August Reaches Its Midway Point

The first half of August 2019 has been unremarkable in the Nation’s Capital.  It has been warmer and drier than average month.  Measurable rainfall occurred on three days so far this month, as compared to eight of the first 15 days of August 2018.  There has been 1.22” of rain so far this month in Washington, D.C. (0.28” below normal). 

What’s accentuated these dry conditions are the warmer than average temperatures.  There have been nine days with high temperatures in the 90s since August 1.  Only two days so far this month have been cooler than average.  That was this past weekend when high/low temperatures downtown were in the mid-80s and mid-60s, respectively.  Washingtonians looking forward to fall were thrilled with the sneak peek of more September-like weather.

While temperatures could touch 90° in the Nation’s Capital this afternoon, the 90s will certainly return tomorrow.  Whether the 90s return today or tomorrow, it will start the next heat wave for Washingtonians.  A “heat wave” is three or more consecutive days of high temperatures that meet or exceed 90°.  There have been 45 days of 90° heat so far in 2019.  That’s already greater than the annual average of 36.  Last year finished with 42 days of 90° heat.  DC’s highest annual total of 90° heat is 67 days that occurred twice: once in 1980 and, more recently, in 2010.

That’s not to say Washingtonians will challenge that record – but approaching 60 days of 90° heat is a possibility.  DC residents should reach 50 such days by the middle of next week.  A significant number of DC’s 90° days this summer occurred in July (22).  What’s made 2019 unusual is while DC has already had more 90° days than average for an entire year, no triple-digit temperatures have occurred.  The hottest temperature so far this year was 99° on July 21.  

Although average high/low temperatures decrease in August and September, 90° temperatures are common in both months.  DC’s last 90° temperature of 2018 didn’t occur until October 4, while the latest 90°/+ on record was on October 11, 1919.  Washingtonians could see 90° heat well into the fall season.  NOAA’s Climate Prediction Center expects above average temperatures to dominate local weather headlines in the DC Metro Area for much of the next six weeks through the end of September.  That said, it will be interesting to see how many 90° days the Nation’s Capital finishes with in 2019.

Summer-like Heat and Humidity Return

Residents of the DC Metro Area have enjoyed a nice stretch of warm and dry weather this past weekend with lower relative humidity and high temperatures in the mid-80s.  However, the weather pattern changed with a high of 92° yesterday and a noticeable increase in relative humidity today.  The area of high pressure that brought the comfortable temperatures and low relative humidity to the Nation’s Capital has shifted eastward.

The clockwise flow around the area of high pressure has helped usher more humid weather conditions into the Mid-Atlantic Region due to southerly winds from the Deep South.  That’s helped create favorable conditions for the prospect of showers and thunderstorms the next few days with the approach of a slow moving cold front.  Although extensive cloud cover will prevent widespread temperatures in the 90s today, DC’s already had 44 days in the 90s in 2019.  That’s nine more than the annual average of 36 days of 90° heat.  Last year’s total was only 42 such days.

DC residents could use a little rain as measurable rainfall has only occurred once this month prior to today.  That’s not to say there haven’t been scattered showers and storms in the DC suburbs so far this month, but the only time rainfall was officially observed in DC was last Wednesday, August 7 (0.9”).  Weather measurements for the Nation’s Capital have officially been made at National Airport since the early 1940s.  Despite having measurable rainfall on only one day so far this month, DC has a rainfall surplus of more than five inches this year.  

July 2019 was DC’s third consecutive wetter than average July and finished with nearly double the average amount of monthly rainfall.  Many area residents and tourists alike have enjoyed the relatively dry weather so far this August.  That’s following the month of July that featured rain on 15 days.  Area lawns and gardens are in need of a good watering in many parts of the DC Metro Area due to spotty rainfall the last few weeks.  

August 2019 has gotten off to a much different start than last August.  Washingtonians had measurable rainfall on six of the first seven days of August 2018.  That’s not to say this month won’t end up becoming DC’s third consecutive wetter than average August.  NOAA’s Climate Prediction Center indicates the Mid-Atlantic Region has a 33% chance of above average precipitation over the next 8-14 days.  However, the outlook for the entire month of August is for near average rainfall in the Nation’s Capital.

NOAA’s Hurricane Update

The Atlantic Ocean Basin also includes the Gulf of Mexico and Caribbean Sea

NOAA just issued an August update to its seasonal hurricane forecast.  When the season began on June 1, NOAA cited El Nino conditions as a primary reason for near to below average tropical activity in 2019 across the tropical Atlantic.  However, El Niño has dissipated and that opened the door to NOAA’s updated hurricane forecast.

In early June NOAA was expecting a near average Atlantic hurricane season (12 tropical storms, six hurricanes and two major hurricanes).  That was consistent with the El Nino conditions at the time.  An episode of El Nino often contributes to below average sea surface temperatures and above average levels of wind shear in the tropical Atlantic.  Both tend to preclude tropical development.

The season has gotten off to a quiet start with only two named storms, including a minimal hurricane.  However, June and July are normally quiet months in the tropical Atlantic.  NOAA’s updated hurricane forecast calls for a total of 10 – 17 tropical storms and 5 – 9 hurricanes, 2 – 4 of which become major hurricanes (Category 3 or greater with sustained winds of at least 111 mph).

Scientists at Colorado State University also tweaked their forecast and are expecting a seasonal total of 14 tropical storms, seven hurricanes and two major hurricanes.  Aside from El Nino conditions dissipating, there has been another key weather headline in the tropical Atlantic.  Atmospheric winds between 5,000 feet and 20,000 feet blew large plumes of dust into the tropical Atlantic from western Africa to west of the Cape Verde Islands.  That helped create some unusually dry and stable air in the same area that tropical storms develop during the height of the season.

There have been active hurricane seasons that got off to slow starts, as this season has.  The 2004 season comes to mind, which had 15 tropical storms, nine hurricanes and six major hurricanes after not seeing the first named storm of the season until August 1.  The overall number of tropical storms and hurricanes that develop is less important than where they develop.  For example, the 1992 Atlantic hurricane season featured only one major hurricane.  But that was Hurricane Andrew which remains one of the costliest and most intense hurricanes to ever impact the United States.  

The National Hurricane Center doesn’t expect any tropical development in the next five days.  The next named storm in the Atlantic Ocean will be “Chantal.”

Final Month of Summer

Strong to Severe Storms a "Slight" Risk in parts of the Mid-Atlantic  (Source: NOAA)

July 2019 was DC’s third consecutive wetter than average July.  While not as wet as 2017 or 2018, this July still finished with 6.49” (almost double the monthly average of 3.73”).  Although no record highs were set, Washingtonians endured 22 days of 90° heat in July, which finished with an average temperature of 82.1°.  That left last month tied with July 1955 for DC’s ninth warmest on record.

August has gotten off to a warm and relatively dry start, with no measurable rainfall yet at National Airport – DC’s official weather reporting site.  That’s not to say parts of the DC Metro haven’t had appreciable rainfall so far this month.  A thunderstorm last evening in Bethesda, Maryland, was its first significant rainfall in more than a week.  The Nation’s Capital averages 2.93” of rain in August.  Ten of the last 20 August’s have been wetter than average, including the last two.

The tropical Atlantic often gets busier during the month of August.  Such was the case in 2011 when a weakening Hurricane Irene brought a few inches of rain to the Nation’s Capital after its landfall in North Carolina.  August can also been a very dry month in Washington, D.C., such as in 1995 and 1998 when less than an inch of rain occurred during the entire month.  In fact, August 1998 ranked as DC’s second driest on record with only 0.59” of rain.

August is the third and final month of “meteorological summer” that began June 1.  Fourteen of the last 20 August’s have been warmer than average in the Nation’s Capital.  To add to that, August has been warmer than July in the Nation’s Capital in eight of the last 20 years.  That’s pretty remarkable considering DC’s average August temperature (combining daily high and low temperatures) is nearly 2° cooler than July (78.1° vs. 79.8°).  Its hottest August temperature on record is 106°, recorded on August 6, 1918 and also on July 20, 1930 – DC’s hottest overall temperature. 

DC’s coolest observed August temperature is 49° and has occurred three times, most recently on August 29, 1986.  Although triple-digit heat typically occurs only every few years in the Nation’s Capital, it happens more frequently than temperatures less than 60°.  DC residents experienced 100° August heat as recently as 2016.  Meanwhile, August temperatures haven’t fallen below 60° in Washington, D.C. since 2004.  NOAA expects this August to finish with near average temperatures and rainfall in the DC Metro Region.

July Comes to a Warm and Wet Conclusion

July will finish warmer than average for a fifth consecutive year in the Nation’s Capital and for the eighth time in the last 10 years.  That’s in large part to DC’s longest heat wave in nearly three years.  It was also a wetter than average month for a third consecutive year.

The 12-day heat wave from July 11 – 22 was the longest Washingtonians experienced since August 2016.  Although no record highs were set, DC had its hottest temperature of 99° (July 21) since August 15, 2016.  There have been 21 days of 90° heat this month in Washington, D.C.  That’s helped July 2019 rank among the warmest on record in the Nation’s Capital.  DC’s average monthly temperature (combining daily high and low temperatures) through July 30 is 82.1°.

The weather pattern was favorable for a lot of warmer than average weather coupled with many very humid days.  That was due in large part to the semi-permanent summer feature known as the “Bermuda High.”  This phenomenon refers to a sprawling area of high pressure that typically sets up during the summer months off the East Coast of the United States near the island of Bermuda.  The southerly/southwesterly winds that blow around the area of high pressure help pump warm and humid air northward from the Gulf Coast toward the Mid-Atlantic Region.  

A series of fronts have also impacted the DC Metro Area during July, bringing frequent showers and thunderstorms to the Nation’s Capital.  Since July 1, a total of 6.48” of rain has fallen in Washington, D.C., well above July’s average of 3.73”. DC’s wettest day this month occurred on July 8 with a daily record 3.44” at National Airport.  However, only 1.05” of rain fell at Dulles Airport on July 8; and a below average total of 2.71” has fallen there this month.  That helps illustrate how localized heavy rain associated with summertime showers and thunderstorms can be.  

As wet as this month has been in the Nation’s Capital, the last two July’s were wetter.  More than 9” of rain occurred in Washington, D.C. in both July 2017 and July 2018.  That was the first time on record that 9” or more rain occurred in consecutive July’s in the Nation’s Capital.  The potential exists for showers and thunderstorms this afternoon as well as the next several days.  August 2019 has the potential to pick up where July left off, with frequent shower and thunderstorm chances.

Warmest July’s in Washington, D.C. by Average Temperature (Combining Daily High/Lows)

1.  84.5° (2011)
2.  84.0° (2012)
3.  83.1° (2010, 1993)
5.  83.0° (1999)
6.  82.7° (2016)
7.  82.6° (1987)
8.  82.3° (1980)
9.  82.1° (1955)
10.  81.9° (1988)

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Most 90° Days in July in Washington, D.C.

1.  25 – 2011
2.  24 – 1993, 1987
4.  23 – 2016, 1988
6.  22 – 2012
7.  21 – 2019
8.  20 – 2017, 2010

Comparing DC Weather Trends in 2019 vs. 2018

U.S. Supreme Court, Washington, D.C.

The first seven months of 2019 have featured remarkable similarities to 2018.  While not as wet as the record-setting 2018 was in the Nation’s Capital, there is a rainfall surplus over the first seven months of 2019.  Check out this new story I co-wrote for the WUSA9 website.

Comparing July 2019 and July 2018

July 2018 was DC’s fourth wettest on record with 9.73” of rain.  This month will also finish wetter than average with 6.48” of rain at National Airport as of July 26.  In fact, Washingtonians experienced a record rainfall total of 3.44” on July 8, 2019, that nearly equaled DC’s entire July average of 3.73”.

A key difference between July 2018 and July 2019 is how streaky the rain was last year compared to this year.  No measurable July rainfall occurred prior to July 17, 2018.  Then, over the final two weeks of last July, Washingtonians experienced three days with an inch or more of rain.  Those three days alone produced a total of 8.14” or more than 80% of the July 2018 total.  By comparison, this month’s rain has been spread out more evenly with measurable rainfall on 11 of the first 26 days of the month.

In addition to both July 2018 and July 2019 being wetter than average, they both were warmer than average in the Nation’s Capital.  DC’s average July temperature (combining daily high and low temperatures) is 81.9°.  So far this month is 2.1° through July 26.  That’s largely the result of the 12-day heat wave from July 11 – 22.  July 2018 also got off to a dry start and was very hot early on with highs in the 90s on eight of its first 12 days.  However, once the rain began falling on July 17, high temperatures reached the 90s only twice the rest of the month.

DC’s weather this July has been fairly streaky as well, with frequent rainfall over the first week of the month followed by the 12-day heat wave.  While eight of the last 10 July’s have been warmer than average in Washington, D.C., no triple-digit July heat has occurred since 2016.  Official weather measurements for the Nation’s Capital are made at National Airport.  It’s important to make that distinction since both Dulles Airport in Sterling, Virginia and BWI Airport in Baltimore, Maryland both experienced 100° heat earlier this month for the first time since July 25, 2016.

DC’s hottest temperature during 2018 occurred on July 16 when it was 98°.  Similarly, DC’s hottest temperature so far this year of 99° occurred on July 21.  High temperatures reached the 90° plateau yesterday for the 32^nd time this year.  That’s pushed the 2019 total for the Nation’s Capital closer to the annual average of 36 days of 90° heat.  High temperatures reached the 90s on 42 days last year.  While above average that was nowhere near DC’s annual record of 67 days of at least 90° (2010, 1980).