Last summer was two degrees hotter than normal, and the mercury touched 90 degrees or higher on 45 days (35 days between June and August). It continued a nearly decade-long stretch of very hot and humid summers, including the three hottest on record from 2010 to 2012.
We think we may be able to break the streak this year, as we have a real shot at our first cooler-than-average summer since 2009. Of course, in the Washington region, even a “cool” summer is very warm and muggy. At least one stretch of punishing heat will be hard to avoid.
Even so, we favor the chances of less heat less often than in many recent years. The chance of 100-degree days is below normal (the average is one such day each summer), and our count of 90-degree days (the average is 36 days in a calendar year, and 31 days between June and August) should be somewhat suppressed.
While temperatures may end up near or below average, we lean toward somewhat above-normal rainfall, which should mean plenty of humidity.
When putting together a summer outlook, we’re less likely to see the signals for extreme warmth or cold (relative to average) that we sometimes see in advance of winter. As such, our outlook is of low-to-medium confidence. This kind of seasonal forecasting is experimental, and errors are possible.
Summer outlook by the numbers
Summer average temperature: Slightly below average, about one degree below the 1981-2010 average.
Temperatures compared to normal month by month:
- June: One degree below average (note that our outlook for June had previously favored slightly above-normal temperatures; however some of the latest model information has shifted a bit cooler. That said, a slightly warmer-than-normal June is within the margin of error, especially given the somewhat warm start.)
- July: Near average
- August: One degree below average
Number of 90-degree days for June, July and August: 15 to 20, compared with a normal of 31
Longest streak of 90-degree days: 5
Number of 100-degree days: 0
Precipitation: Somewhat above-normal rainfall
We considered several factors, described below, in preparing this outlook. It should be noted that any one factor doesn’t necessarily correlate with a particular kind of summer (e.g., warm, cool, dry or wet).
El Niño/La Niña
We are still experiencing a weak El Niño episode that started in late fall, which we expect to persist through the summer. During El Niño, which is an episodic warming of tropical Pacific Ocean waters, we tend to see somewhat above-normal rainfall.
Increased rain often reduces the intensity of summer heat because increased clouds and soil moisture prevent the air and ground from warming up as much.
Other factors and analog analysis
In addition to El Niño, we considered the current phase of the Pacific Decadal Oscillation (neutral) and predecessor weather patterns (from the spring). These inputs informed the identification of years when some of the above factors were present to some extent.
In this case, 2004 — whose summer was cooler than normal — emerged as the best analog. The weather during that summer was given some loose consideration in our projections.
CWG’s 2018 summer outlook (We did not evaluate this outlook but did well calling for the summer to average one degree above normal, and it ended up two degrees above normal. In addition, we projected 35 days at or above 90 between June and August, which was the exact number.)
Jason Samenow contributed to this outlook.