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August 2013: 3rd driest and 3rd coolest of 2000s; Summer 2013 slightly warm and wet

August 2013 wraps up meteorological summer and our typically second-warmest month of the year, but this year was a bit of a treat compared to the last few.

The August average temperature of 77.1F at National was about one degree cooler than the 30-year (1981-2010) normal. The monthly precipitation of 1.34″ was 1.59″ below normal.

But the cool and dry August was not enough to counteract the warm and wet June and July.  Meteorological summer 2013 closed 0.6F warmer-than-normal and 5.3″ wetter-than-normal, impressively enough.

Let’s look at August and the summer’s temperature and precipitation issues more closely:


August 2013 was the coolest since 2004, while the summer as a whole was the coolest since 2009.  Even though the month was the third coolest of the 2000s, we actually saw slightly more hotter-than-normal days (15) than cooler ones (14) for the month.  The cool events just happened to be stronger than the warm ones.  In fact, they were the source of two of the only three record-breaking conditions in August.

* National saw a record low maximum of 74F on August 6 that tied with 1886, 1993, and 2004.

* Baltimore’s BWI slipped down to 53F for a record low on August 16, besting the 54F record from 1979.

* Dulles had one record warm minimum – on my birthday, August 9 (besting 2007’s 74F).

Here are the day-by-day details for the month where our hottest temperature was only 94F compared to last year’s 98F:

Looking sequentially through the 2000s reveals that the hottest (2002) and coolest Augusts (2000) occurred at the start, but it has been a while since we have been quite this cool as many of our readers have discussed.

This summer overall was quite a bit cooler than the last three very hot ones.  It was an impressive 2.6F cooler than the mean of 2010, 2011, and 2012.  For a 92-day period, that is a very noticeable difference, and it helped us steer clear of any 100-degree temperatures for the first time since 2009.  Despite the downturn in temperatures this summer, we can still see a noticeable upward trend in temperatures so far this new century:


August rain was below normal for the first time since May 2013.  And the total of only 1.34″ was the driest month since November 2012’s very small 0.6″ result.  Unlike other recent months, the precipitation anomaly disparity was fairly small among the area airports.  You can see the 2000s ranking and daily readings here:

Our wettest days in August never even achieved a half inch of precipitation with only 0.41″ (Aug 1) and 0.44″ (Aug 13).  Six days in August reported a trace of rain in addition to the nine days with measurable rain shown above.

The summer June-August period was the fourth wettest of the 2000s.  While a slight drying trend is noted through this new century, you could also argue that we have seen a wetter trend since 2007.  See the chart of the summer precipitation progression here:

The weather pattern

The pattern this summer and August, more specifically, was one where the main summer heat focus was over the interior West.  In fact, many areas of the West saw their hottest summer in at least three years as the primary area of heat ridging (upper level high pressure) set up shop there.

In June and July, we had occasional incursions from the Atlantic Bermuda high ridge as shown below (left map), but in August, that feature was weaker-than-normal, allowing our heat events to be weaker and shorter-lived instead.  This is all a huge difference from the 2010-2012 trilogy of summers when heat ridging primarily sat over the Midwest with frequent expansions to the East Coast connecting up with a stronger Bermuda ridge at times.

September 2013

Even though we are officially into the start of the meteorological autumn (and we have many autumn-lovers out there!), the first half of September can frequently carry some summer flavor yet.  Last year, for example, we saw 90s at times in the first half.

This year should be cooler overall compared to last year, but we still may see some brief periods of warmth and humidity, so we cannot completely rule out another 90-degree reading yet.  For now, it seems like September should be near to slightly above normal overall for temperatures with near normal precipitation.

Normal temperatures are very different between the start and finish of the month.  National’s normal high on September 1 of 84F becomes 74F by September 30.  The low temperature normal is 67F on the first and a cool 57F on the 30th.  So we’ll be watching this transition unfold in the weeks ahead.

The National Weather Service again does not have a whole lot to offer here.  But they do put our area closer to the wetter and warmer side overall for the month.  I am personally a bit suspicious that we’ll have a big wet month given the slow tropical Atlantic season and the struggle to get big rain system patterns to the East Coast in recent weeks.

For further information

The National Weather Service publishes nice monthly assessments usually within a week of the close of each month (should be available shortly):

You can click on your closest airport location here:

Reagan National (DCA)

Dulles Airport (IAD)

Baltimore-Washington (BWI)

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Matt Rogers · September 3, 2013

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