Arlene: Northeast a 'virtual sauna bath' -- with more to come

Forty-three states are experiencing temperatures in the 90s this week, and 16 have issued heat advisories or warnings. The heat is expected to break over the weekend. NBC's Tom Costello reports and TODAY's Al Roker gives the forecast.

By Matthew DeLuca, Staff Writer, NBC News

A sweltering heat wave that has pushed temperatures into the 90s from Cape Cod to Oregon will continue Wednesday as emergency crews, power companies and commuters sweat it out with little relief in sight before the end of the week.

The countrywide slow bake is expected to continue until a cold front moves south on Friday over the Plains, making thunderstorms likely, the National Weather Service said. Temperatures along the East Coast looked unlikely to tail off until Sunday, forecasters said.

Low temperatures read like highs; the lowest temperature in New York City since Sunday has been 77 degrees. And the heat index -- the "feels like" factor -- is well into triple digits for tens of millions of people as clear, sunny skies combine with high humidity in many places.

?Plain and simple, this week may feel the worst of any week for this summer in the Northeast,? Accuweather.com meteorologist Alex Sosnowski told Reuters. ?The I-95 region will be a virtual sauna bath.?

People across the country struggled to stay cool, with power companies urging customers to turn off nonessential devices and heat advisories in effect for 16 states including parts of New York, Massachusetts, Rhode Island, Ohio, Pennsylvania and Connecticut.

Every state but Alaska, New Mexico, and Hawaii stood to hit 90 degrees on Wednesday, NBC News meteorologist Bill Karins said.

?A lack of a breeze in the humid conditions at night will make it very rough in urban areas without air conditioning or a fan,? Sosnowski told Reuters.

Andrew Burton / Getty Images

Chess players use umbrellas to find shade in Union Square on July 16, 2013 in New York City.

An 18-year-old New York City Council intern passed out at a mayoral campaign stop on Tuesday morning and waited 30 minutes before she was picked up by an ambulance, NBC New York reported. Area hospitals have seen a 5 to 20 percent spike in heat-related illnesses, according to the station. At one point, mayoral candidate Christine Quinn called the city?s police commissioner to speed up the process.

There was little relief in many cities overnight as temperatures stayed high through the evening hours.

?In this case, it?s the longevity of the heat wave, that poses the biggest concern, rather than the magnitude of the temperatures themselves,? Weather Channel meteorologist Jon Erdman said.

Compounding the sizzling heat in Prince Georges County, right outside Washington, D.C., crews worked to make emergency repairs to a major water main ? but some residents in the Maryland county will not be able to get more than a trickle out of their faucets on Wednesday.

?The more judiciously you use the water, the longer it will last,? Kira Lewis, a spokesperson for the Washington Suburban Sanitary Commission, told NBC Washington. ?We?re reminding folks new water is not coming into the system Wednesday.?

The water outage could last three to five days in the affected area, NBC Washington reported.

Two deaths in the Chicago area on Tuesday were cited as possibly related to the heat, the Cook County Medical Examiner?s Office told NBC Chicago. Temperatures in the city could feel as high as 105 degrees in the city on Friday, forecasters said.

A 78-year-old Kentucky man whose body was found around 9 p.m. local time on Tuesday died from heat exhaustion, the Mason County deputy coroner said, according to NBC affiliate LEX18. Roy Allen Bishop had gone missing from his home about 12 hours earlier.

The near-record temperatures and humidity have already strained the electricity supply in New England, causing the region?s power grid operator to urge people to turn off lights and appliances they do not need as a precautionary measure.

As temperatures soar across the country, NBC's chief medical editor Dr. Nancy Snyderman shares how to beat the heat and stay healthy, and describes who is most at risk from the high temperatures.

?As the heat continues to build throughout the week, electricity demand is expected to increase significantly, which is likely to result in tight system conditions,? said company vice president Vamsi Chadalavada in a release. ISO New England expected to hit a peak power demand of about 27,800 megawatts on Thursday.

Power usage also threatened to reach new highs in New York, where Con Edison said crews have worked around the clock to repair outages to more than 7,600 customers since temperatures began to rise again on Sunday. No usage records have been set yet, the company said, but that could change as the state stood to see no relief on Wednesday or Thursday.

In Maine, where temperatures reached 90 on Tuesday, the spiking mercury had people rushing to purchase new air conditioning units, causing some stores in the Portland area to sell out over the weekend, hardware store owner Tim Currier said.

?It takes good hot weather like this to push people over the edge,? Currier told local newspaper the Portland Press Herald. ?This is the time when everyone starts calling because they?re panicking and can?t find [air conditioners] anywhere.?

Most people will need to wait for the weekend to get more than temporary relief from the stifling heat, however.

?There is relief ahead,? the Weather Channel?s Erdman said. ?Cooler air should arrive in the upper Midwest beginning Friday. By this weekend, the Northeast will receive the cooler air with open arms. All this will come at the cost of severe thunderstorms, however.?