Weather Quiz

Supreme Court of the United States, Washington, D.C.

True or False.

Today’s record high of 92° that occurred on April 16, 2002 is DC’s hottest April temperature on record.

Severe Weather In April

April has the third highest monthly total of tornadoes (155) in the United States, behind only May (276) and June (243).  The United States has averaged 1,253 tornadoes annually for the 20-year period from 1991-2010 according to NOAA.  

There have been a series of deadly tornadoes in April during that time.  An outbreak began yesterday in Texas and continues today in the southeastern United States.  The threat of severe weather caused the final round of the Masters Golf tournament to begin earlier than scheduled this morning.  A severe thunderstorm contains any of the following: 1”/+ in diameter, wind gusts of 58 mph/+ or a tornado.

2017: A severe weather outbreak occurred on April 29, with a total of 295 severe weather reports from Texas to Virginia.  Of those, 25 were tornadoes from Texas to Illinois.  Fortunately, no fatalities were reported.

2011: One of the largest severe weather outbreaks on record in the United States occurred from April 27-28.  During this two-day period, there were a total of 1,054 severe weather reports, with 307 confirmed tornadoes.  Of those, 292 tornadoes occurred on April 27 from Mississippi to New York State and claimed 230 lives.  

These tornadoes were especially powerful with four being rated as “EF-5” tornadoes on the Enhanced Fujita Scale, with wind speeds of over 200 mph.  That was the highest daily total of F5 or EF5 tornadoes since the “Super Outbreak” of 1974 when seven occurred on April 3.  The Fujita Scale was first introduced in 1971, and was updated to become the “Enhanced Fujita Scale” in 2007.

2006: A significant tornado outbreak occurred on April 2, with 86 tornadoes from Iowa to Tennessee.  The tornadoes were part of an overall severe weather outbreak that stretched from Texas to Ohio, with an overall total of almost 900 severe weather reports.  There were 11 reported fatalities during this outbreak.

2002:  Over the span of two days (April 18-19), a total of 540 cases of severe weather were reported.  Only 15 of the 540 severe weather reports were confirmed tornadoes, and none caused any fatalities.

 

It’s interesting to note that for many years hail was considered “severe” when it reached a diameter of ¾” or greater, roughly penny-sized.  However, the National Weather Service raised the criteria for severe hail to 1”/+ in diameter (quarter size) on January 5, 2010.  That was done because a determination was made that significant damage is less frequent with smaller hail.  The result is a net reduction in the overall number of severe weather warnings since 2010.  

Whenever severe weather is a possibility people should rely on their favorite local meteorologists, such as my colleagues and I on the WUSA9 Weather Team for the latest weather watches and warnings.

A Tale of Two Extremes

Washingtonians have experienced June-like warmth over the last two days with high temperatures of 84° and 81° on April 8 and 9, respectively.  While not record-setting, those temperatures are significantly above the average highs for this time of year, which are in the mid-60s.  The Nation’s Capital isn’t supposed to see temperatures in the 80s on a daily basis until May 31.  

DC’s high of 84° on April 8 was the first 80° day in the Nation’s Capital since October 11, 2018 (83°).  It was also the earliest in the season DC has been that warm since April 4, 2011.  It’s worth considering, too, that while weather records in the Nation’s Capital date back to the 1870s, they have been measured at two different locations historically.  Starting in 1871, weather records were kept for Washington, D.C. downtown.  However, that changed in the early 1940s when National Airport opened and where weather records have been kept since.

DC has averaged three to four April days with temperatures in the 80s dating back to 1993.  That ranges from a high of nine such days in April 2017 to no 80° warmth in 1997 or 1998.  Meanwhile, DC residents have had a 90° April temperature roughly every other year over the last 25 years.  DC’s hottest stretch of April weather in recent times occurred from April 16 – 18, 2002, with daily high temperatures of 92°, 95° and 91°.

On the opposite extreme, Washingtonians have had temperatures at or below freezing at the same rate, roughly one such day every other year.  The last time it was </=32° in April was in 2016.  The last time DC residents experienced frigid April temperatures in the 20s was in 2007.

Prior to this week, Washingtonians had not had consecutive 80° days during the first 10 days of April since 2013.  Making this week even more remarkable is the unusually cold first two days of April when high temperatures were more characteristic of late-February, with highs in the low 50s on April 1 and 2.  Over just a six-day span, Washingtonians experienced a swing in temperatures that normally takes 3.5 months.  DC’s high of 50° on April 2 is the average high temperature on February 25 and the April 8 high of 84° isn’t DC’s average high until June 13. 

2019 Hurricane Season Outlook

This past week Colorado State University released its April outlook for the upcoming Atlantic hurricane season.  Led by scientist Dr. Philip Klotzbach, the forecast calls for a slightly below average 2019 season with 13 tropical storms, of which 5 will intensify into hurricanes, including 2 major hurricanes.  An “average” Atlantic hurricane season has 12 tropical storms, 6 hurricanes and 3 major hurricanes (based on averages from 1981 – 2010).

Two primary factors in the below average hurricane outlook for 2019 are the presence of weak El Nino conditions and below average sea surface temperatures in the tropical Atlantic.  One of the byproducts of El Nino is an increased amount of wind shear (differing wind speeds with altitude) that tends to preclude tropical development.  NOAA expects weak El Nino conditions to continue into the summer.  Tropical storms and hurricanes also rely on warm ocean water, typically at least 80° Fahrenheit, to a sufficient depth (usually 200’ or greater) as its energy source, so cooler than average water is not ideal.   

A tropical disturbance becomes a tropical storm when sustained winds around the center reach at least 39 mph.  That’s also when a system gets a name.  A tropical storm becomes a hurricane when sustained winds reach 74 mph.  A hurricane becomes “major” when sustained winds reach 111 mph and that makes it a Category 3 on the Saffir-Simpson Scale.  It’s important not to focus too much on the intensity or “Category” of a hurricane, since they are all capable of being destructive and deadly.  The remnants of Harvey in 2017 and Florence in 2018 both continued to cause devastating flooding in areas well-inland once they were no longer hurricanes.  Tropical storms are also capable of producing widespread and deadly flooding such as during Allison in 2001.  

Tropical storms and hurricanes are warm-core systems that rely on warm ocean water for their energy.  That’s unlike mid-latitude cyclones that rely on differences in temperature to intensify.  Consequently, another key difference between tropical and non-tropical storms is that the strongest winds are concentrated around the center of a tropical storm or hurricane, while the highest winds of a non-tropical mid-latitude storm (like a Nor’Easter), cover a much larger geographic area often considerable distances from the center of the storm.  

Whether or not a hurricane season is busier than average is less important than where the tropical storms and hurricanes track.  For example, the 1992 Atlantic hurricane season was quiet with only one major hurricane.  However, the one major hurricane was Andrew, which remains one of the costliest natural disasters in U.S. history.  While the 2013 – 2015 Atlantic hurricane seasons were quieter than average, the subsequent three seasons through 2018 were each busier than average with a number of destructive and deadly storms.  

The World Meteorological Organization met recently and officially retired the names “Florence” and “Michael” since they were destructive and deadly.  When the 2018 list of hurricane names is used again in 2024, those names were replaced with “Francine” and “Milton.”  Hurricane season begins on June 1 and the first named storm of 2019 will be named “Andrea.”

April Gets Off to a Chilly Start !

The first three days of April felt more like March in the Nation’s Capital.  That coincided with the National Park Service’s declaration that the famous cherry blossoms along the Tidal Basin reached “peak bloom” on April 1.  The chilly temperatures are actually good for the blossoms once they reach peak bloom.  Their primary enemies are gusty winds, hot weather or heavy rainfall.

High temperatures on April 1 and 2 were 51° and 50°, respectively, at National Airport while they remained in the upper 40s at Dulles Airport.  That’s more characteristic of late-February than early April.  It also made for the chilliest first two days of April in the Nation’s Capital since 2011.  Low temperatures remained in the 30s at National Airport on the first three days of the month.  That was the first instance of at least three consecutive days with temperatures in the 30s during April in Washington, D.C. since April 2 – 5, 2013.

It was even colder at Dulles Airport, where low temperatures were in the 20s on the first three days of the month.  Although not record-setting, it was the first time it’s been that cold at Dulles in early-April since the same time period in April 2013.  However, it doesn’t typically remain chilly for very long in April given the increasing sun angle and amount of daylight.  Despite a low temperature of 38° at National and 27° at Dulles yesterday, high temperatures reached the low 70s.

Whenever there is a difference in low temperature of 5° or more between two relatively close geographic locations, one should compare the difference in urbanization and proximity to water between them.  National Airport is on the Potomac River by the Nation’s Capital and the heavily urbanized northern Virginia suburbs.  Meanwhile, Dulles Airport is in the more rural Sterling, Virginia area away from any body of water.  The principal known as the “urban heat island effect” explains how one area in a more rural location will become much cooler than a more urban area on a night with clear skies and calm winds.  Bodies of water can also influence temperature by keeping it a little warmer at night and cooler during the day depending, on wind direction.

Relative humidity was also low yesterday and that helped temperatures to rise significantly.  It’s not uncommon for there to be a large swing in temperature between the daily high and low temperatures on a sunny day with a dry air mass.  There was a 44° temperature swing at National Airport yesterday and a 43° difference at Dulles Airport.  Humidity is “relative” to a given temperature since warmer air can hold more water vapor than cold air.  Think of the atmosphere as a sponge where the higher than temperature is, the larger the sponge.  That’s why a humid day in the summer feels much more oppressive than in the winter.

Feeling More Like March as April Gets Underway

Despite the chilly start to April, the first full month of spring is underway in North America.  That follows March 2019, which featured diverse weather in the Nation’s Capital.  DC’s coldest temperature last month was 22° on March 6, while its warmest temperature was 79° on March 30.  No record high or low temperatures were set, but March 2019 ultimately finished with an average monthly temperature (combining daily high and low temperatures) of 46.8°.  That’s a relatively rare occurrence; the last time DC had a March that finished within 0.1° of average was 1998. 

March 2019 was a wetter than average month, with 4” of rain making it DC’s wettest March since 2015.  It was also the 11^th wetter than average month in the past year dating back to April 2018.  That’s helped create a rainfall surplus of 29.71” in the Nation’s Capital since April 1, 2018.  While not a record for the date, the 1.96” of rain that occurred on March 21 made for DC’s wettest March day since March 8, 1995 (1.97”).

April is an interesting weather month in the Nation’s Capital.  Daily average high/low temperatures range from 62°/42° on April 1 to 71°/52° on April 30.  DC’s hottest April temperature on record is 95°.  That’s occurred four times, most recently on April 17, 2002.  By comparison, DC’s coldest observed April temperature was 15° on April 1, 1923.  Temperatures of at least 90° are slightly more common in the Nation’s Capital than temperatures at or below freezing.  The last time Washingtonians experienced 90° heat in April was the record-tying high temperature of 91° on April 29, 2017. 

Meanwhile, the last time temperatures fell to at or below freezing in April was on April 8, 2007 (29°).  However, this morning’s unofficial low temperature of 34° was unusually chilly.  In fact, 34° is DC’s average low temperature on March 4!  April has been a warmer than average month in Washington, D.C. in 15 of the last 20 years.

While Washingtonians can expect to see an average of 3.06” of rain during April, 12 of the past 20 April’s have been drier than average.  Accumulating snowfall is exceedingly rare in the Nation’s Capital during April and hasn’t occurred since April 7, 1972 (0.6”).  DC’s snowiest April day on record remains April 1, 1924 (5.5”).

Last April was an unusual weather month in the Nation’s Capital, being the first cooler than average April since 2007.  It was also DC’s first wetter than average April since 2014.  According to NOAA, there is a 40% chance that April 2019 will be a warmer than average month in Washington, D.C., with an equal chance for above or below average rainfall.