As Hurricane Barry exits stage left, people living in the eastern half of the United States will face oppressive heat later this week. The cynical or snarky response might be, "Well, it is July and summer." However, the projected heat in the eastern U.S. is excessive and poses a significant danger, particularly in urban areas like Washington D.C., Atlanta, Charlotte, Nashville, Indianapolis, Chicago, and New York. What are the meteorological factors driving this potentially life-threatening heat and just how hot will it be?
Before I answer those questions, a quick review of the "heat index" is required. According to the National Weather Service website:
"It's not the heat, it's the humidity". That's a partly valid phrase you may have heard in the summer, but it's actually both. The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This has important considerations for the human body's comfort. When the body gets too hot, it begins to perspire or sweat to cool itself off. If the perspiration is not able to evaporate, the body cannot regulate its temperature.
If we examine the map above, the colors tell the story for Saturday, July 20th although oppressive heat will actually be in place as early as Friday. Areas represented by brown are projected heat index values in the 100 to 105 degree F range. As bad as that sounds, pink regions are values in the 105 to 110 degree F range. It gets worse. As I review the map more closely, there are large areas (tan) in the 110 to 115 degree F and even some regions of the Mid-Atlantic in excess of 115 degree F (yellow). I am an atmospheric scientist that spent 12 years of my career at NASA, but it does not take a rocket scientist to understand that this is a dangerous level of heat.
As we have seen with previous deadly heat waves in Europe, India, and the United States, heat-related deaths or illnesses are related to excessive maximum and minimum values of temperature or heat index. The map below is equally worrisome for me because it shows the minimum heat index for the continental United States. Large swaths of the eastern U.S. will experience minimum heat index values in the 75-80 degree F range by Friday. At those levels, nighttime provides little relief for people that may not have additional or adequate cooling resources and the body has to work harder to cool itself. Studies have shown that one of the main factors that lead to over 30,000 fatalities during the European heatwave of 2003 was elevated minimum temperatures. One study published in the journal Environmental Health Perspectives noted that people, particularly the elderly, were more susceptible to death after being exposed to consecutive days of elevated nighttime temperatures. Studies show that minimum temperatures have been increasing, on average, in recent decades. The Environmental Health Perspectives study also found that residents of urban areas were particularly vulnerable because of the additional contributions of urban heat islands (cities warmer than surrounding rural areas because of asphalt, other heat-retaining surfaces, lack of vegetation, and waste heat).
So why is it going to be so hot later this week? The weather map below tells the story: A dome of high pressure. High pressure systems are typically associated with hot, stagnant conditions or heat waves. The air also generally sinks in areas of high pressure leading to something called adiabatic warming (as air sinks it is compressed). The National Weather Service Wakefield forecast discussion provides additional context:
A typical mid July/summertime wx (weather) pattern persists through the extended forecast period as high pressure sfc-aloft (surface and above) builds from the SE/mid-Atlantic states on W through the mid MS valley.
Confidence is high right now that this will be the hottest period of the summer so far. Mainly partly cloudy...
This weather pattern comes on the heels of the announcement by NASA, the Japanese, and Europeans that Earth just experienced the hottest June on record. With temperatures like this, we may be having a similar conversation about July.
If you live in the regions affected by this heat and are not acclimatized to it, plan accordingly. Heat is not as telegenic or social media buzzworthy as a tornado or hurricane, but according to National Weather Service date, heat killed more people in the United States last year than any other type of weather. In fact, close examination of the graphic below reveals that heat has been the deadliest weather hazard in the United States for the past several decades.
