Meteorological summer – spanning June to August – has ended. Much of it was fairly pleasant, notwithstanding some of the hottest and muggiest weather at the tail end. Still, the August average temperature of 77.7F at Reagan National Airport (D.C.’s official weather observing location) was a hair cooler than the 30-year (1981-2010) normal of 78.1F.
The near normal August and the somewhat cooler than normal July were not enough to make it a “cool” summer at National thanks to temperatures 2F above normal in June. The overall summer result was 0.4F warmer than normal – slightly cooler than last summer and the coolest in five years.
Whereas National was only 0.4F cooler than normal in August, Dulles and BWI airports were substantially cooler: 3.6F and 2.6F below normal, respectively. For the summer, Dulles and BWI averaged about 2F and 1F cooler than normal. Thus, outside of urban Washington, much of region seemed to have a somewhat cooler than average summer.
August rainfall was slightly above normal at National. The summer overall was about 1″ wetter than normal. It may have seemed wet but consider last summer was about 5″ wetter-than-normal.
Over the last 14 years, summer temperatures have trended somewhat warmer and drier at National, but the last two summers have slowed those trends a bit. Here’s a look at the year-to-year variability in summer temperatures and precipitation:
Temperature in detail
The cool days outnumbered the warm ones in August, but we did get some classic D.C. hot/humid weather before the month’s end. Sunday’s high (August 31) of 96F at National was the hottest since we hit that level back on July 8. We also had a superb share of pleasant, low-humidity results at times. Many of those days were in the first half of the month when the summer can be typically hotter. You can see the daily breakout here:
Precipitation in detail
As usual with summer thunderstorms, you can see huge differences in rainfall totals over relatively small areas. While National Airport managed to eke out a small surplus (compared to normal) this past month, Dulles and BWI saw much bigger gains (after they experienced drier July outcomes than National). Almost half of National’s August rain occurred just on one day (our very wet August 12 episode). Otherwise, we experienced a week of dryness at the end of the month before Sunday’s 0.27″ contribution. You can see the daily breakout here:
No records were recorded this month at National, but we picked up some precipitation records at Dulles and BWI:
August 21: Record rainfall of 1.43″ beating 1973’s 1.42″
The weather pattern
The situation over North America in August was a variable one, but generally we experienced two different weather patterns- a generally cool-prevailing pattern in the first half of the month and then a warmer, more humid situation in the second half. The prevailing pattern was a weak cool upper level dip in the jet stream as shown below. The global atmospheric wind conditions in the first half of August were more reflective of a developing Pacific El Niño pattern (which we saw in most of July too). But the end of August saw more of a hangover La Niña state like we also experienced back in the warmer June pattern. These back-and-forth influences complicated the summer pattern and helped average the overall period very close to normal.
The slightly cooler and wetter August vs. normal helped to push 2014 in the coolest and wettest directions for the 2000s. With August completed, 2014 is running as the third coolest year on record. This is going to be interesting as typically a developing El Niño pattern can deliver a cool autumn to the East Coast (especially by October), so we will see if we can shift ahead of 2009 and 2003 by the end of the calendar year for the coolest of the new century standing. 2012 was way ahead of all other years at this point thanks to a super-warm March and very hot summer that year.
While the summer wasn’t nearly as wet as 2013, the snowy and rainy start to the year is keeping 2014 on track as one of the wettest years of the 2000s. The odds of catching up to 2003 are extremely low without some sort of hurricane impact (this doesn’t even have September 2003’s Isabel factored in yet!). But with a weak El Niño still developing, we still have a shot to maintain this second place status through the balance of the year.
September 2014 outlook: Warm start, but mixed middle to end
The last-minute hotter and more humid summer pattern that just ended August lingers into the first part of September too giving the month a toasty head start. We should see more pattern variability by next week and we could see some more vigorous cool fronts by mid-to-late month. But at this point, there are no signs of any sustainable, stronger early autumn cool-prevailing pattern signatures yet. Historically, October has better odds for bigger cool patterns than September when a slow-evolving El Niño is taking shape. Precipitation for September should be near to above normal again thanks to an active pattern expectation.
Meanwhile, the National Weather Service (NWS) final September outlook favors a warm month overall, with no signal for precipitation. You can look at the NWS final August forecast here.
For further information
The National Weather Service publishes nice monthly assessments about five days into the start of the next month. You can click on your closest airport location here: