For the second time this season, Massachusetts is contending with another oppressive wave of heat.
Temperatures are expected to climb above 90 degrees over the next few days, but it could feel as hot as 104 degrees on Monday, Tuesday, and Wednesday afternoon, according to the National Weather Service.
The heat wave is expected to continue through Wednesday for most of the region before “somewhat cooler” temperatures arrive by Thursday, forecasters said.
Scattered showers and thunderstorms also are possible Tuesday and Wednesday, but there is a “greater risk for unsettled weather” later in the week, forecasters said.
In the Boston and Milton areas, 90 degree days are on the rise — occurring both earlier and more often. The extreme heat falls in line with the trends that meteorologists have been tracking.
The sizzling temperatures forecasted statewide have prompted the National Weather Service to issue a heat advisory through 7 p.m. on Wednesday for the vast majority of Massachusetts as well as northern Rhode Island.
In counties in Western Massachusetts including Franklin, Hampshire, and Hampden, the heat advisory is in place through 7 p.m. on Tuesday. In these areas of the state, the heat index values — or how hot it feels outside when relative humidity is factored in — could reach into the upper 90s on both Monday and Tuesday afternoon.
In response to the forecasted heat wave, Acting Mayor Kim Janey has issued a heat emergency, which is in effect in Boston from Monday through Wednesday.
The heat and humidity will peak on Monday and Tuesday, the National Weather Service said in a tweet. There will be little nighttime relief, according to forecasters, and overnight lows will remain above 70 degrees in urban areas.
Because of climate change, more nights are hot. Over the course of the past several decades, recorded low temperatures of 70 degrees or higher in the Boston and Milton areas have steadily increased.
And as the climate continues to warm up, it’s Boston’s “heat islands” — primarily concentrated in lower-income neighborhoods throughout the city — that will most acutely feel its effects, including heat-related illnesses.
Both the heat and humidity will reach an oppressive level on Monday. Forecasters said there will be plenty of sunshine, as well as gusty southwest winds.
Forecasters said the “stage is set” for “oppressive to dangerous” heat and humidity across most of the region on Monday afternoon, with temperatures starting to “soar” after sunrise.
Temperatures throughout the region will likely be around the middle 90s and higher, forecasters said. But dew points, expected to be near 70 degrees, will make it feel as though it is as hot as 98 to 104 degrees during the afternoon away from the south coast.
Across much of Rhode Island and southeastern Massachusetts, southwest winds will keep temperatures in the 80s to the lower 90s, forecasters said, but it “still will be quite humid.”
In one map from the National Weather Service, Boston is expected to reach an afternoon high temperature of 95 degrees, with several areas of the state including Bedford and Amherst predicted to reach even higher.
Another map from the National Weather Service highlights the heat index values expected on Monday throughout the region. In some areas of Massachusetts, such as Boston, it could feel as hot as 101 degrees outside, while other communities like Greenfield may seem as hot as 103 degrees.
The excessive heat risk forecasted by the National Weather Service on Monday — noted by the yellow coloring on the map below — does not apply to as much of the state as what is expected on Tuesday.
In the areas colored yellow on the map — defined as the “limited” risk level — the National Weather Service says that heat exhaustion is “possible with prolonged exposure.”
The majority of the eastern portion of the state — aside from the south coast — along with parts of Western Massachusetts, is designated as being at limited risk.
During the later part of the day Monday, isolated thunderstorms or showers may be possible in Western Massachusetts, forecasters said.
The mugginess residents are likely to feel throughout the day will continue into the night, forecasters said. Temperatures likely will hover between the low and middle 70s for most areas of the state.
The heat wave is likely to continue through Wednesday, forecasters said.
“Oppressive to dangerous” heat will again be in store for the majority of southern New England on Tuesday, forecasters said. Between the surface winds and abundance of sunshine, temperatures could be a couple of degrees higher on Tuesday than on Monday across much of the region.
Temperatures likely will remain in the middle to the upper 90s statewide on Tuesday, and some record highs may be challenged, forecasters said.
The south coast may also see temperatures climb into the high 80s. As a result, the entire region is under a heat advisory for Tuesday except for the Cape and Islands, forecasters said.
Dew points throughout the region could climb into the 70s, according to a map from the National Weather Service.
Across most locations, it could feel as hot as 104 degrees, forecasters said. Less wind is expected on Tuesday, which will add to the oppressive heat and humidity.
The excessive heat risk for Massachusetts increases on Tuesday, with more of the western and eastern parts of the state expected to be affected. Some areas remain in the limited risk category, but other communities south of Boston are expected to be under “elevated” risk.
Forecasters said there is also a risk for isolated to widely scattered showers and thunderstorms on Tuesday afternoon and evening.
The heat wave will likely continue through at least Wednesday before a “transition to cooler than normal temperatures for Fourth of July weekend,” forecasters said.
A map from the National Weather Service shows temperatures climbing into the upper 90s Wednesday across the state. In Boston, it may be around 97 degrees.
Dew points on Wednesday are also likely to remain in the upper 60s to low 70s.
Forecasters also said that the chance for afternoon and evening showers and thunderstorms “will increase each day during the week.”
The National Weather Service warned that the hot temperatures and high humidity expected may cause heat-related illnesses to occur.
Massachusetts residents are advised to drink plenty of fluids, remain out of the sun, and stay cool over the course of the next few days.
Sabrina Shankman of the Globe Staff contributed to this report.