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.