An ongoing heat wave has been roasting the Plains and is now poised to bake the East Coast this weekend. The high temperatures, which have been in the 90s to low 100s in the Plains, have been accompanied by intense humidity that has made the weather especially miserable, if not downright dangerous.

The National Weather Service sent out excessive heat warnings over a swath of the country from Minneapolis to St. Louis and Chicago to Wichita earlier this week. Those warnings mean the heat index, or the combined effect of the heat and humidity on the body, was between 100°F and 110°F. Any time the level exceeds 105°F (40°C), NOAA considers it a danger day, meaning there is an elevated risk of heat-related illnesses like heat exhaustion. 

heat danger zones

The humidity — courtesy of moisture pulled up from the Gulf of Mexico — has been particularly high in the Plains. One way to measure it, the dew point, has been hovering in the upper 70s during the middle of the week. That value is slightly higher than during a typical summer day in the tropical climate of South Florida. Glencoe, Minn., even recorded a dew point of 80°F on Wednesday.

forecast lows
forecast lows

GFS forecast lows for Friday night.


Another reason for the current push in humidity in the Plains is the extensive corn crop. Corn gives off water as it grows. Corn stalks take in water from the soil during photosynthesis, and the water in their leaves evaporates into the air around it. With cornfields as vast as those in the Plains and Midwest, the effect of that additional evaporation adds up.

That big bubble of heat will send highs into the upper 90s in the Northeast this weekend, but with the high humidity, the heat index will easily exceed 100°F, and perhaps even exceed 105°F on Saturday from New York City southward to South Carolina. The nights will be especially stifling, with temperatures hovering above 75°F overnight; a few locations may not drop below 80°F. Such warmer nights make it more difficult for the body to recover from the high heat of the day, and raise the threat from heat related illness.

Record highs this time of year are around 100°F in several northeastern cities, so not all locations will break records during the day. However, with the high humidity, the nighttime cooling processes are slowed, as moist air does not cool as easily as dry air. So record high minimum temperature records are more likely to be broken than daytime highs. Even so, as late July is often the hottest time of the year, setting new records will not be easy.

Current records going into this weekend (all in °F):

  Saturday (23rd) record warm low Saturday (23rd) record high Sunday (24th) record warm low Sunday (24th) record high
New York 83 (2011) 100 (2011) 80 (2010) 97 (2010)
Philadelphia 83 (2011) 100 (2011) 83 (2010) 98 (2011)
Baltimore 79 (1978) 102 (2011) 82 (2010) 101 (2010)
Washington 84 (2011) 102 (2011) 84 (2011) 101 (2010)

The most intense heat will break after the weekend, but even so, temperatures will remain a few degrees above normal in the Northeast next week.

This hot spell comes right after the news earlier this week that the first six months of 2016 combined to make the hottest first half of a year on record globally.

As temperatures continue to rise from the increase in greenhouse gases, extremely hot summer days, like what the Northeast will see this weekend, will become more frequent. As a result, that combination of heat and humidity will drive up the number of danger days in the summers to come, increasing the risk of heat related illnesses in places unaccustomed to seeing it so often.