The Washington PostDemocracy Dies in Darkness

The Washington area’s first significant heat wave of 2019 peaks heading into the weekend

But the heat has been worse in the past and could get worse in July and August

The GFS weather model forecast for highs on Saturday. Temperatures should be similar Friday.

Washington’s second heat wave of the season is cranking it to maximum power in the next few days. It’s destined to become the most significant of the summer so far.

The heat this summer has been ordinary, but history shows it is likely to ramp up in July and August — and this late June spell could just be an appetizer.

Tuesday and Wednesday marked the start of the heat wave, when temperatures hit 90 and 91 degrees. They’ll tick up a bit more today before peaking Friday and Saturday, when mid-90s or even some upper 90s are possible. Sunday should not be quite as hot but has a strong chance to hit at least 90.

This heat wave is set to become the second of the year, based on an unofficial but widely accepted criterion of three straight days of 90-degree weather. Using this criterion, we also notched a heat wave May 28-30, lasting the minimum three days.

We expect the heat wave to extend to five (if it ends Saturday) or six days (if it ends Sunday). There’s an outside chance that it extends into next week if Monday manages to touch 90, but most forecasts are for highs in the upper 80s.

This heat wave in historical context

Assuming the heat wave ends Sunday, historical data show it is unlikely to be particularly noteworthy. If it’s capped at six days, it won’t even come close to some of our lengthier heat waves. The longest D.C. heat waves on record endured for an incredible 21 days in both 1980 and 1988.

But we’ll have plenty of time to put longer heat waves together in July and August, when they’re typically most frequent, long-lasting and intense. In recent decades, our longest heat waves have all come in July or August (with one beginning in June) and averaged around nine days.

Considering we’ll have notched two heat waves in the books to close June, it seems we have a reasonably strong chance to get as many as five heat waves for the summer, which is the annual average.

We’ve already seen heat surpass the levels in our coolest summers. 1887, 1907 and 2004 did not see a single heat wave, and the longest streaks in the 90s were only two days long.

Long-term trends reveal an increase in the annual number of heat waves, from around three to five.

2019′s heat year-to-date

The heat we’ve observed so far this year has been somewhat less intense than we’re accustomed to in the past decade.

Through Wednesday, the District’s highest temperature so far — 93 degrees on May 29 — is the coolest maximum temperature this late in the year since 2009, when it had hit 92 degrees to this point.

While the heat hasn’t been particularly intense, we’ve had enough steamy days that June temperatures are running a bit above normal.

This blistering conclusion to the month should help the city post a warmer-than-normal June, by perhaps a bit past 1 degree, as calculated by plugging in National Weather Service forecasts.

So far, Washington has hit 90 degrees at least 10 times this year, which is about two days above average to date. The number is the most to date since 2015, when there were 19 by now. The five so far in June is fairly close to average, but with several straight 90-degree days coming, we should end the month ahead of average.

As we sweat in the days ahead, the historical odds seem good that this current heat wave is a warm-up for bigger ones ahead. A not-so-gentle reminder that there’s a whole lot of summer left.