(CNN)More than 10 million people remain under a heat advisory for Sunday across the Northeast as record temperatures could be seen from Washington, DC, to New Hampshire.
Afternoon temperatures could climb into the 90s around the region, with heat index values up to 100 degrees possible, said CNN meteorologist Haley Brink -- about 15 to 25 degrees above normal for this time of year.
The National Weather Service has forecast Boston could see a high temperature of 96 degrees Sunday. The daily record high for the day is 93 degrees, while the monthly record for May is 97 degrees, set back on May 26, 1880.
Boston records are kept at Logan International Airport, which is right on the ocean, so those temperatures tend to be milder than the temperatures farther inland. And if the sea breeze sets up in the right direction, it could prevent temperatures at the airport from reaching records.
In Worcester, Massachusetts, the daily record highs for May 21 and May 22 are 88 and 90 degrees respectively. The city tied the record Saturday at 88, and could surpass 90 on Sunday.
"We're definitely a little bit ahead of schedule," said Matthew Belk, a meteorologist at the weather service office in Boston. "The average first 90-degree day in Boston is June 8. It's a little bit earlier when you get out towards Hartford (Connecticut); May 30 is generally the average first 90-degree day."
Sunday, however, is set to be the last day of extreme heat for a while as a cold front is expected to push into the region this evening, bringing strong to severe storms and cooler temperatures.
A severe thunderstorm watch was in effect for parts of Delaware, Maryland, New Jersey, New York, Pennsylvania, Virginia and West Virginia until 9 p.m. ET, according to the Storm Prediction Center.
Thunderstorms were beginning to develop Sunday afternoon from west Virginia into central New York. The storms were expected to track slowly eastward, posing a risk of locally damaging wind gusts up to 70 mph and hail the size of ping pong balls.
The early season heat wave this weekend is all thanks to high pressure off the Eastern Seaboard resulting in southerly winds pushing hot and humid air across the Northeast -- bringing temperatures 20 to 30 degrees above normal for this time of year.
Records were also set in Richmond, Virginia, Saturday at 95 degrees and Hagerstown, Maryland, at 91 degrees, and records were tied in Philadelphia and at Dulles International Airport at 95 and 92 degrees respectively, according to weather service data.
Half the population feeling the heat
Nearly 170 million people, roughly 52% of the population of the Lower 48, were expected to feel 90-degree heat over the weekend.
On Sunday, New York City is forecast to hit 90, Philadelphia 92, Baltimore 91 and Washington, DC, 93, the weather service said.
"Over half of the US population will see temperatures at or above 90 degrees this weekend, and it's only May," said CNN meteorologist Pedram Javaheri.
For some areas, it isn't just the heat, but also the humidity bringing "feels-like" temperatures in the triple digits.
"Since many outdoor events are planned this weekend in the region, be aware of the heat, and take extra precautions if you work or spend time outside this weekend," said the weather service office in Baltimore/Washington.
Heat a 'sneaky killer'
Even though the calendar may not say it's summer yet, Mother Nature has other plans, so it's important to be aware of the health dangers associated with this heat wave. For example, do not ever leave children or animals in a hot car. Ever.
"Since this is going to be the first heat wave of the year, it's important to be really cognizant of making sure that you're aware of any heat illness or heatstroke-related symptoms and being especially cautious of them and having extra water on hand to combat that and the importance of shade," said Aaron Swiggett, a meteorologist at the weather service office in Raleigh, North Carolina.
Swiggett also pointed out the forecast temperatures are actually for the shade, not in direct sunlight. So keep in mind your forecast high temperature is actually going to feel even hotter in the direct sun.
"Heat is a very sneaky killer," said Chesnea Skeen, a meteorologist at the weather service office in Baltimore/DC. "A lot of people don't see it as a huge threat, but it's actually one of the largest killers as far as extreme weather goes."
Skeen emphasized the importance of taking the heat seriously.
"Make sure you're getting hydrated, staying in the shade, getting out of the sun when possible," said Skeen. "And keeping an eye on folks in your life that are more susceptible to heat, such as young and the elderly and those that might be compromised."
The good news is the heat wave will be short-lived. Once the cold front moves through the Eastern Seaboard on Monday, temperatures will drop back into the 60s and 70s for the Northeast and Mid-Atlantic.