Context on D.C. Area Severe Weather

NOAA's Severe Weather Outlook for May 14, 2018

May is one of the busiest severe weather months of the year in the United States.  NOAA considers a thunderstorm “severe” when it contains any of the following: hail 1” in diameter or greater; wind gusts of at least 58 mph; or a tornado.  There are several other thunderstorm related terms that are important to be familiar with such as a “mesoscale convective system” or “MCS”, a derecho and a squall line.

NOAA defines an MCS as a group of thunderstorms that form a large, long-lasting cluster which can travel over several states.  A “mesoscale convective complex,” or “MCC,” is a type of MCS and can be as large as an entire state.  Unlike a severe thunderstorm, NOAA does not have specific criteria that require an MCS or MCC to be a particular size or last a certain amount of time.  However, there are often severe thunderstorms embedded within an MCC.  Although MCS’s and MCC’s are more common in the Midwest, they can also occur along the U.S. East Coast.

A “derecho” is another type of “MCS” and they cause widespread wind damage over a swath of greater than 240 miles.  A derecho includes a high number of severe wind gusts (greater than 58 mph) over its path and with several wind gusts of greater than 70 mph.  Derecho’s sometimes have wind gusts as high as 80 to 90 mph (equivalent to the sustained winds in a Category 1 hurricane), with occasional higher gusts. 

Longtime D.C. residents may remember the powerful and destructive derecho that occurred on June 29, 2012.  This derecho brought National Airport a peak wind gust of 70 mph, Dulles Airport had a 71 mph wind gust and BWI Airport had a 66 mph wind gust according to the National Weather Service.  Widespread damage and power outages ensued in the D.C. Metro Area during a record-setting heat wave.

Another MCS brought multiple severe wind and hail reports to the Washington-Baltimore corridor on May 14, 2018.  However, unlike the June 2012 derecho, this week’s MCS fell short of meeting the official criteria of a “derecho.”  It was a better example of another type of MCS known as a “squall line.”  A squall line is a line of thunderstorms that produces squally weather with often severe wind gusts.  Monday’s severe weather reports were far less numerous than during the June 29, 2012 event.

Squall lines can last hundreds of miles and sometimes develop a “bow” shape on weather radar.  When a squall line has a “bow echo,” that often means higher wind gusts will occur that are more likely to cause damage.  When meteorologists investigate thunderstorm damage, they look for clues in the storm debris that allows them to determine what kind of severe weather occurred.  Many thunderstorms cause something called “straight-line” wind damage.  Meanwhile, tornadoes tend to cause damage in a more circular pattern.

Longtime D.C. residents may remember some other significant severe weather outbreaks that have occurred in the last ten years.  June 4, 2008 was one of the D.C. Area’s most active severe weather days.  A powerful squall line also caused widespread damage in the Mid-Atlantic Region on July 25, 2010.  A relatively weak EF-0 tornado developed on June 13, 2013 that had an unusually long 17.3 mile track through Montgomery County, Maryland.  Future columns will look at other important severe weather events and terminology.