Beware April’s Severe Weather

NOAA's Map of Severe Weather Reports (April 27, 2011)

This week is the anniversary of two significant severe weather outbreaks in the Mid-Atlantic Region.  Severe weather is defined as wind gusts of 58 mph or greater, hail 1” in diameter or greater, or a tornado.  Severe thunderstorms often have more than one of these things.

2011: There was a total of 2,708 severe weather reports including 492 tornadoes, 1322 severe wind gusts and 876 reports of large hail across the central and eastern United States from April 25 - 28.  The busiest day was April 27, when more than half (292) of the tornadoes occurred.  According to NOAA, four of these tornadoes reached EF-5 status with sustained winds of greater than 200 mph.  Sadly, a total of 231 weather related fatalities were reported on April 27.  

Also, from April 27-28, 2011, there were a total of 19 tornadoes in the DC Metro Region.  The strongest of these was an EF-2 in Rockingham and Shenandoah counties in Virginia.  Part of what made this outbreak so deadly was that a high percentage of the tornadoes occurred during the overnight hours when people weren’t awake to get the latest warnings.

2002: Longtime residents of the Mid-Atlantic Region will recall that Maryland’s strongest tornado on record occurred on April 28.  That’s when the town of La Plata in Charles County was decimated by an F4 tornado with wind speeds of 207-260 mph.  This tornado was on the ground for nearly 70 miles, including 24 miles in Charles County.  It reached its peak intensity while passing through the town of La Plata.  The town has been rebuilt in the years since then and is now vibrant and booming.   

The La Plata tornado was one of 18 tornadoes that developed on April 28 as part of a severe weather outbreak that saw a total of 389 severe weather reports, according to NOAA statistics.  Since, the original Fujita Scale was updated and became the “Enhanced Fujita” (or “EF) Scale in 2007, the La Plata F4 would now be ranked an EF-5 – as strong as tornadoes get.

A key lesson from these two severe weather outbreaks is how important preparedness is.  Knowing what weather watches and warnings mean and what to do in the event of a severe thunderstorm or tornado warning is issued for your area is vital.  The evolution of social media has improved dramatically since 2002 and so has smartphone technology.  Both social media and smartphones are critical for communicating vital weather information from meteorologists to the public.

Weather Quiz

Maryland's State Flag

True or False.

The F4 tornado that occurred in La Plata, Maryland on April 28, 2002, was the strongest tornado on record in the state of Maryland.

Nearing Record Territory For April

Today will mark the first of a likely five-day stretch with temperatures in the 80s in the Nation’s Capital.  In fact, the Nation’s Capital hasn’t had five or more consecutive days with April temperatures in the 80s since 2002.  Temperatures this weekend will approach 90 degrees in parts of the DC Metro Area.  The last time it was 90 degrees in Washington, D.C. in April was in 2013.  The last consecutive 90-degree days in DC during the month of April occurred in 2010.

Despite four significantly cooler than average days (April 22 – 25), this month is still poised to set a new record for warmest April in Washington, D.C., breaking the existing record from 1981.  If this month finishes as the warmest April on record, then it would join February was Washington, D.C.’s second record warmest month of 2017.  This month could also finish drier than average as well, since the Nation’s Capital averages 3.06” of April rainfall and only 2.62” has occurred through April 26.

This month’s warmer and drier than average weather continues a common trend over the last two years.  Since May 2015, Washingtonians have had three of their warmest months on record (May 2015, December 2015 and February 2017) with several months in between ranking in the Top Ten for warmest on record.  Last summer Washingtonians experienced the sixth warmest July and second warmest August on record.  In a rare occurrence, both July and August 2016 finished with the same monthly average temperature (82.7 degrees) at National Airport. 

The sustained warmer than average weather has acerbated the dry conditions in the DC Metro Area.  Since August 1, 2015, the rainfall deficit has grown to more than 13” in Washington, D.C. and more than 11” at Dulles Airport.  Just since October 1 when the “water year” as defined by the U.S. Geological Survey began, the rainfall deficit in the Nation’s Capital is more than 7.5”.  It’s been nearly a year since the last wetter than average month in Washington, D.C. (May 2016). 

The drier than average conditions have lasted so long, that moderate drought conditions have enveloped much of the Mid-Atlantic Region.  The fact this spring hasn’t provided any significant drought relief isn’t good news for gardeners and farmers.  NOAA’s Climate Prediction Center is expecting May to be a warmer than average month in the Mid-Atlantic Region, despite potentially cooler than average temperatures close to Mother’s Day.  Also, there is an equal chance for above or below average precipitation next month.  

Warmest Aprils on Record in Washington, D.C. in degrees Fahrenheit (Source: NOAA)

1.  62.1 (1981)
2.  62.0 (1994), (2017 through April 26)
3.  61.6 (1985)
4.  61.2 (1960)
5.  60.9 (2010)

Off-Season Tropical Storms and Hurricanes

Tropical Storm Arlene, April 2017  (Source: NOAA)

Something very rare occurred in the Atlantic Ocean last week.  Tropical Storm Arlene became only the third tropical or subtropical storm to occur during the month of April.  Arlene was the first April storm since Ana in 2003 and an unnamed subtropical storm in April 1992.  No hurricanes have been observed in the Atlantic Ocean in April.

Arlene was also the strongest April storm on record, with peak sustained winds of 50 mph and a minimum air pressure of 993 millibars.  Air pressure is considered a more accurate measure of tropical storm intensity than wind speed.  There is an inverse relationship between lower air pressure and higher wind speeds in tropical storms and hurricanes.

Hurricane season in the Atlantic Ocean runs from June 1 through November 30.  The period from August 15 to October 15 is typically the most active portion of the hurricane season.  That’s when conditions tend to be most favorable with tropical ocean water at its warmest levels of the year and wind shear at its lowest level.  However, there have been 15 tropical storms in the Atlantic Ocean Basin (including the Gulf of Mexico and Caribbean Sea) outside of hurricane season.  Just last year, Hurricane Alex became the first January hurricane to develop in the Atlantic Ocean since Alice in 1955.

A lot of off-season tropical activity doesn’t necessarily correlate to a busier than average hurricane season overall.  For example, the 2012 Atlantic hurricane season saw two tropical storms develop in May as part of a busier than average season with 19 tropical storms, 10 hurricanes and 2 major hurricanes.  But, Tropical Storm Ana developed in May 2015 and the entire year had only 11 tropical storms, 4 hurricanes and 2 major hurricanes.  That’s slightly below the Atlantic’s 30-year average of 12 tropical storms, 6 hurricanes and 2 major hurricanes.  A “major” hurricane is defined as a Category 3 or higher on the Saffir-Simpson Scale, with sustained winds of 111 mph or greater. 

It’s important to remember that the overall number of storms is less important than whether or not any storms make landfall.  Scientists at Colorado State University are forecasting a 2017 season very similar to 2015, with 11 tropical storms, 4 hurricanes and 2 major hurricanes.  NOAA will issue its seasonal hurricane forecast for 2017 closer to June 1.  The next tropical storm that develops in the Atlantic Ocean will be named “Bret.”