Related: Commentary - July heat was unprecendented | Graphic
National Weather Service summary of D.C. records during July

Congratulations D.C.-area readers. You have just experienced -- or tolerated, survived, and suffered -- the hottest month in recorded history. We did it last July too, but this time we soared just over another degree on average to achieve an incredibly toasty 84.5F at Washington Reagan National Airport. That’s 4.7 degrees hotter than the new 1981-2010 normal or 5.3 degrees hotter than the old 1971-2000 climatological baseline. Also, it is slightly hotter than a normal July for Houston, TX and slightly cooler than a normal July for Dallas, TX. It’s like we were living in a whole different part of the country.

As you might expect for a month like this, precipitation ran below normal, but only .7” below the 1981-2010 normal of 3.70” at National Airport. This puts our running deficit over 4” since January 1st. It is dry, but it could be worse.


July 2011 daily low temperatures in D.C. compared to 2010

While we actually saw the same number of 100-degree days this July compared to last, and both were extraordinarily hot, this one took the cake on pretty much every high-temperature metric. Of those three 100-degree days, one was a record for the date 104F on the 29th. That was the hottest we’ve seen in D.C. since August 17, 1997 when it was 105, and it tied for the 5th hottest all-time. Previously, the second of back-to-back 102s on July 23 also set a new record for the date.

Other notable D.C. area 100-degree plus stats for the month include the all-time record high of 105F at Dulles Airport on July 22. Although the records there only go back to 1963, no other day had topped 104 with the last occurrence in 1988. On the same day, BWI hit 106F. That was the highest on record at the current location, and one shy of the all-time Baltimore record of 107.

A fourteen day streak of 90-degrees or higher, which now carries on into August, helped carve out a significant portion of the record 25 90+ during the month. July 2011 now stands alone in this field, besting two other months with 24 days – July 1993 and July 1987. Last year, we saw 21 days in July that were 90 degrees or higher. Stepping up to 95-degree plus days, July 2011 tied for the most on record with 1999 at 14. 1999 was also the previous high mark for an average daily high temperature, with a 92.8F. This year came in at an incredible 93.6F, which is 5.2 degrees above the new 1981-2010 average (88.4F) for the month.

July 2011 daily low temperatures in D.C. compared to 2010

Like last summer, major contributors to this July’s phenomenally high temperature average (even if less focused on) were the low temperatures. An astounding eight record high minimums were tied or set this year in D.C. Compiling overnight records started early and went into high gear late. On July 12, a record of 77F was set. On the 19th, the low of 79F tied a record. From the 22nd through 24th temperatures barely dipped into and below the mid-80s with records on all three dates including all-time record warm minimum tying 84Fs on the 23rd and 24th. To close the month, an 80F on the 29th tied a record, while an 81F and 80F on the 30th and 31st put July 2011 in the books two more times.

The unheard of seven 80-degree or higher low temperatures not only blew the previous monthly king of three such days (in July 1876 and Julu 2010) out of the water, it also demolished the full-year record set just last year of four. Both the four-day streak and three-day streak now stand as one and two all-time, with two in a row being the most such days prior. Similar numbers continued down to 75-degree or higher lows, with this year tying 2010 for the most at 17 such days. Thanks to some oh-so-brief nicer weather after the Fourth of July, we only managed 26 days 70+ on lows, compared to the high of 29 in 1955. Yet, the big numbers more than made up for it.


As perhaps noted by the lack of much mention, rainy events were few and far between in D.C. itself, with notable exceptions being the unexpected early-morning thunderstorms on July 3rd that were followed by an evening round of severe weather. An additional soaking (the biggest of the month at 1.05”) came to D.C. on the 13th as thunderstorms moved through. Other days throughout the month featured slow-moving storms that caused localized flooding across the region. This type of very hit-or-miss activity that can still impact certain people heavily is relatively normal for a July in D.C., the city just managed to miss out on the worst of it.


There are at least three major reasons why this situation unfolded and why much of the eastern 2/3 of the U.S. is experiencing near -- or at and past -- their hottest Julys on record. The three brutal components for this July were (1) a hot jet stream pattern, (2) a brutal heat supply, and (3) an impressive humidity source.

Jet Stream: If you live on the West Coast of the U.S., you are probably saying “what hot summer?” because like last year, you are experiencing a mostly seasonal to cool summer story. The jet stream has lined up so that it dips farther south on the West Coast (cool upper-level low pressure) and like a see-saw, it rebounds north over the eastern two-thirds of the nation in the form of a big hot ridge of upper-level high pressure (“heat dome of doom”). This pattern type is typical of a negative Pacific Decadal Oscillation (PDO) state, but what has been atypical this summer is the intensity of the warm waters in the North Pacific (south of Alaska). This feature is stronger than anything found historically (back to 1948). You can see this “warm pool” here (those anomalies are 5 or more degrees Celsius above normal):

Sea surface temperatures relative to average during July (red is warm, blue is cold) ( Adapted from NOAA )

Meteorologists can get into the classic chicken-vs.-egg arguments with this situation. Did the hot ridge pattern in the North Pacific warm those temperatures or did those warm temperatures fuel the jet stream? Most at least agree that a large warm pool such as this one offers a positive feedback mechanism to strengthen the pattern we are seeing this summer. Another contributor to the prevailing jet stream pattern is probably the Atlantic Ocean too. When the Atlantic is in its warm, positive AMO (Atlantic Multi-decadal Oscillation) phase like it is this year (look at those warm Atlantic waters on the map above), then we also tend to support more heat ridges in the Eastern U.S. So, the combination of a –PDO and +AMO with a very strong North Pacific warm pool allowed our jet stream to keep the burners on the eastern 2/3 of the nation more often than not.

Heat Supply: In addition to a supportive jet stream pattern, we also had an impressive hot air supply situation this July. Last winter, the tropical Pacific Ocean saw a moderate to strong La Niña pattern (cooler-than-normal waters in the central Tropical Pacific). This usually leads to a dry winter in the Deep South with a dry/warm spring too. Thanks to that, the summer started in Texas and nearby areas with strong drought conditions. Dry soil conditions do not drive jet stream patterns either, but they can make patterns stronger by adding more impact in the way of another positive feedback mechanism. As a result, Texas and Oklahoma are experiencing exceptional periods of sustained 100+ temperatures this summer. Our jet stream this season frequently advances hot air from the mid-continent to the East Coast, and the big hot drought in the Plains and Texas is a seemingly inexhaustible supply of strong heat.

Precipitation anomalies over U.S. during July (red is dry, blue is wet) (NOAA ESRL)

When our big heat dome travels over the super-saturated soils of the Midwest, it can evaporate a great deal of moisture to laden the air with heavy humidity. And this worked very well this summer. Higher humidity leads to the sinister heat index levels that brought us up into the 110s and 120s in the Midwest and on the East Coast.

See Jason’s commentary for possible links to urbanization and global warming.

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

Historical Washington, DC data provided by NOAA and Speedwell Weather (