"Record" heat envelops millions from Midwest to D.C.

• About 61 million people are under heat watches and warnings as highs near 100°F, combined with high humidity, lead to dangerous heat indices between 105°F and 115°F.
• PJM Interconnection, a power grid serving Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and the District of Columbia, issued a maximum generation alert for Tuesday due to the hot weather, warning that conditions could require "all generators to operate at their maximum output capability."
• According to the National Weather Service, the heat risk will reach "extreme" levels from St. Louis to Chicago, northeast to Grand Rapids, Michigan on Tuesday.
• This risk level applies to "anyone without effective cooling and/or adequate hydration," the NWS states, with likely effects to "Most health systems, heat-sensitive industries and infrastructure." At least 11 states are projected to see at least "major" levels of heat risk.
• Such extreme heat can quickly cause heat illness for even the healthiest of individuals, particularly anyone working or exercising outside.

Zoom in: As a weak cold front advances southeast from Canada, the core of the hot air and greatest heat risk is forecast to shift to the southeast through Wednesday, with a high temperature near 100°F in Washington, D.C..

• The National Weather Service forecast office in Chicago is warning of "near record warmth" on Tuesday, with an excessive heat warning in effect for the city.
• The agency is forecasting a high temperature of 98°F in Chicago on Tuesday, which is about 20°F above average for the date. The high temperature in St. Louis is forecast to reach 102°F on Tuesday, and may not fall below 90°F until Saturday.
• By Wednesday and Thursday, the area of hottest temperatures is likely to shift slightly south and east, focusing from roughly St. Louis to Washington, with July-like temperatures and humidity levels creeping up the Mid-Atlantic coast.

Between the lines: Public schools are going back into session or have already started for many cities, shining a spotlight on a lack of air conditioning in some school districts.

• Chicago Public Schools started Monday with heat restrictions in place.
• Typically, heat waves this severe in the Midwest are confined to July into early August, but human-caused climate change has been lengthening the heat season across many parts of the U.S.

• Climate change is also intensifying individual heat waves and raising their likelihood.
• Underscoring the health threat the heat wave poses, this round of scorching temperatures comes amid news that heat deaths in the U.S. last year reached 2,325 people. This was a peak since at least 1999.

Context: The heat is associated with a strong area of high pressure aloft, also referred to as a heat dome, that is slowly migrating from region to region, bringing scorching temperatures to areas under its influence.

• Along the heat dome's periphery, rounds of severe thunderstorms are likely Tuesday through Wednesday, in what meteorologists refer to as a "ring of fire" weather pattern. Such storms knocked out power to thousands in the Twin Cities Monday night, for example.
• The source air mass has already crushed heat records from Texas into the Central Plains.
• Abilene, Texas, hit its hottest temperature on record, at 113°F, on Aug. 21, as records fell elsewhere across the state as well.
• Amarillo, Texas also saw its hottest August temperature, at 108°F, and saw its longest streak of consecutive 100-degree days or above at 10 days.

The bottom line: This heat wave will not be record-setting in terms of duration, but it may tie or break numerous daily and possibly even a few monthly records.

Editor's note: The story is updated with a notice from the PJM Interconnection.