Exceptionally Cool September Weather

 

DC’s coolest September weather in quite some time has come as a shock to the system to many.  That’s because September’s have typically been unusually warm in the Nation’s Capital over the last decade.  Not since 2009 have Washingtonians had a cooler than average September.  As cool as the last few days have been, temperatures will rebound to more seasonable levels later this week and that will allow monthly temperatures to return to near average. 

DC’s average monthly temperature for the first twenty days of September is nearly a full degree (0.8°) below average.  Only seven of the first eighteen days of September 2020 have been warmer than average.  By comparison, there were only three cooler than average days in the Nation’s Capital during all of September 2019. 

My colleagues and I on the WUSA9 Weather Team expect high temperatures to remain the 60s in the Nation’s Capital this afternoon.  That would be the third consecutive day with highs in the 60s.  That hasn’t happened during the month of September since September 16 – September 18, 2011.  Having high temperatures of less than 70° during the month of September has been rare in the DC Metro Area in recent years, with a total of only five such days during the last five years.

Low temperatures have been quite cool, also.  Frost and freeze advisories have been in effect for areas west of the immediate DC Metro Area the last two mornings, with lows falling to the 30s and 40s.  Baltimore tied its September 20 record low of 42° that occurred twice previously in the 1950s.  The coolest observed temperature the last few days at Dulles Airport was 41° on September 20 – just 1° from tying the record low of 40° set in 1979.  DC’s coolest temperature measured at National Airport was 46° on September 20.  However, the Nation’s Capital hasn’t set a record low temperature during the month of September since 1988.  That can be largely attributed to the significant increase in the intensity of the urban heat island effect in that time.  Today was the third consecutive September morning low temperatures fell to the 40s at National Airport. 

The last time daily low temperatures fell to the 40s in the Nation’s Capital on consecutive days during September was in 2000.  Low temperatures were in the 40s in the Nation’s Capital on four out of five days from September 26 – September 30, 2000.  So, it would be accurate to say this is the coolest it’s been in September in 20 years.

Large temperature swings are common in the DC Metro Area during the fall when a dry air mass is in place.  Such will be the case this week when high temperatures reach the 70s again tomorrow after three consecutive days in the 60s, with highs around 80° again on Wednesday. 

DC's Coolest September Weather Since ?

 

Following a rainy stretch of weather in the Nation’s Capital from early July through mid-September, residents are in store for a treat.  A prolonged stretch of sunny and dry weather is underway.  However, this stretch of fall-like weather has gotten off to a chilly start as a cool area of high pressure from Canada is dominating local weather headlines. 

Consequently, Washingtonians are enduring the coolest stretch of September weather in more than a decade following this morning’s lows in the 40s.  Check out this new story I co-authored for the WUSA9 website for details.

Feeling Like Fall

 

The coolest weather of the early fall season has arrived following the rain from the remnants of Hurricane Sally.  Not only will high temperatures remain in the 60s into next week, but low temperatures will range from the 40s in the suburbs to the 50s within city limits.  That’s more characteristic of mid-October than mid-September.  By comparison, DC’s average high temperature both today and tomorrow is 78°. 

Having three consecutive September days with highs in the 60s is rare in the Nation’s Capital.  You have to go back to 1995 to find the last time Washingtonians had four consecutive September days with high temperatures below 70°.  Despite the warm start to September 2020 with highs in the 90s on two of its first four days, temperatures have been much closer to average since then. 

Despite cooler than average weather this weekend, NOAA’s Climate Prediction Center expects warmer than average temperatures to return for much of the final week of September.  That said, having only two 90° days this month would be DC’s fewest total since 2011 when there weren’t any 90° September days.

Fortunately, there will also be an extended period of dry weather in the Nation’s Capital following what has been an exceptionally rainy period.  A total of 19.72” of rain occurred in Washington, D.C. between July 1 and September 18.  That’s 10.91” above average for the period and has produced a year-to-date rainfall total of 40.33”.  DC’s averages 39.74” of rain during the entire year, so any additional rainfall would push 2020 even further above average. 

Since July 1, there have been a total of six days with at least an inch of rain.  That includes three days with at least 2” of rain in the Nation’s Capital that set daily rainfall records (July 7, August 4 and September 10).  While Hurricane Isaias was responsible for DC’s record rainfall on August 4, the rainfall on the other two dates was the result of slow-moving thunderstorms.

Measurable rainfall occurred in the Nation’s Capital on 36 of the 80 days between July 1 and September 18.  Despite being significantly wetter than average, both July and August were much warmer than average.  September 2020 has also been wetter than average, but unlike the last two months should finish within 1° of average.  DC Area residents should take advantage of the ample sunshine, fall-like temperatures and dry weather for much of the upcoming week. 

Sally is the Latest Gulf Coast Hurricane

 

NOAA's Estimated Rainfall Totals for Hurricane Sally

Hurricane Sally regained Category 2 intensity shortly before it made landfall early this morning with sustained winds of 105 mph near Gulf Shores, Alabama.  Now that Sally has made landfall, its forward speed – or lack thereof – is critically important.  While the hurricane is gradually weakening over land, the widespread heavy rain will lead to devastating flooding.  The flooding will be made worse by Sally’s slow movement and flash flood watches have been issued now as far north as southern Virginia. 

There have been other significant autumn hurricanes that have made landfall near where Hurricane Sally did. 

Michael (2018) – Unlike Sally, Hurricane Michael didn’t slow down as it neared the U.S. Gulf Coast.  Rather, Michael continued moving north and intensified right up until it made landfall near Mexico Beach, Florida, as a Category 5 hurricane with sustained winds of 160 mph.  Michael became only the fourth Category 5 hurricane to make landfall in the United States and was both damaging and deadly.  It was the strongest hurricane of the 2018 season and the first Category 5 hurricane to make landfall in the United States since Andrew in 1992.

Georges (1998) – Although peaking as a Category 4 hurricane while east of the Caribbean Islands, Georges made landfall as a strong Category 2 hurricane with sustained winds of 110 mph near Biloxi, Mississippi on September 28.  It was a moderate to high impact storm along the U.S. Gulf Coast.  However, its legacy in the Caribbean was particularly severe where it was a deadly storm that left a widespread swath of damage.

Opal (1995) – Opal was the strongest hurricane of the active 1995 season and strongest Atlantic hurricane, overall, since Andrew in 1992.  Hurricane Opal tracked over the exceptionally warm water of the Loop Current in the Gulf of Mexico.  That combined with favorable atmospheric conditions allowed Opal to rapidly intensity to Category 4 status with sustained winds of 150 mph and a minimum central air pressure of 916 millibars (mb). 

Opal was the strongest hurricane on record in terms of lowest air pressure to not achieve Category 5 intensity, surpassing 1985’s Hurricane Gloria (145 mph / 919 mb at peak intensity).  Fortunately, Opal began an eyewall replacement cycle shortly after reaching peak intensity early on October 4.  That led it to weaken to Category 3 status before making landfall in the Florida panhandle later on October 4.  Opal’s remnants brought heavy rain and severe weather to the DC Metro Area on October 5.

Tropics Getting Active

 

Not a coincidence that the tropics have gotten quite active the last few days with mid-September being the climatological peak of the Atlantic hurricane season.  Tropical Storm Sally is expected to become a minimal hurricane before it makes landfall along the US Gulf Coast, while Paulette has impacted Bermuda as a borderline Category 1/Category 2 hurricane.  It’s a good time to remember some significant September hurricanes that have battered the United States.

Florence (2018) – At peak intensity, Florence was a powerful Category 4 hurricane over the open Atlantic with sustained winds of 150 mph and minimum central air pressure of 937 millibars (mb).  Standard sea level air pressure is approximately 1013.25 mb.  Fortunately, Florence weakened as it approached the North Carolina coast where it made landfall September 14 as a Category 1 hurricane. 

However, because its forward speed had slowed dramatically it brought significant flooding rainfall to areas well-inland, even after it was no longer a hurricane or tropical storm.  Meteorologists compared the flooding from Florence as similar to what Hurricane Harvey brought Texas in 2017.  The DC Metro Area saw record rainfall and flooding from the remnants of Hurricane Florence on September 17-18, 2018.

Irma (2017) – Irma was the first of two Category 5 hurricanes (Maria) of the destructive and deadly 2017 Atlantic hurricane season.  At peak intensity, Irma had sustained winds of 180 mph and a minimum central air pressure of 914 mb.  Hurricane Irma was also at Category 5 intensity for three consecutive days - a new Atlantic record.  Irma was the first Category 4 hurricane to make landfall in Florida since Charley in 2004, and Florida’s first major hurricane since Wilma in 2005. 

Ivan (2004) – The strongest hurricane of the active 2004 season, Ivan was another Category 5 hurricane with peak sustained winds of 165 mph and a minimum central air pressure of 910 mb.  Hurricane Ivan weakened to Category 3 status before its landfall near Pensacola, Florida.  It caused a significant amount of damage not only where it made landfall, but in areas well inland as well.  Ivan set a new record for spawning the highest number of tornadoes in the United States.  Its remnants brought severe weather to the DC Metro Area on September 17, 2004.

Isabel (2003) – Isabel was the first Category 5 hurricane in the Atlantic Ocean since 1998.  Hurricane Isabel reached its peak intensity on September 11, with maximum sustained winds of 165 mph combined with a minimum central air pressure of 915 mb.  Fortunately, Isabel weakened to Category 2 intensity with sustained winds of 105 mph before making landfall in North Carolina on September 18.  Despite weakening, Isabel caused major flooding and damage across much of the Mid-Atlantic and the Northeast.  In the DC Metro Area, Isabel caused approximately one million power outages.

Weather Quiz

 

True or False.

When Tropical Storm Sally was named today, September 12, it became the earliest “S” storm on record in the Atlantic Ocean.