(wainscotte via Flickr )

With a warmer than average August coming to a close, we say goodbye to meteorological summer and hello to meteorological fall, which runs from September through November. Last month’s most notable weather was probably the often-oppressive humidity, but in terms of actual moisture we ended much drier than average. Fortunately we had a vast surplus of rain in June and July, so summer ended wetter than normal in the D.C. region.

We had a wide range of temperatures across the D.C. region in August, with Reagan National Airport slightly hotter than normal, Dulles International slightly cooler, and BWI right around normal.

National got a new thermometer on Aug. 10, which brought the location’s readings closer to Dulles and BWI for the final 2/3 of the month, but the warm bias in the first third likely kept Washington, D.C., warmer than the rest of the region. We should see more parity among the airports in our next monthly assessment. It was also a dry month — the second driest since 2000 at National — with rainfall amounts universally below average, offsetting the wetter totals we saw in June and July (see below for a meteorological June-August summer assessment).

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August temperature and precipitation

National’s average August temperature of 79.3 degrees was the 22nd warmest on record of all-time and the 9th warmest of the 2000s. The hottest all-time August for the location was in 1980, when the average temperature was a hothouse 82.8 degrees.

The measly 1.16 inches of rain we saw at National was the second driest of the 2000s, but 15th driest of all time. The driest August on record was in 1962, when just 0.55 inches of rain fell over the course of the month.


Last month was highly varied at National with the hottest temperature being 96 degrees on Aug. 4, and again on Aug. 17. The coolest temperature was a reading of 63 degrees just a few mornings ago on Aug. 27. The wettest day was Aug. 24, when we picked up 0.41 inches as the months only true thunderstorms rolled through the Beltway. Twenty three of 31 days (just over three weeks’ worth!) saw no precipitation at National.

Records

There were no records set at any of the three airports in August.

The weather pattern

The August pattern featured a cool trough of upper-level low pressure near the Midwest area with some extension to the East Coast. It was enough to keep our area temperature closer to normal overall for most locations, but the core of the cool trough was focused toward the Midwest and Tennessee Valley rather than us. The South and West saw the hottest temperatures more frequently.


U.S. temperature departures from normal for the month of August. (Commodity Weather Group)

Average upper level pressure pattern for the month of August. (NOAA/ESRL)

Summer 2015 (June-August)

The conclusion of August wraps up what we call “meteorological summer” and to put that in context, I also looked at temperature and precipitation at National Airport. For the third summer in a row, we did not reach 100 degrees in the Washington, D.C., region even though the heat index got there a few times.

Since all three months were warmer than normal, temperatures worked out to be the fourth warmest of the 2000s and the fifth warmest of all-time. Unfortunately, National’s thermometer was likely running on the warm side for the majority of summer, which probably played a role in the overall ranking, and NOAA’s policy is to not correct the data when sensors are replaced. Despite that issue, we are still seeing a general warm-up in summer temperatures at National in the past 15 years.


On the precipitation side of things, National picked up an impressive 18.11 inches of rain compared to an average summer of 10.44 inches, so the very dry August did not overcome the seasonal surplus. Compared to other years, it was the third wettest of the 2000s and the 14th wettest of all-time. Summer precipitation is not showing a clear trend — it’s flat overall, but rising in recent years.


Here is the national temperature anomaly map for meteorological summer.  The hottest areas were in the South and West:


2015 so far

With the addition of a warm August, 2015 to-date is still the eighth warmest year since 2000. We continue to be nowhere close to the warmest on record — 2012 — which had a stunningly warm March and a very hot summer. This year is holding steady in the middle of the pack among the past 15 years, just a bit warmer than the 30-year average.


For precipitation, the drier August knocked us down a notch to fourth place since 2000, from third last month. 2003 is still far and away our wettest year overall thanks to lots of big storms, but at least we are still running wetter-than-normal for the year!


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…

Reagan National (DCA)

Dulles International (IAD)

Baltimore-Washington International (BWI)