Downtown Denver skyline at dusk with the City Park lake in the foreground. (iStock)

It is not even July, and an epic summer heat wave is just getting started in the Lower 48. Record-high temperatures are expected across the Central and Eastern U.S. through the weekend. After a brief pause, forecast models suggest the heat dome will build even more next week.

The most intense heat Thursday was reported in the High Plains. Denver tied its all-time hottest temperature, which was most recently set June 26, 2012.

By 11:31 a.m. Thursday, the Mile High City had already reached 100 degrees, which surpassed the record for the date. The previous record for June 28 was 99 degrees in 1986.

It only took three more hours for Denver to climb to 105 degrees, which tied the city’s all-time hottest temperature. Denver has reached 105 degrees four other times in its historical weather record that dates back to 1878.

Several other records were set or tied for June 28 in Colorado:

  • Akron — 101 degrees (previous record 101 in 1963)
  • Boulder — 98 degrees (previous record 96 in 1931)
  • Fort Collins — 101 degrees (previous record 97 in 2013)
  • Greeley — 102 degrees (previous record 101 in 2011)
  • Limon — 100 degrees (previous record 99 in 1970)

The most extreme heat was spreading east into the Mississippi Valley and Great Lakes on Friday. In northern Wisconsin and Michigan’s Upper Peninsula, temperatures will be more than 20 degrees hotter than normal. Highs will reach at least the 90s across much of the Midwest. Factoring in  humidity, the heat index will exceed 115 degrees in many places.

In northern Illinois, the National Weather Service is warning people to limit outdoor time and check on elderly and sick neighbors. The Weather Service expects the heat index to reach 105 degrees in downtown Chicago, but it will climb to 115 or higher southwest of the city.


As we wrote on Thursday . . .

The ridge of high pressure will be at its strongest Saturday and Sunday as it shifts focus to the East. Temperatures will climb well into the 90s, and the heat index will reach triple digits. Sunday will be the hottest day for New York City, where the afternoon temperature will soar to 97, and the heat index will top out at 105.

In Washington, highs could reach at least 90 for nine days in a row, potentially longer, beginning Friday. The capital’s longest streak of 90-degree days is 21, which occurred in 1980 and 1988.

Saturday highs (and heat index)

  • Washington — 95 degrees (99)
  • New York City — 92 degrees (94)
  • Boston — 91 degrees (93)

Sunday highs (and heat index)

  • Washington — 96 degrees (102)
  • New York City — 97 degrees (105)
  • Boston — 89 degrees (95)

Record highs will probably be broken over the next several days. This list, compiled by weather.com, includes some of the more notable records on the chopping block:

  • Saturday: Detroit (96 degrees); Rochester, N.Y. (96); Burlington, Vt. (93)
  • Sunday: Concord, N.H. (99); Syracuse, N.Y. (94); Burlington (96); Worcester, Mass. (94); Buffalo (93); Caribou, Maine (89)

This heat wave will be brutal because of the temperature, but it is packing an extra punch because of the extreme humidity.

The dew point — a measure of how much moisture is in the air — will be in the low 70s across most of the area affected by the heat wave, making it even more oppressive and potentially dangerous. Combined with high temperatures, high dew points lead to extreme heat index. Sometimes referred to as the “feels-like” temperature, heat index takes into account the humidity and is more representative of how hot the air feels than the temperature alone.

Given the forecast for highs in the 90s and 100s alongside dew points in the 70s, the National Weather Service issued widespread heat advisories and excessive heat warnings Thursday, covering 15 states in the central United States.

A weak low-pressure system and associated cold front could provide modest relief early next week, but forecast models anticipate high pressure to return quickly. Both the European and the American (GFS) models predict extreme high pressure to continue across the eastern half of the country through next weekend.