Stifling heat wave settles in for the long haul

• Human-caused climate change is making heat waves more likely, longer lasting and more intense.

Zoom in: A heat dome is building in intensity across the Ohio Valley and lower Great Lakes. It is forecast to shift east-northeastward this week, and potentially peak at record intensity at this time of year for New England.

• Heat domes are strong ridges of high pressure that feature sinking air beneath them, which causes air to sink, temperatures to increase and squelches rain chances. Air circulates around them in a clockwise direction.
• Typically, the hottest air is found beneath them. But in this case, the most sizzling conditions are likely to be found to the west and northwest of the center of the high pressure area, where winds blowing from southwest to northeast will transport unusually hot and humid air.
• Late this week, the center of the heat dome is likely to meander south-southwestward and weaken somewhat, eventually becoming elongated across a large expanse of real estate.
• This will leave much of the U.S. smothered in hotter-than-average conditions.

Threat level: According to WeatherBELL Analytics, by Sunday, about 265 million people in the U.S. are forecast to see air temperatures reach or exceed 90°F, with many of them experiencing heat indices of about 105°F.

• Big cities to be affected include Chicago, Columbus, Albany, Syracuse, Pittsburgh, Washington, D.C., New York City, Hartford and Boston.
• Even northern Maine, which typically is just emerging into warmer conditions at this time of year, is likely to see temperatures soar into the upper 90s°F this week.
• NWS forecast offices are pointing to the long-lasting nature of the heat wave. They also cite the fact that it is occurring so early in the season and societal factors — some schools are still in and the federal Juneteenth holiday occurs on Wednesday — as increasing the risks of significant human health impacts.

What they're saying: "Heat Risk Impacts show Major impacts from Tuesday all the way through next Sunday," wrote the NWS forecast office in Philadelphia on Sunday.

• At the NWS forecast office in Caribou, Maine, forecasters are concerned about record warm overnight minimum temperatures and daytime highs well into the 90s through midweek. They said in an online forecast discussion that any airflow off the Atlantic won't cool the state significantly:
• "Needless to say it is going to be hot and [a] one or two degree difference here or there will not mean a hill of beanhole beans during this heatwave!" forecasters wrote Sunday.
• Forecasters at the NWS office in Boston noted that temperatures touching 100°F in spots "would challenge all time record highs for the month of June."
• Meteorologists there are confident that many records will be set for hottest overnight minimum temperatures for this time of year.

By the numbers: NWS forecasts for Wednesday and Thursday show highs in the mid-to-upper 90s stretching from Missouri to northern Maine.

• Chicago's O'Hare International Airport reached a record daily maximum temperature of 97°F on Monday, according to NWS data.
• In Washington, the National Weather Service forecasts highs at between 93°F to 99°F during the Monday through Saturday period, with the hottest temperatures arriving later this week.
• Philadelphia may be slightly hotter, at 95°F to 99°F.
• It's possible that some locations in the Mid-Atlantic and Northeast will see temperatures climb to or above 100°F late this week, given the computer model guidance, which has trended warmer in recent days.
• D.C. hasn't hit 100°F since 2016.

Zoom out: Dozens of daily high temperature records are in jeopardy from this heat wave, as are June hot streak milestones and hot overnight minimums. Some monthly records could be set as well.

• The NWS has hoisted excessive heat watches and heat advisories for 60 million people Monday through Friday in the Midwest and Northeast.
• Its new joint heat risk product, developed with the CDC, shows "Extreme" health risk levels moving from the Midwest and Ohio Valley to the Northeast during mid-to-late week.
• At Levels 3 and 4, which correspond to "Major" and "Extreme" risk, the extreme heat will affect those without adequate hydration or access to effective cooling, NWS warns.

What's next: The NWS Weather Prediction Center said in a forecast discussion Monday that the heat wave will likely continue beyond Friday, as the heat dome slides slowly slides south and elongates from west to east, but does not diminish completely.

• The NOAA climate outlook for June 29 to July 12 shows an above average likelihood of hotter than usual conditions across much of the country, including areas affected by the ongoing heat wave.

Editor's note: This a breaking news story. Please check back for updates.