NWS issues heat advisory starting Wednesday, with high heat, humidity peaking Thursday and Friday

A mass of hot and humid air about to park above Long Island has prompted a heat advisory starting Wednesday for all of Long Island except for southeastern Suffolk, the National Weather Service said.

Earlier Tuesday, the weather service had limited the heat advisory to northern Nassau and northwestern Suffolk, but the advisory was expanded Tuesday night. It goes from 11 a.m. to 8 p.m. Wednesday and Thursday and could last through Friday and even Saturday.

"Hot and humid conditions are expected through the end of the work week, with heat index values ranging from the mid 90s to mid 100s through Friday," the weather service said in an updated forecast Tuesday night. "The heat may linger into Saturday depending on the timing of the cold frontal passage."

A heat advisory is issued by the weather service when a combination of temperature and humidity make it feel like it is 95 to 99 degrees for two or more consecutive days — or 100 to 104 for any length of time.

"We are bracing for the arrival of high temperatures that can become unbearable and dangerous for many of our residents," Nassau County Executive Laura Curran said in a statement Tuesday, announcing the opening of cooling centers.

Seniors and young children are especially vulnerable, the weather service said, as are those with chronic health problems, mental health conditions and those without home air conditioning. Outdoor work should also be reduced, if possible.

Excessive heat can lead to heat stroke or other heat-related illness.

A high of close to 90 is forecast Wednesday, the weather service, with excessive humidity — and possible thunderstorms. Temperatures are expected to soar into the low 90s both Thursday and Friday.

As a result of the elevated heat risks, Curran announced the opening of three countywide cooling centers on Wednesday: Cantiague Park, 480 West John Street, Hicksville, from 10 a.m.-8 p.m.; Community Room, Mitchel Field Administration Building, 1 Charles Lindbergh Boulevard in Uniondale from 10 a.m.-6 p.m.; and, the Wantagh Administration Building, 1 King Road in Wantagh from 10 a.m. - 6p.m.

Officials also advise you drink plenty of fluids, avoid outdoor exposure, wear a hat and loose-fitted clothing and check often on at-risk relatives.

Symptoms of heat-related illness include headaches, dizziness, fatigue, anxiety, nausea, vomiting and low pulse rate. Other symptoms may include cold, pale and clammy skin or hot, flushed dry skin, officials said.

Officials also warn never to leter children or pets in your vehicle, even for brief periods, since interior temperatures can rise dramatically in short periods of time -- even with windows left cracked open.

In anticipation of high electricty demand, PSEG Long Island said in a news release that it may choose to activate its voluntary Smart Savers Thermostat program to reduce loading on the system and create future savings for customers. About 31,000 customers have voluntarily enrolled in this program, said PSEG.

"With more people working from home, it’s more important than ever that we all do our part to conserve energy and reduce overall peak demand," said Michael Sullivan, senior director of Transmission and Distribution, PSEG Long Island. "PSEG Long Island has made preparations for this situation and our personnel will work according to plan to provide the electricity we all need. By working together, we can avoid exceeding load forecasts and ensure that everyone’s air conditioning stays on as we weather this heat."

With Matthew Chayes