making Landfall (9-2-16)  Source: NOAA

Hermine ended a record streak of more than 1,000 days without a hurricane in the Gulf of Mexico.  Despite being a minimal Category 1 storm, Hermine was the first hurricane to make landfall in the state of Florida since 2005.  Hermine’s legacy, however, will depend on how significant coastal flooding and beach erosion will be up and down the East Coast. 

Hermine will linger off the East Coast for the next several days, resulting in the potential for a sustained period of gusty winds, storm surge and wind-driven rain over several hundred miles.  That’s unlike the majority of systems, such as a tropical system or a Nor’Easter, that affect a given area for only several hours.  Hermine is a different type of system altogether since the atmospheric steering currents are not allowing it to simply head out to sea.  Instead, Hermine will linger creating potentially catastrophic coastal flooding and beach erosion.

Earlier today, the National Hurricane Center (NHC) designated “Hermine” as post-tropical cyclone since it no longer has the same characteristics of a hurricane or tropical storm.  But the NHC will continue to issue advisories on the system.  That’s in contrast to Sandy in 2012 when tropical storm and hurricane advisories were discontinued after it was declared a post-tropical (or non-tropical) system.

Fortunately for Mid-Atlantic residents, the destructive effects of Hermine have been largely confined to areas of the DelMarVa Peninsula.  That’s unlike some other systems of the last 10 years.  On August 27, 2011 Irene made landfall as a Category 1 hurricane in the Outer Banks of North Carolina.  It tracked northward and brought the DC Metro Region – and much of the Mid-Atlantic and Northeast – significant rainfall, damaging winds and coastal flooding.  Irene, unlike Hermine, was a relatively fast moving system and didn’t linger.

Other tropical systems that have impacted the United States in early September include Tropical Storm Hanna which made landfall in South Carolina on September 6, 2008.  It created dangerous surf and brought appreciable rainfall to much of the eastern United States.  Also, Ernesto made landfall in North Carolina as a strong tropical storm on September 1, 2006.  It tracked northward and brought flooding rains and gusty winds to much of the Mid-Atlantic Region, including Washington, D.C.  September is the busiest month of the hurricane season so having systems like “Hermine” and others – though inconvenient for Labor Day beach plans – is not unusual. 

What Can We Expect in September?

Before talking about September, it’s worth reflecting on August’s personality.  Last month featured 23 days at/above 90 degrees at National Airport which broke the 1980 record of 22.  August 2016 also finished with an average temperature of 82.7 degrees (just behind the 1980 record of 82.8 degrees).  Washingtonians also experienced their second highest average daily high temperature of 91.7 degrees for the month of August – narrowly behind the August 1980 record of 92 degrees.  In Washington, D.C., the highest number of August 100 degree days since 1930 occurred last month with three (August 13-15).

Now that we’ve turned the calendar to September, the start of meteorological fall, many are eager for the cooler weather associated with that.  Average high and low temperatures in Washington, D.C. drop significantly from 84/67 degrees on September 1, to 74/57 on September 30.  The hottest September temperature on record in Washington, D.C. is 104 degrees (September 7, 1881), while September’s coolest temperature is 36 degrees (September 23, 1904).

As the Mid-Atlantic Region has gotten more developed in recent decades, no record low temperatures have been set in Washington, D.C. in September since 1963.  I recently wrote about the effect increased urbanization has had on keeping warmer overnight low temperatures.  That can be especially significant when forecasting winter weather if temperatures downtown remain in the mid to upper 30s, while temperatures in the suburbs cool to at or below freezing.

National Weather Service data indicates there have been recent September extremes in Washington, D.C.  For example, there was a record high of 99 degrees on September 24, 2010; while September 2011 was significantly wetter than average (3.72”) with 8.84” of rain.  Some will recall that most of the rain that occurred in September 2011 was from the remnants of Tropical Storm Lee.  September is traditionally the busiest month of the Atlantic hurricane season and a number of tropical systems have impacted the Mid-Atlantic Region, including Hurricane Floyd in 1999, Hurricane Isabel in 2003, and the remnants of Tropical Storm Lee.

Tropical systems can be tricky to forecast since their tracks can change often and unlike thunderstorms, tropical systems are large in diameter and can effect areas hundreds of miles from their centers.  That’s what made forecasting Hurricane Hermine so tricky.  While the forecast tracks given by the computer models have vacillated a number of times in recent days, “Hermine” will make landfall in the Florida panhandle late tonight.  Hermine is also significant for being the first hurricane in the Gulf of Mexico since Ingrid in 2013, a record stretch of 1,081 days.  My colleagues and I on the WUSA9 weather team will keep you posted.