We’ve already gotten a healthy dose of summer this year, as we experienced an extremely warm and wet May. There are signs that the wet pattern will continue for the next week or two, though temperatures briefly look to be around or below normal. But we all know better than to be lulled into a sense of complacency.
In the D.C. metro region, summer always wins. And now that meteorological summer is officially upon us, we present our annual summer outlook.
Overall, we expect a summer similar to the last, warmer than normal, but nothing memorable as far as D.C. summers are concerned. That said, it will still be hot and muggy, with plenty of 90-degree days and warm, stuffy nights.
Our outlook shares a lot in common with those issued by private companies and the National Weather Service. There is a relatively strong consensus for a summer that is slightly warmer than normal but not historically hot.
A word of caution: 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 has low-to-medium confidence (and others issued should be considered in the same vein). This kind of seasonal forecasting is experimental, and errors are possible.
Summer outlook by the numbers
- Summer average temperature: slightly above normal, about 1 degree above the 1981-2010 average.
- June: one to two degrees above average.
- July: one degree above average.
- August: about average.
- Number of 90-degree days for June, July and August: 35, compared with a normal of 31.
- Longest streak of 90-degree days: eight.
- Number of 100-degree days: one.
- Precipitation: slightly above normal rainfall.
Methodology
The following factors are among those considered in preparing the 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 and La Niña
After a weak La Niña event this past winter, characterized by cooler-than-normal ocean temperatures in the equatorial Pacific Ocean, sea surface temperatures have been gradually warming, and we are experiencing neutral conditions — in between La Niña and El Niño (characterized by warmer-than-normal waters).
While we don’t expect official El Niño conditions in the summer, we do expect continued warming in the Pacific as we head toward a potential El Niño event by the winter. Often, this progression toward El Niño exerts a gradual cooling effect as the summer wears on.
Pacific Decadal Oscillation
After a four-year stretch where the Pacific Decadal Oscillation (PDO) has been in a positive phase, we are finally seeing signs of weakening. We expect the PDO to be around neutral or perhaps slightly negative. Summers with a negative PDO in Washington sometimes put a check on big heat in the region (although, note that the PDO was negative during our hottest summers on record from 2010 to 2012).
Spring conditions
After particularly chilly conditions in March and April, we are closing out a very warm and wet May. We looked at past years that had a similar progression and considered the summer conditions that followed in formulating our outlook.
Persistence
Twelve of the past 13 summers have been above normal in Washington. It is hard to bet against a warmer-than-average summer, especially with Washington’s urban heat-island effect consistently elevating overnight lows, even in air masses that are yielding average afternoon highs.
Analog years
The analog years, or the years when some of the above factors were present to some extent, are 1911, 1939 and 1986. The weather during those summers was given loose consideration in formulating our outlook.