July was slightly cooler and wetter than normal in D.C.

August 1

The fantastically polar end to July helped shift the month’s temperature to the cool side of normal in D.C.  The average temperature of 79.4F at Reagan National Airport (DCA), D.C.’s official weather observing site,  is about four tenths of a degree cooler than the thirty-year climatology. At DCA, it was the coolest July since 2009 and wettest since 2010- so nothing too exceptional here.

Precipitation registered a surplus of about 0.95″ at DCA, but we did not receive much rain at all in the second half of the month.  Both Dulles and Baltimore-Washington International reported below normal precipitation (and temperatures).

You can see how temperature and precipitation for 2014 ranks in terms of this current century.  There appears to be some long-term upward trend for temperature even though the last three July periods have edged cooler than its prior.  No discernible pattern is seen for precipitation.



 

Temperature in detail

We saw lots of temperature volatility in July, but the hottest weather was early in the month and coolest in the second half.  A national pattern shift occurred in early to middle July as the jet stream dove south carving out a deep trough into the central, southern, and eastern parts of the U.S.  The atmosphere showed signs that it was starting to respond to the developing El Niño, which tends to favor cooler patterns (most recently in 2009).  July 2014 recorded only nine days in the nineties at DCA, just about one in three days compared to an average of just over 14.


 

Precipitation in detail

Monthly rainfall at DCA was robust, but less so at the surrounding airports.  In all cases, rainfall was less common in the second half of the month; so with the strong summer sun, we are starting to see some signs of drier conditions in our vegetation.   Rain chances are picking up again here in early August and the correlation with the developing El Niño should favor seasonal to wetter conditions through the balance of summer.


Records

No records were recorded this month at National, but we had mostly cool records elsewhere:

Washington Dulles

July 13:  Record warm minimum of 72F tying 2006

July 17:   Record low of 55F matching 1976

July 29:  Record low of 52F besting 1978′s 53F

July 30:  Record low of 48F(!) beating 1981′s 51F

Baltimore-Washington International

July 5:   Record low of 55F ties 1979

July 18:  Record low of 57F beating 1976′s 58F

July 25:  Record low of 57F beating 2008′s 59F

July 29:  Record low 59F tying 1978

July 30: Record low 55F besting 1997′s 56F

The weather pattern

While June showed very little of it, we finally started to see some El Niño atmospheric response in some key indicators and sure enough, the pattern shifted cooler.   The location of cool troughing around the Midwest (or dip in the jet stream) was also very similar to the prevailing pattern from last winter (triggering comparisons to our polar vortex pattern).   You can see the prevailing pattern below (the big difference from the winter was the lack of upper level warm ridging over Alaska – it was over western Canada and the western U.S. instead):

july2014pattern

2014-to-date

The current calendar year continues to run wetter and cooler than normal overall with July certainly helping that cause.  With the slow-emerging El Niño pattern, we should continue to expect a cooler/wetter lean in the pattern for the balance of summer and even into the autumn period.  This makes sense since we’re seeing the coolest/wettest readings since the last one in 2009.   You can see the latest January to July ranking estimates here:



August 2014 outlook:  Limited heat chances

The current thinking is that August should continue to lean toward seasonal or the cooler side for the overall month, but don’t be surprised to get some brief periods of warmer/muggier weather at times intermixed with periods of cooler days.  I believe August could be very close to normal or slightly cooler with seasonal to slightly above normal precipitation.  The first five to seven days of August should run cooler and wetter than normal overall, at least.

Meanwhile, the National Weather Service (NWS) final August outlook doesn’t have much to tell us.  Our taxpayer dollars have yielded an August forecast that is EC for both temperature and precipitation.  EC means Equal Chances of above, below, and normal 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:

Reagan National (DCA)

Dulles Airport (IAD)

Baltimore-Washington (BWI)

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Brian McNoldy · August 1