Weather Quiz

What notorious October hurricane brought Washington, D.C. its highest wind gust (98 mph) on record?

A.  Gloria

B.  Opal

C.  Hazel

D.  Agnes

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Answer to Weather Quiz question from September 17.

D.  Wilma was the last hurricane to make landfall in Florida (October 2005) prior to Hermine. 

Severe Weather in the Autumn

Ivan's Severe Weather Outbreak in the Mid-Atlantic Region  (Source: NOAA)

Although most common during spring and early summer, there is a secondary, smaller peak of severe weather in the United States during the fall.  This is largely attributed to the change in seasons, as well as severe weather associated with landfalling tropical systems.  The National Weather Service defines “severe” thunderstorm as one that contains any of the following: wind gusts of at least 58 mph, hail 1” in diameter or greater, or a tornado.

When a tropical storm or hurricane makes landfall, it often spawns weak tornadoes as the circulation of the storm interacts with the topography of the land area.  However, these tornadoes aren’t usually very strong, but are potentially dangerous since their approach can be obscured by heavy rainfall.  The Mid-Atlantic Region has had a variety of fall severe weather during the last 15 years.

For example, on September 8, 2012, a damaging line of thunderstorms passed through the DC Metro Region and brought a multitude of severe wind gusts.  These storms led to a number of downed trees and power lines.  Several funnel clouds were observed, but it was determined that none of the funnel clouds touched down and became tornadoes.  Meanwhile, two tornadoes developed during the same severe weather outbreak in the New York City Metro Area – one in Brooklyn and one in Queens.

 

Washington, D.C. residents recently marked the 15^th anniversary of the deadly severe weather outbreak of September 24, 2001.  On that day several tornadoes touched down in the DC Metro Region, including F3 tornado (with winds of approximately 200 mph) in College Park, MD.  Tragically, it caused two fatalities when a pair of sisters were caught in their car.

The Mid-Atlantic Region has seen the remnants of several tropical systems which have caused significant severe weather outbreaks.  The most significant came in 2004 when the remnants of Hurricane Ivan spawned a large number of tornadoes from North Carolina to Pennsylvania.  The remnants of Hurricane Opal also brought three tornadoes to the state of Maryland on October 5, 1995.

Although uncommon, severe weather can occur in the Mid-Atlantic Region during the autumn.  Fortunately, this month hasn’t featured any significant severe weather outbreaks in this part of the country.  In fact, this month could rank among the driest Septembers on record in the Nation’s Capital.

Hot and Dry

This month is on the verge of finishing among the Top 5 hottest and driest September’s on record in the Nation’s Capital.  That shouldn’t surprise area residents considering how hot and dry the months of July and August were.  Although the seasons changed and it’s officially autumn, this month has remained very summer-like. 

Washington, D.C. has had some of the hottest September weather in decades this year.  There have been seven days at or above 90 degrees, including four days that were at least 95 degrees.  That includes a streak earlier this month of three consecutive days of 95-degree heat.  The last time Washingtonians experienced such hot September weather on consecutive days was in 1985. 

Area residents also endured the hottest September day since 2010, when it was 98 degrees on the 8^th.  Also, today is the 6^th anniversary of when the high temperature reached 99 degrees at National Airport.  That was DC’s hottest September temperature since September 2, 1980 (101 degrees).  Just yesterday, Washington, D.C’s high temperature reached 90 degrees.  Although that wasn’t a record high for the date, the average high for September 23 is just 77 degrees.

Through September 23, Washington, D.C.’s average monthly temperature (combining daily high and low temperatures) is 78.4 degrees, according to the National Weather Service.  Although that average will come down a bit during the final week of the month with more seasonal temperatures on tap, it won’t fall enough to keep this month from finishing among the warmest Septembers on record. 

Many Washingtonians have likely noticed, too, just how dry the last few months have been in the Mid-Atlantic Region.  September has a rainfall deficit of 2.3” at National Airport, while the yearly deficit is 3.77”.  The combination of below average rainfall and hot weather – 53 days of 90 degree heat since Independence Day (58 for the year) – have led to abnormally dry conditions for parts of the DC Metro Region on the Drought Monitor Index. 

The relatively recent hot and dry summers of 1999 and 2011 in the Nation’s Capital also featured significant rainfall deficits.  They came to an abrupt end with the arrival of decaying tropical systems.  However, there aren’t any tropical systems on the horizon that might bring the area any relief from the continuing dry weather.  My colleagues and I on the WUSA9 weather team are tracking the potential for a few showers late Monday into Tuesday, but no significant rainfall is expected the rest of the month.

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Driest Septembers on Record (DCA)

1.  0.11” (2005)

2.  0.14” (1884)

3.  0.2” (1967)

4.  0.32” (1977)

5.  0.51” (2016 – through 9/23)

6.  0.53” (1941)

7.  0.6” (2007, 1986, 1906)

10.  0.63” (1954)

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Warmest Septembers on Record in Washington, D.C. (Combining Daily Highs and Lows)

1.  78.2 (1881)
2.  77.1 (1980)
3.  76.0 (1930)
4.  75.5 (2010)
5.  75.4 (1998)

2016 Hurricane Season

Hugo making landfall in South Carolina

The tropical Atlantic is often most active in late September.  This season has already had 12 tropical storms, 4 hurricanes and 1 major hurricane.  That’s near the seasonal average but below NOAA’s forecast for 12 – 17 tropical storms, 5 – 8 hurricanes and 2 – 4 major hurricanes.  The 12^th named storm of 2016, Tropical Storm Lisa, was named yesterday but isn’t expected to become a hurricane or impact the United States.

The last three hurricane seasons have been quieter than average in the tropical Atlantic.  That’s not to say there haven’t been significant storms such as Hurricane Gonzalo in 2014 and Hurricane Joaquin last year.  However, as a whole, the last three seasons have featured the least amount of tropical activity in the Atlantic Ocean since the early 1990s.  That’s before the uptick in tropical activity that began in 1995.  (I recently wrote a column about the 20 year average of 15.4 tropical storms, 7.4 hurricanes and 3.5 major hurricanes through 2015.)

Although the final three years skew the numbers downward slightly, the casual observer may not realize how active the period from 1995 – 2015 was in the tropical Atlantic unless you look at the previous 20-year period.  From 1974 through 1994, there was an average of only 9.7 tropical storms, 5.3 hurricanes and 1.7 major hurricanes each year in the Atlantic Ocean.  A “major” hurricane is defined as a Category 3 or higher on the Saffir-Simpson Scale.  There was a major episode of El Nino in 1982-1983 that helped reduce the number of Atlantic tropical storms.

Hurricane experts point to the “Atlantic Multi-decadal Oscillation (AMO),” which is a natural decades-long cycle of changes in sea surface temperatures of the North Atlantic Ocean than can influence the multi-year cycles of above and below average activity in the tropical Atlantic.  The quieter-than-average period that occurred in the tropical Atlantic during the 1970s and 1980s coincided with large-scale development in Florida and other coastal areas. 

This period of below average tropical activity contributed to a period of complacency and lax enforcement of building codes.  So, when a devastating hurricane like Andrew came along in 1992, catastrophic damage occurred in south Florida.  Despite the quieter-than-average period from 1974 – 1994, Andrew was one of a handful of damaging U.S. hurricanes to occur along with Hurricane Frederic in 1979, Hurricane Allen in 1980 and Hurricane Hugo in 1989.

By coincidence, tonight is the 27^th anniversary of Hurricane Hugo’s landfall in South Carolina, which remains one of the most destructive hurricanes in U.S. history.  While the last few hurricane seasons have been more like the quieter seasons so common from the 1970s to the early 1990s, they also showed us that a damaging or deadly hurricane can occur even during an otherwise quiet season.  Although Hermine became the first hurricane to make landfall in Florida in more than a decade earlier this month, no major hurricane has impacted the United States since 2005.  So it’s important not to let your guard down during a quieter-than-average hurricane season.

Say Good-Bye to Warm and Dry Weather?

View of Key Bridge and Georgetown from Roosevelt Island

Residents of the Mid-Atlantic Region have surely noticed area lawns and gardens have been struggling in recent weeks.  That can be attributed to the frequent record heat since early July, combined with a growing rainfall deficit.  Following the sixth warmest July and second warmest August on record, September has seen more of the same.  

Through September 18, the Nation’s Capital has had six days with high temperatures in the 90s this month, including four days of at least 95 degrees.  Washingtonians also experienced their hottest September day since 2010 when it reached 98 degrees at National Airport on the September 8.  While it has averaged over three days of 90 degree heat in September since 1993, the Nation’s Capital has averaged just under one day at or above 95 degrees over the same period.  So, having four such days this month is rather remarkable.

Dating back to last September, nine of the last 12 months have had below average rainfall in Washington, D.C.  Not since May has there been a month with above average rainfall in the Nation’s Capital.  To add to that, this month has had only 0.05” of rain so far.  Washington, D.C. has a rainfall deficit of 4.48 inches since March 1 and Dulles Airport’s deficit is 4.84 inches.

The combination of prolonged above average temperatures combined with below average rainfall have led to “abnormally dry” conditions across much of Virginia and North Carolina according to the National Drought Monitor Index.  Conditions are more extreme in the northeastern United States, where a significant portion of New York and New England are experiencing severe drought.   

Although D.C. area gardens and lawns have gotten quite dry, the water tables are in good shape.  Hot and dry summers aren’t uncommon in this part of the country.  The summer of 1999 was very similar to this summer in the Nation’s Capital.  In fact, 1999 was so dry that parts of Virginia, Maryland and Washington, D.C. had water restrictions.  Fortunately, the remnants of Hurricane Dennis and Floyd brought an abrupt end to the dry conditions in September 1999.  

Historically, Washingtonians experienced their driest September in 2005 when only 0.11” of rain fell during the entire month; and that was followed by its wettest October.  This helps illustrate that the current dry conditions could change quickly.  This year’s rainfall deficit isn’t especially large or insurmountable.  My colleagues and I on the WUSA9 weather team are monitoring the prospect for rainfall tonight and tomorrow across much of the Mid-Atlantic Region.