WASHINGTON – After what has largely been a cool start to meteorological summer in the northern and eastern U.S., temperatures are expected to swing in the opposite direction shortly after astronomical summer arrives.According to the FOX Forecast Center, a stout ridge of high pressure will move eastward across a large swath of the country, bringing sweltering heat to major cities from Chicago to Detroit to the major urban centers along the Interstate 95 corridor on the East Coast.The prime time for this heat wave will be from Sunday through Thursday, when hundreds of millions of Americans will experience temperatures ranging anywhere from a few degrees above average to 20 degrees above average.While relatively few locations may actually hit the 100-degree mark, the combination of high temperatures and elevated humidity levels will make it feel significantly hotter. Heat indices, which measure how the temperature feels to the human body when factoring in humidity, are forecast to soar well into the triple digits in many areas.7 FACTS ABOUT HEAT WAVESAmong the areas expected to be affected are Chicago, Detroit, Pittsburgh, Philadelphia, New York City and Washington.New York City has only reached the 100-degree threshold three times during the month of June, while Philadelphia has done so nearly a dozen times, with the most recent instance occurring in the mid-1990s.The National Weather Service is expected to issue a range of heat-related alerts in the coming days. These may include Heat Advisories, Extreme Heat Watches and Extreme Heat Warnings, depending on the expected severity and duration of local conditions. The alerts are intended to warn residents about the dangers of prolonged exposure to high heat, which can lead to serious health impacts such as heat exhaustion, heatstroke and dehydration.Heat waves pose a particularly high risk to vulnerable populations, which include children, the elderly, outdoor workers and those without access to air conditioning. In communities with heat response plans, cooling centers are usually opened for residents to take advantage of when temperatures reach dangerous levels.ALASKA’S FIRST-EVER HEAT ADVISORY ISSUED AS FAIRBANKS SEES TEMPS SOARTemperatures are expected to return to more typical levels by the final days of June and early July as the ridge of high pressure treks eastward, allowing for the entrance of frontal boundaries, which will help induce more cloud cover and rainfall.The heat likely won’t be gone for long, as the hottest days of the season usually occur in mid- to late July, as the effects of the year’s direct sunlight and solar radiation are felt across the Northern Hemisphere.