A boy plays in a fountain in Washington, D.C. during a heat wave. Image: Jim Watson/AFP/Getty Images
From New Orleans to Portland, Maine, the heat and humidity has hit oppressive levels for the second long stretch this summer. In New York City, for example, entire blocks smell like hot garbage, and the air feels like a wet blanket, with heat indices approaching 110 degrees Fahrenheit...
While the actual air temperatures during this heat wave are not record-breaking in most areas, it is the humidity that stands out as the weather event's defining feature.
Trajectory model showing the source region of New York City's air on Friday. Air at low levels is flowing from the Atlantic Ocean, across the Southeast, and then up the East Coast. Image: NOAA.
Anything above 65 degrees Fahrenheit is enough to make people feel uncomfortable, and once the dew point climbs above 70 degrees Fahrenheit, the humidity feels oppressive.
At times on Friday and Saturday, Washington, D.C., Philadelphia and New York could see dew points as high as 80 degrees Fahrenheit, which is almost unheard of for this far north.
Or, to put it another way, this is about as humid as it gets in parts of the Amazon.
In many areas, the heat and humidity during the day is not being offset by relatively cool weather at night, leading to greater public health concerns.
This is part of a trend toward higher overnight low temperatures as the climate warms. Urban heat island effects from the insulating influence of paved areas and buildings also plays a role, though a comparatively minor one relative to global warming.
For example, Reagan Washington National Airport made it five weeks — from July 5 through August 8 — without ever seeing the thermometer fall below 70 degrees Fahrenheit. This 35-day stretch beat the previous record of 32 days, set in 1980.
As meteorologist Robert Henson of Weather Underground points out, the average daily minimum for the lower 48 states was the warmest on record for June and July combined.
"Warmer nights are a hallmark of a climate being heated by added greenhouse gases, and it’s long been recognized that nights should generally warm more than days, and winters more than summers, as climate change proceeds," Henson wrote.
The eight years with the warmest average daily minimum temperatures have all occurred during the 21st century, according to Henson's analysis of government temperature data.
"Urban heat islands are no doubt helping to increase overnight lows in large metropolitan areas; however, the nationwide extent of the trend toward warm nights goes well beyond this effect," he wrote.
Chart showing average minimum temperatures across the lower 48 states in June and July since 1880. Image: NOAA/NCEI
The humidity means that any showers and thunderstorms that form will dump torrential amounts of rain, and that is exactly what is happening in the South on Friday, with extensive flooding taking place in Louisiana and Mississippi as a weather disturbance swirls across the northern Gulf of Mexico.
The Weather Service is predicting that "Significant to catastrophic flash flooding" will continue due to this "unusual heavy rain event."
Water rescues took place on Friday near Baton Rouge as rainfall totals eclipsed 10 inches, with up to 10 inches more on the way, according to the National Weather Service.
Images being shared on social media show a steadily worsening flooding situation, particularly just east of Baton Rouge. Some rivers are cresting at record high levels.
Bermuda High plus record ocean heat
The high humidity can be traced back to two large-scale factors. One is a pattern of atmospheric circulation, and the other lies in the oceans.
A strong-willed Bermuda High has parked itself near that island, with the clockwise flow of air around it causing southwest winds to pump moisture into the Mid-Atlantic and Northeast like a fire hydrant.
Animation showing total precipitable water in the atmosphere, revealing a plume of moisture (orange) up the East Coast of the U.S. Image: University of Wisconsin
As these winds pass over the Atlantic Ocean, and other southwesterly winds move over waters of the Gulf of Mexico, they are picking up heat and moisture from unusually mild ocean waters.
Water temperatures along the Atlantic coast and in parts of the Gulf of Mexico are record warm for this time of year. This is helping to fuel the flooding in Louisiana, as well as the dangerous heat along the East Coast.
Fortunately, this weather pattern is expected to be relatively short-lived, with the potentially deadly heat and humidity receding from New England by Monday, and decreasing in the Mid-Atlantic early next week as well.
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