A Warm and Wet May

May will finish as a significantly warmer and wetter than average month in the Nation’s Capital for a second consecutive year.  With the exception of a few days in the middle of the month, May 2019 has been warmer than average in its entirety.  After a hot stretch of 90s on four out of five days through May 30, today will be a relatively benign weather day with more comfortable temperatures and no severe weather in the DC Metro Area. 

This May’s average temperature (combining daily high and low temperatures) in the Nation’s Capital is 70.8° through May 30.  That’s significantly warmer than NOAA’s 30-year average of 66°, but doesn’t rank among DC’s warmest May’s.  It’s interesting to note, however, that if you include this May, five of DC’s 10 warmest May’s have occurred since 2004, including in 2015 and 2018.

This month has actually been more like May 2018 than May 2015.  That’s because May 2015 was a drier than average month with DC getting less than half its May average.  On the other hand, last May was DC’s sixth wettest on record.  That can largely be attributed to a very warm start to last May, while the second half of May 2018 was very wet with four days that had an inch or more of rainfall.  By comparison, DC’s wettest day this month was May 5 (0.65”).

The warmest temperature this month was 93° on May 29, while DC’s coolest temperature was 50° on May 13 and May 15.  Aside from the three-day period from May 12 – May 14 when high temperatures were in the low 60s (typical of late-March or early-April), May 2019 has been warmer than average.  Although temperatures were in the 90s on four out of the last five days, no record highs were set in the Nation’s Capital.  However, Dulles Airport tied its record high of 90° on May 28.

With yesterday’s rainfall, DC has had 4.97” of rain since May 1.  That’s almost an inch above the monthly average of 3.99”.  Measurable rainfall has occurred on 13 days this month, with the wettest being May 5 (0.65”).  Aside from the warmer and wetter than average conditions this month, the major weather headline in the DC Metro Area has been the frequency of severe weather.  On May 23 and May 30, confirmed tornadoes occurred in Howard County, Maryland.

D.C.’s Warmest May’s (National Airport “DCA” - in Degrees Fahrenheit)

1.  73.2° (2015)
2.  73.0° (1991)
3.  72.4° (2018)
4.  71.8° (2004)
5.  71.6° (1944)
6.  71.4° (2012)
7.  70.5° (1880)
8.  70.1° (1953)
9.  70.0° (1911)
10.  69.8° (1959)

Average: 66.0°

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D.C.’s Wettest May’s on Record (DCA) - Source: NOAA

1.  10.69" (1953, 1889)
3.  10.66" (2008)
4.  10.60" (1886)
5.  8.87" (1948)
6.  8.73" (2018)
7.  8.05" (2009)
8.  7.77" (1989)
9.  7.06" (2003)
10.  6.99" (1946)

Average: 3.99”

Weather Quiz

Tropical Storm Arlene, April 2017  (Source: NOAA)

True or False.

With “Andrea” named last week, the Atlantic Ocean has had at least one named storm prior to the official June 1 start to the hurricane season the last five years.

How Strong Can Tornadoes Get ?

United States Annual Tornado Averages  (Source: NOAA)

Tornadoes are ranked on the Enhanced Fujita Scale that starts with a minimal EF-0 that has winds of 65 – 85 mph.  The strongest tornadoes are ranked an EF-5 and have winds of greater than 200 mph.  Even though wind speeds in an EF-5 tornado are lower than what had been required in an F5 tornado (261+ mph) on the original Fujita Scale, EF-5 tornadoes remain rare. 

All tornadoes, however, are potentially dangerous regardless of intensity.  At least two EF-3 tornadoes occurred late last night in Ohio where extensive damage and a fatality occurred.  Meteorologists are still investigating the damage so the EF-3 ratings are preliminary.  Some of the deadliest and most destructive tornadoes on record have occurred during the month of May. 

2013:  The last EF-5 tornado in the United States occurred on May 20 in Moore, Oklahoma.  It had a long and destructive track as part of a larger, three-day severe weather outbreak.  The strongest tornadoes often develop from something called a “supercell thunderstorm,” which is a long lasting thunderstorm that has a rotating updraft.  

One of the supercell thunderstorms that developed during this outbreak spawned the Moore, OK tornado.  Although not at EF-5 intensity during its entire track, it reached its peak intensity near the town of Moore.  According to NOAA, it caused two dozen fatalities and remains the third costliest tornado on record having caused $2 billion in damage (unadjusted for inflation). 

2011:  The town of Joplin, Missouri experienced a devastating EF-5 tornado on May 22.  According to NOAA, the Joplin EF-5 remains the costliest tornado on record having caused $2.8 billion in damages (unadjusted for inflation).  With a total of 181 fatalities, it was also the single deadliest U.S. tornado since 1947.  The spring of 2011 was an unusually active severe weather season for the United States with a total of six EF-5 tornadoes.  That was the highest annual total since 1974.

1999:  The last observed F5 tornado in the United States before the Fujita Scale was updated or “enhanced” in 2007 occurred on May 3.  The town of Moore, Oklahoma also saw impacts from this devastating tornado that at the time was the third costliest tornado in the United States.  However, the $1 billion in damages it caused (roughly $1.4 billion in 2015 dollars) makes it “only” the sixth costliest tornado in 2019.

1997:  Some might remember the Jarrell, Texas F5 tornado that occurred on May 27.  This was the last F5/EF-5 tornado that developed in the state of Texas and was both destructive and deadly with 27 fatalities.  The Jarrell tornado was unusual since it tracked in a South by Southwest direction, instead of the more common Easter by Northeast direction in North America.

How are El Nino and the Atlantic Hurricane Season Related ?

Hurricane Florence, September 2018  (Source: NOAA)

NOAA issues its 2019 seasonal hurricane forecast on May 23.  There are a variety of factors involved in such a forecast, including whether or not El Nino is present and prevailing sea surface temperatures.  Check out this story I wrote for the WUSA9 website that discusses the upcoming Atlantic hurricane season that gets underway on June 1.

2019 Hurricane Outlook for the Atlantic Ocean

NOAA released its 2019 forecast for the hurricane season that begins June 1 in the Atlantic Ocean.  It calls for a near average hurricane season with 9 – 15 tropical storms, 4 – 8 hurricanes and 2 – 4 major hurricanes.  An “average” Atlantic hurricane season features 12 tropical storms, 6 hurricanes and 3 major hurricanes.  A tropical storm has sustained winds around its center of 39 – 73 mph while a hurricane has sustained winds of 74/+ mph.  A hurricane becomes “major” when it reaches Category 3 intensity on the Saffir-Simpson Hurricane Wind Scale with sustained winds of at least 111 mph.

On May 9, NOAA’s Climate Prediction Center issued an “El Nino Advisory” and expects a high chance that El Nino will continue through the summer and into the fall.  That’s a large factor NOAA considered in its seasonal hurricane forecast.  That’s because El Nino conditions, characterized by a warming of the equatorial Pacific Ocean, have worldwide repercussions.  Most notably, during moderate to strong episodes of El Nino, conditions in the tropical East Pacific (off the west coast of Mexico) are more favorable for tropical storm development.  

Conversely, during El Nino years conditions in the Atlantic Ocean tend to be less favorable for tropical storm development with cooler than average sea surface temperatures and an increased amount of wind shear.  Hurricanes typically require warm ocean water of sufficient depth, commonly 80° Fahrenheit to a depth of 200’, to develop.  They also require little or no wind shear (differing wind speed and direction with altitude) for the hurricane circulation to intensify.  In issuing its forecast, NOAA cited above average ocean temperatures in the tropical Atlantic Ocean and Caribbean Sea as well as a more active West African monsoon.  Both of these factors are known to favor a more active Atlantic hurricane season.

The last moderate or strong El Nino occurred in 2015-2016.  The 2015-2016 El Nino was one of the strongest on record, comparable to the 1982-1983 and 1997-1998 episodes.  These three major El Nino events all had discernible impacts on the Atlantic hurricane season.  While the 1982 (6, 2, 1) and 1997 (8, 3, 1) Atlantic hurricane seasons were much quieter than average, the 2015 season was closer to, but still below average with 11 tropical storms, 4 hurricanes and 2 major hurricanes.

By comparison, weak episodes of La Nina made weather headlines in 2016-2017 and 2017-2018.  Consequently, the 2016 and 2017 Atlantic hurricane seasons were busier than average.  A total of 17 hurricanes and 10 “major” hurricanes developed during these two seasons, above the two year average of 12 and 6.  Three Category 5 hurricanes also developed, including Matthew (2016), Irma (2017) and Maria (2017).  Fortunately, none of them made landfall at that intensity but were each deadly and destructive.  Despite the weak El Nino that developed last year, the 2018 Atlantic hurricane season was still an active one with 15 tropical storms, 8 hurricanes and 2 major hurricanes.  Hurricane Michael developed last season and became only the fourth Category 5 hurricane on record to make landfall in the United States. 

It’s important to remember that a hurricane season does not have to be an active one for an intense or destructive hurricane to develop.  The 1992 Atlantic hurricane season comes to mind that occurred during a strong episode of El Nino.  Consequently, it was a below average hurricane season with only 7 tropical storms, 4 hurricanes and 1 “major” hurricane.  However, that one hurricane was Andrew which made landfall in Florida as a Category 5.  It was the first Category 5 to make landfall in the United States since Camille in 1969 and would remain the costliest natural disaster in US history until 2005.  The next tropical storm that develops in the Atlantic Ocean will be named “Barry.”

An Early Start to the Hurricane Season ?

Hurricane Alex, January 2016  (Source: NOAA)

The National Hurricane Center is watching an area of disturbed weather east of the Bahamas.  That’s where a tropical disturbance as a 70% chance of becoming the Atlantic’s first tropical storm of 2019 according to NOAA.  An Air Force Reserve reconnaissance aircraft is scheduled to investigate the disturbance later today.

Although hurricane season doesn’t official begin in the Atlantic Ocean until June 1, having a named storm develop early has become a regular occurrence in recent years.  At least one named storm has developed in the Atlantic in each of the last four years and in five of the last seven since 2012.  A system becomes a “tropical storm” and is given a name when the sustained winds around its center reach 39 mph.  If sustained winds reach 74 mph, then the tropical storm becomes a hurricane.

On occasion, a named storm has characteristics of both a tropical system and a non-tropical area of low pressure.  Under such circumstances, the system is classified as a “subtropical storm.”  That’s what happened with Tropical Storm Alberto last May, which began as a subtropical storm in the Gulf of Mexico.  However, it eventually became a fully tropical storm before it made landfall. 

When “Alberto” developed last May, it was the fourth consecutive year that a named storm formed prior to the official start of the hurricane season.  That tied the previous record for most consecutive years with a tropical storm prior to June 1 was three (1951 – 1954).  There have been four other instances where a tropical storm has developed prior to June 1 in consecutive years.  

In January 2016, Hurricane Alex developed south of the Azores and was the first Atlantic hurricane during the month of January since Alice in 1955.  Some may wonder if having a named storm prior to the official start of hurricane season is a reliable indicator of whether or not a given season will be an active one.  Quite simply, it isn’t.  That’s because tropical storms and hurricanes are complex systems that require a series of conditions to develop.  And if conditions are favorable for a tropical storm to develop prior to June 1, then that isn’t a guarantee that conditions will remain favorable for an active hurricane season as a whole.

The 1992 Atlantic hurricane season was a quieter than average season with only seven named storms.  However, a subtropical storm developed in April 1992.  According to the National Hurricane Center, 1992 was the first time since 1981 that a tropical or subtropical storm developed in the Atlantic Ocean Basin (that also includes the Gulf of Mexico and Caribbean Sea).  Eleven years passed before another storm developed in the Atlantic prior to June 1 when Tropical Storm Ana was named in April 2003.  Unlike the 1992 season, Ana was the first storm of what became an active 2003 Atlantic hurricane season.  The first named storm of 2019 will be given the name “Andrea.”