Officials warned Wednesday of a "public health emergency" when residents lacking heat are exposed to cold temperatures -- forecast for the low 30s -- this weekend.
All 17 Red Cross shelters on Long Island will remain open, a spokesman said, and visitors are welcome for any duration, even if it's just to warm up.
He said dozens hit the streets Wednesday afternoon.
David Stark, a meteorologist with the National Weather Service in Upton, said daytime temperatures Thursday and Friday will be in the high 50s, with lows in the 40s at night, but the weekend will be cooler, with highs in the lower 50s and lows in the mid-30s.
Monday and Tuesday are expected to be similarly cool, with a chance of showers Tuesday.
"It's going to feel more like November," Stark said.
Israel said he's worried about the pace of power restoration.
"This started as a flood and wind emergency and graduated to a power emergency," he said. "Now, it has become a public health emergency. When temperatures dip into the 30s, it becomes a threat to people's health and safety."
Suffolk County Executive Steve Bellone agreed, saying, "We understand that people without power are in a difficult circumstance."
The very young and the elderly are at risk in such chilly temperatures, said Gino Farina, program director in the department of medicine at Long Island Jewish Medical Center.
Farina said there is no universal cutoff at which it is too cold to sleep, but temperatures in the 30s and 40s will be a challenge, he said.
"That's a bit too cold for you to get a comfortable sleep unless you have a bunch of blankets and are next to somebody else and can share the heat," he said.
Long Islanders could experience hypothermia if they are exposed to cold temperatures for a long stretch, he said. Symptoms include an accelerated heart rate and elevated blood pressure as the body tries to warm itself. If a person's temperature continues to drop, they can become confused, lack coordination, "become more lethargic and can lapse into a coma."
Farina said he's concerned about people using their stoves to heat their houses and lighting fires without cleaning their chimneys, which can lead to carbon monoxide poisoning.
Craig Cooper, a spokesman for the Red Cross, expects the number of people using shelters will increase as the air cools.
He said the shelter population has steadily decreased from a high of 2,000 Monday night.
Cooper said some residents already "spend the night at the shelter and get out of their cot, go to work and then come back."
With Kevin Deutsch