Eduardo Velev cools off in the spray of a fire hydrant during a heatwave on July 1, 2018 in Philadelphia, Pennsylvania. Credit: Jessica Kourkounis/Getty Images

Extreme heat and humidity will continue through the end of the holiday week across the eastern U.S., before it shifts westward, roasting the Midwest and Great Plains, potentially setting records and intensifying ongoing wildfires all the way to California.

Why it matters: Heat is the top weather killer in the U.S. during most years. According to the Centers for Disease Control and Prevention, more than 600 people are killed every year due to heat-related illnesses in the U.S.

Details: Shortly before noon on Monday, the heat index — which is a combination of the air temperature and humidity — in Washington, D.C. had already surged past 100 degrees Fahrenheit. Same for Philadelphia, Baltimore and parts of New York City. The air temperature is forecast to flirt with the century mark from Albany, New York to New York City southward to Washington, with heat indices reaching as high as 110 degrees Fahrenheit or higher, prompting the National Weather Service to issue excessive heat warnings.

Animation showing the heat dome (as indicated by 500 millibar height anomalies) located over the Northeast on Monday, and then shifting West over time. Credit: tropicaltidbits.com/European model.

Historical context: The actual air temperatures during this heat wave are not smashing all-time records in most cases, since they're only 10 to 20 degrees Fahrenheit above average for this time of year. However, the combination of heat and humidity, plus the long duration of this event, make it a public health threat.

  • Daily high temperature records have fallen in some places, along with records for the highest overnight low temperature.
  • Research has shown when air temperatures fail to cool below about 80 degrees Fahrenheit overnight, the risk of heat-related illnesses, including heat stroke, increases.
  • On Sunday night into Monday morning, the temperature failed to drop below that 80-degree threshold in New York City and other locations along the East Coast.

What's next: The heat should peak along the East Coast on Monday, but high temperatures all week are likely to reach or exceed 90 degrees Fahrenheit from the Mid-Atlantic to New England, which may put this event in the top 10 for long duration heat events.

Meteorologists define heat waves as occurring once the temperature hits or goes above 90 degrees for at least three consecutive days. Many locations in the East are on their fourth or fifth such day as of Monday, with no relief expected until next weekend.

The big picture: This heat wave has been triggered by an unusually intense, sprawling area of high pressure, also known as a "heat dome," that is parked across the eastern third of the nation. The clockwise flow of air around it is drawing hot, sultry air northward from the Gulf of Mexico, bringing unusually humid conditions as far north as Quebec and Ontario. This high pressure area is expected to slide to the west with time, ensuring that much of the lower 48 states eventually experience this event.

  • For the period of June 21 through June 29, as the heat wave was beginning, the U.S. set or tied 165 record high temperatures, along with 411 record warm overnight low temperatures.
  • This compared to just 62 daily record cold temperatures and 36 record lows during the same period.

Be smart: Peer reviewed studies show that heat waves are already becoming more likely and more intense across the globe as the overall climate warms due to rising amounts of greenhouse gases in the air. The connection between global warming and heat waves is the best understood and least controversial of any type of extreme weather events. One trend that's been most closely tied to climate change is an increase in overnight low temperatures.

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