You won't need a calendar to mark the start of summer in the Northeastern United States.
The National Weather Service forecasts potentially record-breaking hot temperatures just as the season officially begins today, the summer solstice and longest day of the year.
Readings are expected to approach or top 100 degrees today and Thursday in cities including Philadelphia, New York and Boston. Health officials warned residents to drink water, stay out of the sun and in air conditioning, and to check on elderly neighbors and pets.
After enjoying relatively mild June temperatures, Philadelphia may see the mercury soar to 97 and 99 degrees midweek.
"You're talking about almost 15 degrees above normal," said Kristin Kline, a Weather Service meteorologist in Mount Holly, N.J.
Normally, the high for Philadelphia at this time of year is about 84 degrees -- closer to today's predicted low of 80 degrees. The city's highs in the next couple of days could break decades-old records of 98 degrees, set in 1931, and 99, set in 1923.
The wilting heat also will hit Boston-area residents hard. Triple digits are forecast for today -- 101 degrees -- followed by 99 Thursday, the Weather Service said. Current record highs for these dates are 98 and 95 degrees, respectively.
New York City's 1.1 million public school students are still in session for another week, and just 64 percent of classrooms are air-conditioned. Temperatures are expected to hit 97 in the city both days, about 20 degrees hotter than it was Tuesday in Central Park.
Students were being advised to wear light clothing and drink plenty of water, and schools have been told to limit outdoor playtime, city Education Department spokeswoman Marge Feinberg said.
Officials will be setting up 114 "cooling centers" at facilities across Philadelphia, officials said.
In Rhode Island, all regular public buses and trolleys will be free today due to anticipated air quality issues.
Moderate relief from the high mercury should come this weekend.