A Soggy and Warm Start to Autumn

This month has become DC’s fifth rainiest September on record with 9.73” at National Airport.  Measurable rain has fallen on a record 16 days since September 1.  To the pleasure of many, the final weekend of September is dry with partly sunny skies.

This month is also DC’s sixth consecutive wetter than average month; the longest such streak in the Nation’s Capital since 2003.  That’s remarkable when you consider the unusually dry streak Washingtonians endured prior to April 1.  During the 32-month period from August 2015 through March 2018, Washington, D.C. had 25 drier than average months.  In that time, DC accrued a rainfall deficit of 16.42”.  Dulles Airport in nearby Sterling, Virginia built a similar rainfall deficit of 11.58” during the same 32-month period.

However, over the last six months there has been a dramatic change from a very dry to a very wet weather pattern.  September’s 9.73” of rain made this month the third of 2018 that DC has had at least 8” of rain for only the second time on record.  DC’s wettest day this month occurred on September 7 when 2.89” fell at National Airport.  

The 9.73” of September rainfall also pushed DC’s rainfall total since January 1 to 49.83”.  Since the Nation’s Capital averages only 39.74” of rain for the entire year, 2018 has already exceeded that.  If DC were to simply have average rainfall the rest of the year, then the Nation’s Capital would finish with 59.45” – making 2018 DC’s fourth wettest year on record.

September 2018 has also been warmer than average with a monthly temperature (combining daily high and low temperatures) of 75.7° as of September 29.  The September average is only 71°.  That leaves DC well positioned for this month to also rank among the warmest September’s on record.  Making that especially impressive is the record number of rainy days that Washingtonians have had in September.  

September got off to a very hot start with a five-day heat wave and highs in the low-to-mid 90s through September 7.  DC’s hottest September temperature of 95° occurred on September 4.  Temperatures largely remained warmer than average thereafter, with DC’s coolest temperature of 59° not occurring until September 27.  In a rare occurrence, that was actually the only time temperatures fell below 60° this month.

DC’s Five Wettest September’s  (Source: National Weather Service)

1.  17.34” (1934)

2.  12.36” (1975)

3.  10.81” (1876)

4.  10.27” (1999)

5.  9.73” (2018)

DC’s Ten Warmest September’s

1.  78.2° (1931)

2.  77.1° (1980)

3.  76.0° (2016, 1930)

5.  75.5° (2010)

6.  75.4° (1998)
7.  75.0° (2005, 1970)
9.  74.9° (2015)
10.  74.3° (1973)

DC’s Five Wettest Year’s

1.  61.33” (1889)

2.  60.83” (2003)

3.  60.09” (1878)

4.  58.17” (1886)

5.  57.54” (1948)

Weather Quiz

Category 5 Hurricane Irma in 2017  (Source: WUSA9)

The rarest of storms, Category 5 hurricanes occur only infrequently.  What year holds the record for most Category 5 storms in the Atlantic Ocean ?

A.  1961

B.  2005

C.  2007

D.  2017

Coastal Development vs. Hurricane Frequency

Hurricane Irma near peak intensity on September 5, 2007  (Source: NOAA)

NOAA recently issued an “El Nino Watch” with a 50% to 55% chance of an El Nino event developing by the end of November.  But, that increases to 65%-70% during the 2018-2019 winter season.  El Nino conditions typically make environmental conditions less favorable in the Atlantic Ocean for tropical storm and hurricane development.

According to NOAA, an “average” Atlantic hurricane season has 12 tropical storms, 6 of which become hurricanes, including 3 major hurricanes (Category 3 or higher on the Saffir-Simpson Scale with sustained winds of 111/+ mph).  However, the Atlantic Ocean has had an uptick in frequency of tropical storms and hurricanes since 1995.  During that time, the Atlantic Ocean’s seasonal average has been nearly 15 tropical storms and 7 – 8 hurricanes, including between 3 and 4 major hurricanes.

By comparison, the Atlantic Ocean generally had a quieter than average cycle in terms of overall number of tropical storms and hurricanes during the previous 23-year period from 1971 - 1994.  The seasonal average during this time was between 9 –10 tropical storms, including roughly 5 hurricanes with 1 – 2 major hurricanes.  The Atlantic Ocean had no more than 3 major hurricanes in any season from 1971 to 1994, while the Atlantic had 4 or more major hurricanes 11 times since 1995.

This predominantly quieter than average period in the tropical Atlantic from the 1970s through the early 1990s is significant since that’s when a significant amount of coastal development occurred in Gulf Coast States and up the East Coast.  With a below average number of tropical storms and hurricanes, it was more common for a significant amount of new construction to be built below the standard building codes for hurricanes as a cost-saving shortcut.  Unfortunately, that’s proven to be more costly in the long-run.

Hurricane Andrew was the only major hurricane of the 1992 Atlantic hurricane season, but it was a Category 5 storm that was the costliest natural disaster in U.S. history for more than a decade.  Andrew helped expose the fact that building codes either weren’t followed for the most part or were inadequate.

Coastal development in the United States has only continued to increase in recent decades.  That has coincided with the busier than average cycle that has been underway in the Atlantic Ocean since the mid-1990s.  Consequently, landfalling tropical storms and hurricanes have been more damaging than in previous decades.  For example, Florida and the East Coast had more landfalling major hurricanes in the 50 year-period through 1965, than in the following 50 years, but the coastal population and degree of development was appreciably less prior to the 1970s and 1980s.  

The fact is that a hurricane season doesn’t have to be active to be destructive.  For example, the 2010 Atlantic hurricane season featured a slew of tropical storms and hurricanes, but the United States only experience minimal impacts.  However, quieter than average seasons have produced destructive storms like Alicia in 1983 and Andrew in 1992.  Hurricane season runs from June 1 through November 30.

Tropical Tornadoes

Mid-Atlantic Tornadoes from Ivan's remnants (9-17-2004)  Source: NOAA

As Hurricane Florence demonstrated, a hurricane and its remnants have far-reaching impacts for areas well-inland.  

Aside from storm surge-related flooding at the coast and inland flooding from heavy rainfall, hurricanes often spawn tornadoes as well.  Tornadoes caused by tropical systems aren’t usually very strong, but a heightened danger surrounds them because they are often obscured by heavy rain.  Some tropical systems have produced more tornadoes than others, as described below.  

2018:  The majority of the damage caused by Hurricane Florence resulted from widespread flooding in the Carolinas and parts of Virginia.  However, the National Weather Service office in Wakefield, Virginia recently confirmed that 10 tornadoes occurred in the Richmond, VA area on September 17, as the remnants of Florence passed through.  The strongest was an EF-2 tornado on the Enhanced Fujita Scale with wind speeds of 115-125 mph.  These tornadoes occurred three days after the storm initially made landfall in North Carolina. 

2004:  Hurricane Ivan was a long-lasting Cape Verde hurricane that at peak intensity was a Category 5 storm with sustained winds of 165 mph.  It made landfall on the U.S. Gulf Coast on September 16 as a Category 3 hurricane.  Ivan had far-reaching impacts, including in the Mid-Atlantic Region where its remnants sparked a tornado outbreak on September 17, 2004, resulting in dozens of tornadoes.  According to NOAA, Ivan was the top tornado producing hurricane on record with a total of 127 tornadoes in the United States.  (Hurricane Beulah had been the previous record-holder with 115 confirmed tornadoes in 1967.)

2004:  Hurricane Frances made landfall on the East Coast of Florida as a Category 2 hurricane less than two weeks before Ivan’s landfall.  Like Ivan, Frances created a large and far-reaching tornado outbreak over several days.  Over 100 tornadoes were spawned by Frances and its remnants between September 4 and September 8.  Of these, roughly two dozen occurred in the Mid-Atlantic Region.  Hurricane Frances remains in third place behind Ivan and Beulah for the total number of U.S. tornadoes it caused (103).

1995:  Longtime DC-Area residents may recall Hurricane Opal’s local impacts in early October 1995.  Opal made landfall as a Category 3 hurricane near Pensacola, Florida on October 4.  No longer a hurricane or tropical storm a day later, its remnants spawned three tornadoes in Maryland.  One each occurred in Charles, Prince Georges and Anne Arundel Counties.  No fatalities were reported.

DC’s Wet 2018 Continues

September 2018 got off to a wetter than average start in the Nation’s Capital even before the remnants of Florence impacted the DC Metro Area.  A total of 47.35” of rain has occurred in the Nation’s Capital since January 1.  That’s the third highest total for the time period in Washington, D.C., according to the National Weather Service.  Washington, D.C. averages 39.74” of rain (including liquid snow-equivalent) for the entire year.  

DC had such a wet spring and summer that it’s easy to forget that 2018 got off to a drier than average start.  January 2018 and March 2018 were a combined 3.43” drier than average.  Ordinarily having two drier than average months over a three-month period isn’t that unusual.  However, it is significant when you consider that 23 of the 29 months (prior to January 2018) were also drier than average in the Nation’s Capital.

There has been a huge turnaround in the overall weather pattern since the end of March.  Washingtonians have now had six consecutive wetter than average months since then.  Prior to the current streak, Washington, D.C. hasn’t had six or more consecutive wetter than average months since 2003.  There have been a total of 17 days with at least an inch of rain in Washington, D.C. through September 20, for the second highest such total on record behind 1886 (19).

DC’s wettest month of the year was July with 9.73” of rain.  That eclipsed the 9.15” of rain which occurred at National Airport in July 2017.  This is the first time that more than 9” of rain fell in consecutive July’s in the Nation’s Capital since weather records began in 1871.  Making these rainfall totals even more significant is that neither July featured any rainfall from a decaying tropical system.  One of DC’s wettest days on record occurred on July 21 when 4” of rain fell at National Airport.  That was DC’s highest daily rainfall total since September 30, 2010 (4.66”).  

If Washington, D.C. were to see merely average rainfall during the final three months of 2018 then the Nation’s Capital would finish with an annual total of 56.97”.  That would make 2018 DC’s fifth wettest year on record and wettest since 2003 (60.83”).

NOAA’s most recent three-month precipitation outlook is for above average precipitation for the Mid-Atlantic Region.  Assuming that holds true, then DC Area residents can expect wetter than average conditions to continue and for 2018 to finish as one of the wettest years on record.