The First Third of Summer Comes to a Close

June 2019 has been a unique weather month in the Nation’s Capital.  It will finish as DC’s thirteenth wetter than average month out of the last 15.  It will also finish as DC’s tenth consecutive warmer than average June.  In fact, this month will end today with high temperatures in the 90s for a sixth consecutive day.

The first half of June was somewhat cool in Washington, D.C. with high temperatures in the 70s on seven days.  That’s compared to only one day that reached 90°.  The second half of June, however, had eight days in the 90s, including today.  This month’s hottest temperature of 96° occurred yesterday.  Although above the June 29 average high of 88°, it was nowhere near the record high for the date of 104°. 

June 29 was also DC’s hottest day since July 16, 2018, when it was 98°.  Today’s high temperatures in the low 90s made for DC’s longest heat wave of the year so far at six days.  Temperatures tomorrow will remain below 90° so the current heat wave will end at six days before 90° heat returns Tuesday.

However, the six-day heat wave is significant since it not only represents DC’s first June heat wave since 2017, but also DC’s longest one since another six-day event in 2015.  The last time Washingtonians experienced a longer June heat wave was an 11-day stretch from June 19 – 29, 2010.  June 2010 was DC’s hottest on record and was the first of three consecutive June’s that triple-digit heat occurred in the Nation’s Capital.  A “heat wave” is a minimum of three consecutive days with high temperatures of at least 90°.

June 2019 finishing more than a degree above average (+1.0° through June 29 with above average temperatures today) is interesting given the frequency of rainfall this month.  It rained on 20 out of 30 days in Washington, D.C., with two exceptionally wet days.  More than an inch of rain occurred on June 13 and June 18, accounting for almost half of June’s 4.27” of rain.  NOAA expects July to be warmer than average with near average rainfall in the DC Metro Area.

Weather Quiz

When was the last time 100° heat occurred in the Nation’s Capital during the month of June ?

A.  1997

B.  2002

C.  2010

D.  2012

June Coming to a Hot Conclusion

High temperatures today will soar to the low to mid 90s for a fourth consecutive day in the Nation’s Capital, making for the longest stretch of 90° June heat in more than two years.  DC’s first heat wave since late May, has come at a fitting time the week before the Independence Day holiday.

Washington, D.C. averages 36 days of 90°/+ temperatures every year.  Today will be DC’s twelfth day of 90° heat in 2019.  For a third consecutive year, the Nation’s Capital is having a heat wave that began during the last week of June and will continue into early July.  Two years ago, Washingtonians had a six-day heat wave from June 29 through July 4, 2017, with the hottest temperature being 94° on June 30.  Meanwhile, from June 29 through July 5, 2018 DC residents endured a similar heat wave with a high of 97° on July 3.

Although those heat waves began in June, they didn’t become an official heat wave until early July.  A “heat wave” is a minimum of three consecutive days with high temperatures of at least 90°.  Prior to this week, DC’s last June heat wave was June 11 – 14, 2017.  As hot as the weather was during the aforementioned periods, Washingtonians haven’t had a record high temperature in June in more than five years, since June 17, 2014 (97°).  DC residents haven’t experienced triple-digit June heat since June 29, 2012 (104°).

Average temperatures rise in Washington, D.C. throughout the month of June and into July so it’s not unusual for there to be heat waves this time of year.  The hottest time of the year in the Nation’s Capital, based on average temperatures, is July 7 – 22 when DC’s average high/low temperatures are 89°/71°.  My colleagues and I on the WUSA9 Weather Team expect high temperatures to remain in the 90s through the Fourth of July holiday.  That would make the current heat wave 10 days long and DC’s longest since the summer of 2016.

Some safety tips to remember during periods of extreme heat are to exercise and spend time outdoors early in the day when temperatures are cooler.  It’s also important to drink plenty of water and use sunscreen.  Your pets need a lot of water, too, and it’s vital to never leave your children or pets in a parked car.  Light colored, loose fitting clothing is also more comfortable on hot, sunny days.  The late afternoon and early evening hours are the hottest part of the day – so that’s a great time to spend in an air-conditioned environment.

More Summer-like Heat ?

A “heat wave” is defined as a minimum of three consecutive days with high temperatures of at least 90°.  Today is poised to be the start of the first heat wave of the month in the Nation’s Capital.  There have been a total of three 90° days in Washington, D.C. so far this month with none occurring on consecutive days.  That’s compared to an average of between seven and eight 90° June days in the Nation’s Capital since 1993.

Despite only three 90° days so far this month, June’s average temperature is slightly above average.  That’s due in large part to a series of warmer than average mornings, as opposed to very hot daytime temperatures.  There have actually been more days with highs in the 70s (seven) than days in the 90s.  That can partially be attributed to the fact that it has rained on 16 of the first 25 days of June.

June 2019 is DC’s 13^th wetter than average month since April 2018.  Another impact that having a lot of rainy days has is a higher degree of soil and surface moisture.  That, in turn, helps keep temperatures slightly cooler as the ground level moisture evaporates, as opposed to the dry conditions that ensue after a prolonged stretch of dry weather.  There has been 4.02” of rain so far this month, with more than half of this total having occurred on two days (June 13, June 18) when DC received more than an inch of rain.

The last time June had more days with highs in the 70s than the 90s was in 2009.  Not surprisingly, that was the last time Washingtonians experienced a cooler than average June.  However, June’s weather has virtually no bearing on what the rest of the summer will be like.  For example, while June 2009 was a wetter than average month, both July and August 2009 were much drier than average.  Last June was also the first time since 2009 that DC didn’t experience a single heat wave.  Although there were a total of six 90° days in June 2018, no more than two occurred on consecutive days.  DC’s last June heat wave occurred more than two years ago from June 11 – 14, 2017.

What makes this week different, however, is a shift in the weather pattern.  As is typical during the summer months in the Mid-Atlantic Region, a sprawling area of high pressure will dominate local weather headlines for much of the upcoming week.  High pressure is characterized by sinking air which not only suppresses cloud development, but also helps warm the air at ground level.  That combined with a relatively dry week will create ideal conditions for plenty of 90° heat.  Although no record heat is expected, this week’s heat could be significant for simply being DC’s first – and only – June heat wave of 2019.