Near-record moist tropical air brings extreme heat and humidity to the eastern half of the US
Washington, DC matched daily heat records for three consecutive days on August 12 through 14. In Philadelphia, the extreme heat and humidity led to three heat-related fatalities. In New York City, several utilities reported power outages as electricity usage spiked, leaving thousands of customers without power amid the heat.
The National Weather Service issued excessive heat warnings for New York City and surrounding areas from August 12 through 14, with air temperatures near 100°F (38°C) and heat indices up to 110°F (43°C). The NWS also issued heat advisories in many major cities including Baltimore, Washington DC, Boston, Portland, Columbus, Charleston, Austin, Fort Worth, and Houston.
The extreme conditions since August 10 are due to a hot and wet tropical weather system, which gained heat and moisture over near-record warm seas in the Gulf of Mexico and Atlantic. The system propelled heat indices (which measure the stress on the body's ability to cool itself) to soar into the triple digits across a wide swath of the Eastern US.
Along the central Gulf Coast, a measure of how much water there is in the air called "precipitable water content" (which is the depth of water in a column of air if all the water were rained out) reached the top percentile of historical values, often in the 2.5 to 2.75 inch range (3 to 3.5 standard deviations above normal). The animation to the right shows the precipitable water over the Gulf of Mexico and Atlantic Coast from August 9 through 12 that has added moisture to the air.
In New Orleans, the precipitable water was determined to be 2.78 on August 10, which ranks among the top-five highest levels on record in the city for August. What's even more remarkable, is that this reading is higher than those observed during many past hurricanes, which are known for raising atmospheric moisture content.
Observed changes in heat extremes
Due to global warming, the most extreme heat events now impact a global area 10 times greater than in the period 1951-1980.
The impact on moderate heat waves is also dramatic, with a seventy-five percent share of moderate heat events now attributed to climate change.
The connection between climate change and extreme heat events is well understood and well documented, reports the US National Academy of Sciences. The Intergovernmental Panel on Climate Change (IPCC) reports that: “Overall, the most robust global changes in climate extremes are seen in measures of daily temperature, including to some extent, heat waves.” 
Global warming is the most obvious, well-documented effect of climate change. As a result, the signal is very strong so we can more easily detect it amongst noise of natural variability compared to other types of extreme events. - Stephanie Herring, NOAA 
Many urban areas across the globe have witnessed a significant increase in the number of heat waves, with the largest number of heat waves occurring in the most recent decade studied, 2003-2012.
Heat waves have generally become more frequent and intense across the US in recent decades, with western regions setting records for numbers of these events in the 2000s. Recent multi-month extreme heat events in the US are unprecedented since the start of reliable instrumental records in 1895. There has also been a dramatic increase in nighttime temperatures in the US, reducing the number of critically important relief windows during heat waves. These changes are consistent with the changes projected by models of a climate warmed by greenhouse gas emissions.
Climate change driving humid heat waves, elevating the risk of heat stress
Climate change is amplifying the intensity of extreme heat through increased humidity. A warmer atmosphere can hold more water vapor, and the global atmosphere has become moister due to warming. The fifth Intergovernmental Panel on Climate Change (IPCC) report states that humans have contributed to observed increases in atmospheric moisture content since 1960.
The increase in surface air moisture content is consistent with changes in atmospheric temperature and a physical relationship (known as the Clausius-Clapeyron equation), which states that the air holds about 7 percent more moisture per 1°C of warming.
According to Kevin Trenberth, a Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research, “Water vapor has increased at rates consistent with Clausius-Clapeyron for the period 1987–2004 (1.3 percent per decades), and the relationship with changes in sea surface temperatures is sufficiently strong that it is possible to deduce an increase of about 4 percent in total column water vapor over the oceans since the 1970s.”
Observed moisture increases are largest in the tropics and in the extratropics during summer over both land and ocean.
The fingerprint of climate change has been found in the increase of wet bulb temperature since 1973, driving heat stress globally and in most land regions analyzed. The northern hemisphere is tending toward increasingly warmer and more humid summers, and the global area covered by extreme water vapor is increasing significantly.
Global warming dramatically increases frequency of the most extreme heat events
Extreme heat events, such as this heat wave, are the kind of weather events that increase the most as the climate warms. The more extreme the heat wave, the more likely it is due to the change in the climate. Global warming has already driven a dramatic surge in the frequency of the most extreme heat events.
Because of the way global warming shifts the climate, the very most extreme events are the weather events most affected by climate change. As the average global temperature rises and the climate shifts, temperatures that were extreme under the old climate are found closer to the middle of the new temperature range. Under the earth's climate system events closer to the midpoint occur much more frequently than events closer to the extremes, as shown in the graphic on the right. Thus a small shift in climate leads to a dramatic increase in the frequency of temperatures at the high end. The shifting bell curve also leads to the occurrence of never-before-seen extremes in high temperatures.
...it is the rarest and the most extreme events - and thereby the ones with typically the highest socio-economic impacts - for which the largest fraction is due to human-induced greenhouse gas emissions.