June Delivers a Severe Weather Punch

NOAA's Severe Weather Outlook for June 22

June is one of the most active severe weather months in the United States.  It averaged 243 tornadoes over the 20 year period ending in 2010 according to NOAA.  Although Texas, Kansas and Nebraska are the three states with the highest average number of tornadoes in June, Maryland and Virginia both average 1 to 2 tornadoes.  In fact, a tornado was confirmed in Howard County, Maryland, just yesterday.  “Severe weather” is defined as a thunderstorm with any of the following hail 1” in diameter or greater, wind gusts of at least 58 mph or greater, or a tornado. 

Some other terminology to be familiar with are “weather watches” and “weather warnings.”  A severe thunderstorm “watch” is issued if conditions are expected to be favorable for severe thunderstorms in a given area during a given time.  A watch usually lasts for several hours.  But, if the situation is more urgent and a severe thunderstorm is imminent or is occurring, than a severe thunderstorm “warning” is issued.  A “warning” typically lasts for only a few minutes to an hour.  If a severe thunderstorm is producing a tornado, than a tornado “warning” is issued.  In this case, only a tornado warning is issued in order to minimize confusion.

Large severe weather outbreaks during the spring are commonly related to the position of the jet stream.  Spring storms typically develop in conjunction with a trough – or dip – in the jet stream.  This helps create favorable conditions for supercell thunderstorms (i.e. long lasting thunderstorms that rotate) from which the strongest tornadoes tend to develop.  However, during the summer when the jet stream retreats northward into Canada, severe weather in the United States tends to be more dependent on smaller scale weather systems.  This includes clusters of thunderstorms known as mesoscale convective complexes.  “MCCs” are long-lasting clusters of showers and thunderstorms and can produce severe wind gusts and hail.  

There were over 400 reported cases of severe weather nationally, on June 4, 2008, with dozens of confirmed cases of severe wind gusts, hail and a few tornadoes in the Mid-Atlantic Region.  Fortunately, there were no fatalities but there was extensive damage and record rainfall at both National and Dulles Airports.  More recently, on June 1, 2012, another severe weather outbreak occurred with nearly three dozen confirmed tornadoes from Pennsylvania to North Carolina.  

Severe weather can be tough to predict since each outbreak is unique.  That underscores the importance of keeping up to date on the latest weather forecasts and taking shelter in an interior room when a severe thunderstorm or tornado warning is issued for your location.  You can rely on my colleagues and I on the WUSA9 weather team for the latest weather forecasts both on-air and on the free WUSA9 app for your mobile devices.

Forecasting Severe Weather

Although not as common as in other parts of the country, the Mid-Atlantic Region does have severe thunderstorms on occasion.  Severe thunderstorms require a unique set of elements to develop such as a warm and humid air mass combined with a lifting mechanism such as a cold front.  When cooler and drier air displaces warm and humid air, strong to severe thunderstorms often ensue.  That’s different than the typically short-lived showers and thunderstorms that develop on a hot and humid afternoon or near the beach during the summer.  Specific criteria that makes a thunderstorm severe include wind gusts of at least 58 mph, hail at least 1” in diameter or a tornado. 

Since so many elements go into forecasting thunderstorms and severe weather – such as atmospheric stability, the presence of a lifting mechanism (like a front), as well as local geography – the forecast needs to be refined multiple times to ensure accuracy.  To give members of the public an idea of when and where thunderstorms may occur, NOAA’s Storm Prediction Center issues “Convective Outlooks” for the United States.  Their outlooks are great at providing a benchmark a few days in advance of any potential severe weather.  However, it’s critical that people pay attention for updates to their local weather forecast since so many, ever-changing variables are involved in forecasting showers and thunderstorms.

This past week, the Mid-Atlantic Region had a significant severe weather outbreak with multiple reports of severe wind gusts and hail.  In fact, unusually large baseball-sized hail occurred in Loudoun County, Virginia.  NOAA’s Convective Outlook for June 16 showed an “enhanced risk” for severe weather from southwest Pennsylvania to west-central Virginia.  The map of where severe weather occurred corresponded remarkably well to the outlook meaning this severe weather event was well-forecast.

Earlier this month, there was another instance where a large portion of the Mid-Atlantic Region was under an “enhanced risk” for severe weather.  The area at “enhanced risk” was larger on June 5 than it was on June 16.  It included the major metropolitan areas of Washington, D.C., Baltimore, MD and Philadelphia, PA.  Unlike June 16, different weather conditions on June 5 caused most of the severe weather to occur outside the DC Metro Region.  While it’s important to notify the public to the potential for thunderstorms on a given day, it’s impossible to say a thunderstorm will develop or track over a particular town more than a few hours in advance.

A common misconception is that the entire area under an “enhanced risk” for severe thunderstorms will see it.  What it actually illustrates is that a particular part of the country has the right combination of elements for a greater than average or an “enhanced” chance for severe thunderstorms.  People can understandably get upset if they change their plans when there is an elevated potential for a given type of weather that doesn’t ultimately occur in their town.  However, people also need to realize that there is an inherent amount of uncertainty in weather forecasting.  That said, the accuracy of weather forecasts have improved greatly – even in the last ten to twenty years – as the forecasting tools such as the computer forecasting models and weather radar have improved.

Summer Heat in Washington, D.C.

Although Washington, D.C. can experience torrid heat in June, it isn’t as common as in other parts of the country.  There have been good examples of both hot and dry weather as well as just delightful weather during the month of June in the Nation’s Capital – including some of the coolest June weather in years earlier this month.

Triple digit heat is rarity in the Nation’s Capital only occurring every few years.  It occurred most recently during the very hot summer of 2012.  In fact, Washington, D.C.’s hottest June temperature on record of 104 degrees occurred on June 29, 2012.  That was the last time it was 100 degrees in June in the Nation’s Capital.  Prior to 2010, there was no triple digit heat during the month of June in Washington, D.C. since 1997.

It’s also interesting to note that seven of the ten June’s from 1997 through 2006 were cooler than average in the Nation’s Capital.  Meanwhile, eight of the following nine June’s through 2015 (with the exception of 2009) were warmer than average according to National Weather Service data.  That shows how streaky the weather can be.  On June 9, 2016, Washington, D.C. had their coolest June temperatures since 2006, when it was 53 degrees at National Airport and 47 degrees at Dulles Airport.

Washington, D.C. averages 36 days per year with temperatures at or above 90 degrees.  The record for most 90-degree days is 67 that occurred twice in 1980 and in 2010.  The fewest number of 90-degree days in a given year is 7 – that last occurred in 1905.  Washingtonians had their first 90-degree heat of 2016 on June 11, when it was 96 degrees at National Airport.  The last time the first day of 90-degree heat in the Nation’s Capital didn’t come until June was in 2003.  Prior to June 11, the last time it was at least 90 degrees in Washington, D.C. was on September 9, 2015. 

Longtime Washingtonians won’t mind a late start to summer since they’ll remember June 1994 for its stretch of 14-consecutive days with temperatures of 90 degrees or hotter.  More recently, June 2010 was the warmest June on record in the Nation’s Capital.  For those of you who thought 96 degrees was too hot this past Saturday, than I should point out residents of Phoenix, Arizona, have excessive heat warnings in effect starting on Sunday and continuing into next week.  That’s because Phoenix temperatures could approach record levels near 120 degrees.  Meteorological summer began June 1 and runs through August 31.