A potentially deadly combination of heat, humidity and stagnant, polluted air

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'Dangerous heat wave is building' as temperatures spike in central, eastern U.S.

WashingtonPost,

18 July, 2019

A potentially deadly combination of heat, humidity and stagnant, polluted air has engulfed more than half of the Lower 48 states, with air temperatures climbing toward the century mark everywhere from the Plains to the Midwest, Mississippi River Valley, and eastward to the heavily populated Washington to Boston corridor.

"A widespread and dangerous heat wave is building in the central and eastern U.S.," the National Weather Service said Thursday.

Heat advisories and warnings affect 154 million Americans. In many major population centers, the heat index - how hot it feels factoring in the humidity - is forecast to peak around 110 degrees between Friday and Sunday. The actual air temperature is expected to reach at least 95 for over half the population of the Lower 48 over the next several days.

The trigger for this heat wave is a sprawling, strong high pressure area, also known as a "Heat Dome," building across the U.S. Another high pressure area in the Western Atlantic, which is known as the "Bermuda High," is also a key player, since the circulation around these weather features is pumping hot and humid air from south to north.

Hurricane Barry's remnants have added to the misery buy bringing a surge of sultry, swamp-like tropical moisture that has blanketed the heat wave zone. As a measure of that moisture, many locations along the East Coast reported dew points of 80 degrees on Wednesday, about as high as such temperatures go in non-tropical locations.

Such extremely humid conditions are expected to continue through Sunday, particularly along the eastern seaboard.

The most memorable aspect of this heat wave will be the lack of relief at night, especially in urban areas where the urban heat island prevents temperatures from falling quickly overnight.

According to the National Weather Service, overnight low temperatures will be in the mid to upper 70s to 80 degrees, and "Dozens of high minimum temperature records are forecast to be set, with a few record high maximum temperatures possible as well."

The Weather Service projects 123 record warm low temperatures to be tied or broken.

For example, the overnight low temperature at New York's Central Park observing station is not forecast to drop below 80 degrees on Saturday and Sunday, and could tie or break the daily records of 82 degrees on both nights. Washington is forecast to have three consecutive nights with low temperatures in the low 80s, from Saturday morning through Monday morning.

Such high overnight lows will exacerbate the public health threat from this event. Heat is typically the No. 1 weather killer in the U.S. each year, and heat-related illnesses spike when overnight lows stay warm, depriving the human body of a break from heat stress.

Most vulnerable groups to heat-related illnesses include the elderly, chronically ill, children, and outdoor workers.

As the climate warms due to human activities, numerous studies show that heat waves such as this one are becoming more common and intense, as well as longer-lasting.

An expansive study in the journal Proceedings of the National Academy of Sciences last year found a climate change fingerprint in heat waves worldwide. Specifically, it showed that climate change has heightened the chances for record heat across more than 80 percent of the surface area of the globe that has sufficient weather data available. (This research excluded parts of the developing world, where weather monitoring networks are more sparse.)

In addition, a sweeping climate assessment published by the Trump administration last year found extreme heat events are on the increase in the United States and have been since the 1960s. Interestingly, summer nights have warmed nearly twice as fast as summer days in the United States, according to NOAA data. This makes heat waves a more formidable threat to public health.

In Washington, for example, lows of 80 degrees or higher have occurred 32 times since 2010, which is higher than the number of instances from 1872 through 2009.