Hot/Humid Weather + Cold Front = Trouble

NOAA's Severe Weather Outlook for June 19

A large portion of the DC Metro Area is under an “enhanced risk” for severe weather today as a cold front heads eastward, says NOAA’s Storm Prediction Center.  Given how dry June has been, any rain would be welcome, but not dangerous or destructive thunderstorms.  A thunderstorm becomes severe when it contains at least one of the following: wind gusts in excess of 58 mph, hail 1” in diameter or greater, or a tornado.  Heavy rain and lightning make a thunderstorm dangerous, but not “severe” per se.  

A severe thunderstorm “watch” is issued when severe weather is expected in a particular region during a particular time.  A “warning” is issued for a smaller region (such as a county) when said weather condition is either occurring or is imminent.  Also, a tornado warning issued instead of a severe thunderstorm warning when a tornado is occurring.  

June is one of the more active severe weather months in the Mid-Atlantic Region.  One of Maryland’s strongest tornado on record was the Frostburg F4 that occurred on June 2, 1998.  More recently, several tornadoes developed in the DC Metro Area as part of a massive severe weather outbreak on June 4, 2008.

There hasn’t been much active weather, locally, so far this month.  This month has also been warmer than average. While Washingtonians have had five days of 90° heat so far this June, that isn’t a high number for the Nation’s Capital.  Washingtonians have experienced some sweltering Junes in recent years, with four Junes since 2010 having at least 10 days at or above 90°.  The record for consecutive June days at/above 90° in Washington, D.C. is 14, which occurred in 1994.  The last cooler than average June in Washington, D.C. was in 2009.

This afternoon’s severe weather potential is significant.  It’s important to quickly seek shelter in an interior room away from windows should threatening weather approach.  My colleagues and I on the WUSA9 Weather Team will keep you posted on the latest weather watches and warnings this afternoon and evening. 

Historic Territory

Great Falls, Maryland

No measureable rainfall has occurred in the Nation’s Capital through June 15.  That ties this month with 1967 and 1976 for driest first half of June on record.  Weather records in Washington, D.C. date back to 1871, so that’s an especially impressive feat.  

There have been several days this month with scattered showers and thunderstorms in the Mid-Atlantic Region, but no widespread precipitation.  The combination of several hotter than average days and less rainfall makes the wetter than average May seem distant.  This month’s rainfall deficit is nearing 2”.  

Longtime residents know June can be a very wet month in the Nation’s Capital.  Three of the five wettest Junes on record in Washington, D.C. occurred since 2003.  Just two years ago, Washingtonians saw 11.94” of rain officially measured at National Airport.  There were 18 days with measureable rainfall in Washington, D.C. during June 2015.

June can be both a hot month and a wet month as was the case in 2015.  That’s when a total of 12 days reached or exceeded 90° and contributed to making the month nearly 3° warmer than average.  The Nation’s Capital has averaged between 7 and 8 days in the 90s during the month of June since 1993.  This month has already had 6 days in the 90s, with the likelihood of adding to that total during the Father’s Day weekend.  Meanwhile, June’s weather doesn’t necessarily correlate to what the rest of the summer will be like.  

The first “heat wave” of 2017 occurred earlier this month when temperatures climbed into the 90s for four consecutive days from June 11 through June 14.  While some record highs were tied or broken, this week’s heat wave doesn’t rank among the historic June heat waves in this part of the country.  For example, a 14 day streak of 90° occurred in June 1994, and in June 2010, there was an 11 day heat wave.  Moreover, that 11 day heat wave largely contributed to making June 2010 the hottest on record in the Nation’s Capital.

The 14-day heat wave in June 1994 included a record high of 101° on June 15 in Washington, D.C.  By comparison, there were only 5 days in the 90s last June.  NOAA’s outlook for the rest of June is for an above average potential for warmer and wetter than average conditions in the Mid-Atlantic Region.

Weather Quiz

True or False.  On this date in 1994, Washington, D.C. experienced its third highest June temperature on record of 101°F.

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Answer to May 31 Weather Quiz question.

True.  Several recent hurricane seasons have gotten off to an earlier than average start in the Atlantic Ocean.

What is a “Heat Wave?”

Keeping Cool on the Water

A “heat wave” is loosely defined by most people as a few hot days.  That’s a rather subjective definition as no two people are likely to have the same definition of what constitutes “hot” weather.  So it’s important to use the official definition of a heat wave from the National Weather Service. They define a “heat wave” in the Nation’s Capital as any stretch of 3 or more consecutive days at or above 90°. 

Today is the second day out of what will become the first heat wave of 2017 in the DC Metro Area.  In fact, record heat is possible both today and tomorrow as high temperature approach the mid to upper 90s.  Official weather measurements for Washington, D.C. are made at National Airport (DCA).  Located on the Potomac River, temperatures there can be influenced by the water temperature. 

That’s why wind direction is an important factor to consider regarding National Airport temperatures.  For example, a southerly wind tends to keep temperatures in the spring and summer a few degrees cooler as the wind blows off the relatively cool waters of the Potomac River.  However, a westerly wind largely negates any influence of the Potomac River on DCA temperatures.  While the Potomac has a cooling influence on temperatures during the summer months, it has the opposite effect in the winter.  Consequently, there is often a difference of several degrees between temperatures at National Airport and areas downtown when the wind is blowing off the Potomac River.

Area residents will be lucky to have the cooling influence of being on or near the water today as temperatures approach the record high of 95° this afternoon.  It’s true that today’s record is the lowest record high temperature of the entire summer (June 1 – August 31) in the Nation’s Capital so that makes it an easier record to eclipse.  The Nation’s Capital has twelve June record highs of 100° or hotter.  Temperatures aren’t forecast to reach the century mark this week, but the combination of temperatures in the 90s with higher dewpoints and relative humidity will make the heat index feel at or above the century mark on Tuesday.

That means drinking extra fluids and staying indoors in the air conditioning as much as possible is extra important for people and their pets.  This week’s heat wave is only expected to last a few days which while significant for being 2017’s first such stretch of hot weather in the Nation’s Capital, it isn’t an historic heat wave.  June’s longest heat wave remains a 14-day stretch of 90°/+ heat in 1994.  Overall, the last heat wave to occur in Washington, D.C. was on September 6 – 10, 2016.

                                                  Record High Temperatures

June 12

DCA: 95° (2002)
IAD: 96° (1986)
BWI: 96° (1986)

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June 13

DCA: 96° (1954)
IAD: 94° (1994)
BWI: 97° (1956)