Just How Warm Was May 2015 in the Nation's Capital?

May is a beautiful time of year in the Nation's Capital

No sooner did we change the calendar to June than the relatively dry weather that dominated much of last month change.  And it changed dramatically with most of the DC Metro Region getting rainfall in excess of 1” to 3” yesterday and last night (Washington, D.C, and Baltimore, MD both set daily rainfall records yesterday).  

However, that doesn’t change the fact that May 2015 was not only one of the driest to occur over the last 20 years but was also the warmest on record in Washington, D.C.  To read more about how and why last month featured record warmth in the Nation’s Capital, check out this story I wrote for the WUSA9 website.

The 2015 Hurricane Season Gets Underway

Tropical Storm Ana (May 9, 2015)  Source: NASA

Today is the first day of the 2015 hurricane season in the Atlantic Ocean.  An average hurricane season features 10 named tropical storms, 6 hurricanes and 2 major hurricanes.  A system becomes a tropical storm when sustained winds around the center of the storm reach 39 mph.  A tropical storm becomes a hurricane when sustained winds reach 74 mph and a hurricane is classified as “major” once sustained winds reach at least 111 mph and it becomes a Category 3 on the “Saffir-Simpson” Hurricane Intensity Scale.  A system is given a name when it becomes a tropical storm.

NOAA recently released their forecast for the 2015 hurricane season.  They predicted 6 to 11 tropical storms, of which 3 to 6 will become hurricanes and 2 of those will become major hurricanes.  That would make 2015 an average or slightly below average season.  Tropical meteorologists at Colorado State University are also calling for a below average hurricane season in the Atlantic Ocean with only 7 tropical storms, 3 hurricanes and 1 major hurricane. 

NOAA cites the presence of El Nino as a primary factor for expecting an average or quieter than average season.  “El Nino” refers to the unusual warming of the tropical East Pacific Ocean off the west coast of Mexico and Central America.  This helps create unfavorable conditions in the tropical Atlantic including increased wind shear – or wind speeds that change direction with height. 

A quieter than average hurricane season doesn’t necessarily mean it won’t be a damaging or deadly hurricane season.  For example, although 1992 was a quieter than average hurricane season with only 7 tropical storms, it featured one of the worst hurricanes in U.S. history.  Hurricane Andrew was only the third Category 5 storm on record to make landfall in the United States when it came ashore in south Florida in August 1992.  Last season was also quiet in the Atlantic Ocean with 8 named tropical storms.  However, 2014 saw the Atlantic’s first Category 4 hurricane (Gonzalo) since 2011.

In a rare occurrence, the first tropical storm of the 2015 season already developed before this season officially began on May 8 when “Ana” was named.  Tropical Storm Ana made landfall in South Carolina.  June and July are typically quiet in the tropics as the ocean water continues to warm to the 80 degree threshold they require to develop.  Mid-August to Mid-October is typically the busiest period of the hurricane season.  Hurricane season ends on November 30.  NOAA will update their seasonal forecast in early August.

My colleagues and I on the WUSA9 Weather Team will keep viewers apprised of the latest weather conditions in the tropics both on-air and online.  The next tropical storm that develops will be named “Bill.”

Weather Quiz

Great Falls, MD  (May 2015)

What is the warmest May on record in the Nation’s Capital?

A. 1991

B. 1944

C. 2004

D. 2015

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Answer to Weather Quiz question from May 15.

True.  The hurricane season begins earlier in the Eastern Pacific than in the Atlantic Ocean because conditions typically become more favorable there a little sooner.  The first hurricane of the season, Andres, is currently spinning more than 700 miles southwest of Cabo San Lucas and should remain out to sea.

How Strong Do Tornadoes Get?

Where the Strongest Tornadoes have formed since 1950  (Source: SPC-NOAA)

The United States averages more tornadoes than any other country in the world due to its unique geography.  The strongest tornadoes form from large, long-lasting thunderstorms that rotate and are known as “supercells.”  Most supercells form during the spring when cold and dry air masses frequently collide with warm and humid air masses.  Landfalling tropical systems often spawn weaker tornadoes as well. 

Like Category 5 hurricanes, F5 and EF5 tornadoes are exceptionally rare.  Since 2000, there have been only nine F5/EF5 tornadoes in the United States.  The last EF5 tornado to occur in the U.S. was on May 20, 2013 (in Moore, Oklahoma).  It’s important to note that in 2007, the Fujita Tornado Intensity Scale was updated and is now known as the “Enhanced Fujita Scale” so tornadoes are ranked as an EF0 – EF5 instead of an F0 to an F5.  The original tornado intensity scale was designed by Dr. Ted Fujita of the University of Chicago.

On May 27, 1997, one of the most powerful tornadoes on record struck the town of Jarrell in central Texas.  Of the 20 tornadoes that formed 18 years ago today, the Jarrell, F5 was not only the strongest and most destructive, but was the deadliest with 27 fatalities.  This tornado was especially rough for farmers who lost approximately 300 cattle to the F5.  The Jarrell F5 was roughly ¾ of a mile in diameter and had a relatively long track of 7.6 miles.  It formed from an especially powerful supercell thunderstorm and was the last F5 to form in the state of Texas. 

Since 1950, there have been two days with multiple F5/EF5 tornadoes.  During what was dubbed the “Super Outbreak” on April 3, 1974, there were almost 150 tornadoes reported nationally.  Of those, seven were ranked an F5.  An even larger tornado outbreak occurred on April 27, 2011.  That’s when there were nearly 300 confirmed tornadoes with four ranked at EF5 intensity.  Although tornadoes occur more frequently in the United States than in any other country, an F5 or EF5 tornado are relative rare while days with multiple EF5’s are rarer still. 

There have been no observed F5/EF5 tornadoes in the Mid-Atlantic Region.  A relatively weak tornado (an F1) touched down in Olney, MD, on this date in 2001, but caused only minor damage.

Approaching Record Territory

Memorial Day Weekend marks the unofficial start to summer

Washington, D.C. could have its warmest May on record as the final week of the month is poised to be significantly warmer than average across the Mid-Atlantic Region.  This month has already had a streak of 10 consecutive days of temperatures 80 degrees or warmer.  There have also been three days with temperatures in the 90s since May 1.  That’s remarkable since the average high in the Nation’s Capital doesn’t even reach 80 degrees until May 31.  This month has already had warmer than average temperatures on 17 of its first 23 days.

The warmest May on record came in 1991 when the average monthly temperature (combining daily highs and lows) was 73 degrees.  According to the National Weather Service, this month’s average temperature is 71.2 degrees through May 23 in the Nation’s Capital.  That puts us in good position to reach the 1991 record with high temperatures approaching 90 degrees starting tomorrow.  It’s hard to believe that such hot weather is on tap this week since the weather has been so comfortable the last few days. 

The same area of high pressure that’s brought the Mid-Atlantic Region stellar weather with comfortable temperatures and low relative humidity will move off the East Coast becoming a Bermuda High.  They are known as “Bermuda Highs” since that’s where they are typically centered.  Since the wind flow around an area of high pressure in the Northern Hemisphere is clockwise, the Nation’s Capital experiences warmer, more humid conditions when it’s on the west side of one.  For example, when the Bermuda High is in its typical summer position off the U.S. East Coast, it pumps warm, humid air northward from the Deep South into the Mid-Atlantic Region. 

As the temperatures increase this week, the relative humidity will also increase so it will feel quite sultry and uncomfortable out during the second half of the week.  The first heat wave of 2015 is possible this week.  A “heat wave” is defined as three consecutive days with temperatures of at least 90 degrees. 

In addition to being a warm month, it had also been a relatively dry month in the Nation’s Capital with only 1.91” of rain.  May averages more than twice that amount of rainfall in Washington, D.C.  This month will finish among the warmest May’s on record, but Washingtonians will have to wait and see if it will supplant May 1991 for the record.  My colleagues and I on the WUSA 9 Weather Team will keep you apprised of the latest weather forecasts this week as the weather heats up.