DC’s cool August weather

 

A fine conclusion to August at the Avenel Park, Potomac, Maryland

August 2023 will finish as DC’s first cooler than average August since 2017.  There were only seven days of 90-degree heat for the second lowest total of the last decade.  By comparison, the nation’s capital had 15 days in the 90s last year. 

Washington, D.C. has averaged between 11 and 12 August days in the 90s over the last 30 years.  However, this month will finish with only 11 warmer than average days.  What’s contributed to making this month cooler than average is the northwesterly wind flow from the Great Lakes Region.  August is traditionally DC’s second hottest month of the year behind only July.  Despite being a cooler than average month, August will still finish as DC’s second warmest month of 2023.

August will also finish wetter than average with 3.71”.  Washingtonians average 3.25” of August rainfall, according to NOAA.  There has been a wide fluctuation in August rainfall over the last five years in the nation’s capital from as little as 1.99” in 2019 to as much as 9.07” in 2021.

August is the third and final month of meteorological summer in North America.  July was warmer and wetter than average while June finished cooler and drier than average.  However, DC’s summer weather isn’t a reliable indicator of what kind of weather will ensue during the autumn.

August’s most destructive hurricanes

 

Hurricane Katrina at peak intensity, August 2005 (Source: NOAA)

The hurricane season often gets much busier during the month of August.  Below are some of the high impact August hurricanes that have made landfall in the United States in recent years.

Ida (2021): Ida reached peak intensity over the northern Gulf of Mexico as a powerful Category 4 hurricane with sustained winds of 150 mph.  It made landfall at Category 4 intensity in Port Fourchon, Louisiana and ranks as one of the strongest hurricanes to ever make landfall in Louisiana.  It caused widespread damage not only along the US Gulf Coast but inland areas as well. 

The DC Metro Area had heavy rain and severe weather due to the remnants of Ida.  A September 1 rainfall record of 4.13” was reported at BWI Airport.  Several tornadoes were also reported in the DC Area, including an EF-2 in Anne Arundel County that caused a lot of damage in suburban Annapolis, Maryland.  Ida ranks as the sixth costliest hurricane in U.S. history with $82.4 billion worth of damage (2023 dollars), according to NOAA.  Ida also claimed nearly 100 lives in the United States.

Laura (2020): Laura was the first major hurricane of the record 2020 season.  It made landfall as a powerful Category 4 on August 27 in Cameron, Louisiana.  Its sustained winds were 150 mph at landfall.  Laura’s impacts also reached areas well inland as far north as Arkansas and Kentucky.  Laura ranks as the 15^th costliest hurricane in U.S. history with $27.2 billion in damage.  It was also responsible for 47 direct deaths, according to NOAA.

Harvey (2017): Harvey was a Category 4 hurricane with sustained winds of 130 mph at peak intensity.  When Harvey made landfall in Texas it was the first major hurricane to make landfall in the United States since Wilma in 2005.  Unfortunately, Harvey was a slow moving storm that left widespread damage both at the coast as well as across inland areas. 

Devastating flooding ensued with 4’ rainfall totals in spots.  Harvey remains the second costliest hurricane on record in the United States, having done approximately $155 billion in damage (adjusted for inflation).  Dozens of fatalities resulted from Harvey, according to NOAA.

Katrina (2005):  A powerful Category 5 hurricane over the Gulf of Mexico less than 48 hours before making landfall, many feared Katrina was the doomsday scenario for the U.S. Gulf Coast.  Fortunately, Katrina weakened to a Category 3 hurricane before making landfall east of New Orleans.  However, because Katrina was such a large and intense hurricane, its storm surge was still commensurate with a stronger hurricane. 

The worst case scenario for a hurricane in New Orleans would be if a Category 4 or 5 storm were to make landfall just west of the city.  That’s because the strongest winds and most dangerous conditions are on the right or eastern side of the hurricane.  Nevertheless, Katrina became the costliest hurricane in U.S. history, according to NOAA.  Katrina caused approximately $193.8 billion in damage adjusted for inflation, with a death toll of more than 1,800 in the United States.

Atlantic hurricane season update – 2023

 

A busy tropical Atlantic (Source: NOAA)

The Atlantic hurricane season spans six months of the year from June – November.  NOAA recently released its August update to its seasonal hurricane forecast.

It calls for an above average Atlantic hurricane season with 14 – 21 tropical storms (sustained winds 39 – 73 mph), of which 6 – 11 become hurricanes (74 mph or greater), including 2 – 5 major hurricanes (Category 3 or greater with sustained winds of at least 111 mph).  That’s a significant increase from their original June forecast that called for 12 – 17 storms, 5 – 9 hurricanes, with 1 – 4 major hurricanes. 

 

Making NOAA’s forecast particularly unique is the unusual set of environmental conditions that exist.  NOAA’s Climate Prediction Center said on August 10: “El Niño is anticipated to continue through the Northern Hemisphere winter (with greater than 95% chance through December 2023 -February 2024).”  El Nino conditions correlate with lower than average sea surface temperatures in the tropical Atlantic, combined with an above average amount of wind shear.  Both create less favorable conditions for tropical storm formation.

However, despite the presence of El Nino, much of tropical Atlantic has unusually warm sea surface temperatures.  According to Colorado State University hurricane expert, Dr. Philip Klotzbach, “Most of the tropical and subtropical Atlantic remains near or at record warm levels, favoring Atlantic #hurricane activity.”  Traditionally, the presence of El Nino has correlated to average or below average tropical development in the Atlantic such as during the 1982, 1997 or the 2015 seasons. 

That’s what makes NOAA’s updated forecast for an above average season so unusual.  Despite El Nino becoming more pronounced during June and July, NOAA increased the number of tropical storms and hurricanes they expect this season.  Their scientists expect that the warmth of the Atlantic will offset, at least to some extent, the El Nino conditions.

Before yesterday, there were only five storms in the Atlantic so far this season.  There was an unnamed subtropical storm in January, along with three tropical storms and a short-lived Category 1 hurricane as of July 24.  However, over the last 24 hours the National Hurricane Center has been tracking three tropical storms (Emily, Franklin and Gert) and a tropical disturbance in the Gulf of Mexico that could develop further. 

September is the busiest month of the hurricane season, so it makes sense the tropics have become more active in late-August.  That’s exactly what NOAA was expecting when they issued their updated forecast for above average activity.

Last season the Atlantic had three November hurricanes illustrating how the tropics can remain active until late in the season.  That’s unusual, though, as environmental conditions normally become less favorable in late-October with cooler waters and higher levels of wind shear.  Hurricane season runs through November 30.