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.

Where has Hurricane Season Already Begun?

Hurricane Marie, Eastern Pacific Ocean (August 2014), Source: NOAA

Although hurricane season doesn’t begin in the Atlantic Ocean until June 1, it is already underway in the Eastern Pacific Ocean.  The area of the Pacific Ocean known as the “Tropical Eastern Pacific” is located north of the Equator, west of Mexico and east of Hawaii.  Hurricane season there starts two weeks earlier on May 15 and ends on the same date as in the Atlantic on November 30.

The National Hurricane Center in Miami monitors both the Atlantic Ocean Basin and the Eastern Pacific Ocean for tropical development.  That’s because systems that develop in both areas can affect the United States.  The Gulf Stream is a warm ocean current that allows tropical systems to maintain their intensity as they trek along the East Coast.  A good example of that is Hurricane Fran in 1996 that made landfall in North Carolina as a Category 3 hurricane.

On the West Coast of the United States the California Current has the opposite track of the Gulf Stream.  It keeps cooler water from the northern Pacific running along the California coast, which often precludes any tropical systems from making it that far north.  Tropical storms and hurricanes that form off the West Coast of Mexico can affect the United States in other ways such as creating rough surf when an intense storm develops.

The Eastern Pacific Ocean averages 15 named tropical storms, 8 of which become hurricanes and 3 of those become “major” hurricanes.  That’s compared to average in the Atlantic which are 11 named storms, 6 hurricanes and 2 “major” hurricanes.  A “major” hurricane is defined as at least a Category 3 on the Saffir-Simpson Hurricane Intensity Scale with sustained winds of at least 111 mph.  Although no named storms have developed yet this year in the Eastern Pacific Ocean, the Atlantic Ocean has already seen its first named storm of 2015.  Tropical Storm Ana made landfall in South Carolina on May 10 and brought heavy rain and winds.  The first named storm in the Eastern Pacific will be Andres.

While 2014 was a quieter than average season in the Atlantic Ocean, the Eastern Pacific Ocean had a much busier than average season.  The Eastern Pacific had 22 named storms, including 16 hurricanes of which 9 became “major” hurricanes last year.  In fact, the 16 hurricanes tied the Eastern Pacific record for most to develop in a season.  The most damaging Eastern Pacific hurricane last year was Odile, which made landfall in Cabo San Lucas, Mexico.

Mexico’s national weather organization, the Servicio Meteorológico Nacional, is calling for a busier than average 2015 season with 19 named storms, 11 hurricanes and 4 “major” hurricanes.  On the other end, tropical meteorologists at Colorado State University are calling for a below average hurricane season in the Atlantic Ocean with only 7 named tropical storms, 3 hurricanes and 1 “major” hurricane.

A Preview of Summer

Beautiful Spring Weather in Germantown, MD  (May 2015)

If you think this month has been warmer than average in the Mid-Atlantic, then you would be correct.  In fact, temperatures have been warmer than average on 13 of the first 16 days of the month in the Nation’s Capital.  That includes the first 90 degree day of 2015 in Washington, D.C., that came on May 12.  Through May 12, the month was an astounding 9 degrees warmer than average.

What’s helped make this May so warm was the stretch of 10 consecutive days with highs in the 80s.  That’s very unusual in May in the Nation’s Capital because the average high temperature doesn’t reach 80 degrees in Washington, D.C. until May 31.  Although no daily record highs have been set this month, the monthly average temperature (combining the daily high and low temperatures) at National Airport through yesterday was 71.2 degrees.  That would make this May the 5^th warmest on record.  The warmest May on record in the Nation’s Capital came in 1991 (when the average May temperature was 73 degrees).

Although today’s average high temperature is 76 degrees downtown, today’s actual high was 87 degrees at National Airport.  That’s the average high on June 24 in the Nation’s Capital.  Temperatures will return to more seasonal levels on Wednesday in the mid to upper 70s.  That will help keep this month’s average temperature among the warmest May’s on record.  According to NOAA’s Climate Prediction Center, the national temperature outlook for the next one to two weeks calls for continued warmer than average temperatures in the Mid-Atlantic Region. 

Until yesterday’s thunderstorms brought appreciable rainfall to the Mid-Atlantic Region, the DC Metro Region had been in the midst of an exceptionally dry month.  In fact, the Drought Monitor Index indicates that “abnormally dry” conditions exist across much of Eastern Pennsylvania and North Central Maryland.  Although May averages 3.99” of rain, National Airport has observed only 0.82” of rain so far this month (with the bulk of that occurring yesterday). 
  

Scattered thunderstorms are possible through Tuesday when a cold front will pass through the Mid-Atlantic Region ushering in more seasonal temperatures and less humid air for the second half of the work week.  Although scattered thunderstorms can bring heavy rain where they occur, no widespread rainfall is expected. 

According to the National Weather Service, May has to finish with an average temperature of at least 70.5 degrees to be the 5^th warmest on record in the Nation’s Capital.  My colleagues and I on the WUSA9 Weather Team will keep you apprised on the latest weather forecasts.