September technically kicks off the first month of meteorological autumn, which runs through November 30. But for the Washington, D.C., area, September is often a combination of two seasons: lingering summer and incoming autumn.

Last year was a perfect example — we began the month with summery mid-90s and then by the third week experienced lows in the crisp 50s. Normal high temperatures for September start at 84 degrees but by the end of the month our average is down to 74.

September 2014 was warmer than normal (2.8 degrees above average) and drier than normal (2.61 inches below average). A similar theme is expected this month, with potential variations due to the strong El Niño. Our current estimate is for above normal temperatures and below normal precipitation.

September average temperature forecast: 71.5 to 73 degrees
(Average: 71.0 degrees)

September precipitation forecast: Less than 3.5 inches
(Average: 3.72 inches)

Here are some key factors involved in our September forecast:

Continuity: After a wet and warm June-July situation, the pattern shifted to drier-than-expected in August. As our calendar turns to September, it looks as though this same pattern continues for at least a few more weeks, which is more than enough time to dominate the month’s final outcome. Warm, high pressure ridging prevents rainfall opportunities, along with the influence from El Niño.

El Niño: Most of the strongest El Niños have led to a drier-than-normal September in Washington, D.C. Some of the rains we receive in September are associated with the Atlantic hurricane season, and El Niños tend to reduce the number and intensity of storms in the Atlantic basin. This limits the amount of rainfall we can expect from storms that strike the Gulf of Mexico or Southeast coasts — even the weaker systems like tropical storms or depressions.

Since this current El Niño story is competing for one of the strongest on record, I decided to rank the five strongest September El Niño readings on record to see what happened here in Washington:

On average in these strong El Nino years, the temperature was almost ½ a degree warmer than normal. The precipitation is more variable, but four out of the five years were dry. All four of the drier years were over an inch drier than normal. We “never say never in weather,” so we’ll watch for any stray tropical activity to give us some extra rain, but El Niño will be a big hurdle to clear.

Long-range guidance: Below is the most recent long-range forecast from the National Weather Service’s CFS model (average of the past five days of runs). This guidance favors above normal temperatures (about 1 degree above normal) and near-normal precipitation. The CFS was too wet for the East Coast for its August outlook, so I tend to think we need to be a bit cautious for September too.

Climatology considerations: Since Reagan National Airport adjusted its sensor earlier this month, the results at the station have been much closer to its neighbors at Dulles and BWI, so I didn’t feel the need to make any sort of bias corrections on this September outlook.

National Weather Service

Like last month, the National Weather Service again favors “EC” for the D.C. region, which means equal chances of normal, below, or above normal precipitation and temperature, noting they don’t have enough confidence to go warm or cool, wet or dry for our specific area. You can read the Weather Service’s forecast discussion here.  They will be issuing their final call for the month ahead late this afternoon and you can find it here.


August outlook verification

The August outlook was only half-correct. The temperature forecast worked well directionally (it was warmer than average), but came in slightly stronger than expected. The precipitation expectation was inaccurate unless we get some last-minute surprise precipitation later on Monday (highly unlikely to be enough anyway). The prevailing pattern turned drier than normal by the second week of the month and the quieter tropical season issue is probably something that feeds into drier September thinking too.

August temperature forecast: 77.6 to 78.6 degrees

Actual: 79.2 degrees (pending results from Aug. 31)
Average: 78.1 degrees

August precipitation forecast: 3 inches or more

Actual: 1.16 inches (pending results from Aug. 31)
Average: 2.93 inches

I’d give the August outlook my second “D” in a row after very successful May and June outlooks. What do you think? Tell us in the comments.