The National Weather Service kept its heat advisory running through 7 p.m. tonight for both halves of Nassau and the western half of Suffolk, after a Sunday that qualified for one too. The air temperature this afternoon will reach about 91. The number the advisory is built on is the heat index, the combination of temperature and humidity that describes what your body actually has to shed, and that is forecast to reach 98.

Where the advisory applies, and where it does not

The advisory covers four forecast zones: Northern Nassau, Southern Nassau, Northwest Suffolk and Southwest Suffolk. That takes in essentially everyone from the Queens line to about the middle of Suffolk County. The East End is not included today. The forecast index east of the county's midsection stays under the trigger, helped by the water that surrounds progressively more of the Island as you move east.

The trigger itself is specific, and knowing it makes the advisory easier to read. The Upton forecast office issues a heat advisory when the index is expected to sit between 95 and 99 for two or more consecutive days, or to reach 100 to 104 for any length of time. Today is the second consecutive day in that first category. The advisory went up Sunday afternoon and was extended through Monday evening for the same four zones before most people were awake this morning.

The shape of the day

The morning and early afternoon are the heart of it: mostly sunny, the temperature climbing toward 91, the index peaking in the mid to upper 90s after midday. Late afternoon brings the possible exit. There is a 40 percent chance of showers and thunderstorms after 5 p.m., and whatever falls will knock the temperature down without doing much about the humidity. The overnight low only makes it to about 75, which is the kind of night that keeps a house warm into the next morning.

Tuesday is barely better on paper. The forecast is mostly sunny and again near 91, with a heat index up to 95, just under the two-day advisory shape rather than clearly out of it. Whether Tuesday gets its own advisory is a call the forecast office will make as the numbers firm up. Wednesday is the real break in the pattern, with a high near 88 and the index no longer the headline.

What to do with the next ten hours

Stripped of boilerplate, the advisory's practical content comes down to a few things. The people at meaningful risk are seniors, anyone with a chronic health or mental health condition, and anyone in a home without air conditioning, which by afternoon can be hotter inside than the street is outside. The advice for that last group is not to tough it out at home but to spend the hottest hours somewhere cooled, and to keep doing so on each hot day rather than treating one visit as done.

For outdoor work, the advisory leans on federal workplace safety guidance: frequent rest breaks in shade or air conditioning, and moving anyone overcome by heat somewhere cool immediately. Heat stroke is an emergency that gets a 911 call, not a rest break.

Nassau County's own machinery uses nearly the same threshold as the weather service. The county opens its temporary weather space when the heat index holds at 95 or above for two consecutive hours, a bar today will clear by early afternoon. That space is the former Family Court building at 1200 Old Country Road in Westbury, and it operates around the clock. The county posts its full list of 2026 cooling centers on the Extreme Weather Centers page of its website, which is the place to look for one closer than Westbury.

The pattern behind the advisories

This is the sort of heat event that does its damage quietly. Nothing today will break a record, and 91 in the second week of August does not sound like news. The advisory exists because of the stacking: a second consecutive day at an index near 98, on top of a night that never cooled off, is measurably harder on a body than either day alone, which is exactly why the two-day rule is written into the criteria. The cumulative version of heat is what fills emergency rooms, and it is also the version that a reader can plan around.

So the planning version of today looks like this. Errands and anything physical go into the morning. The beach and the pool work, with more water than feels necessary. The afternoon belongs indoors wherever the air conditioning is, and the evening thunderstorm, if it shows up, is the signal that the worst of it is over. Tuesday gets the same treatment on a slightly lower flame, and Wednesday the Island gets its summer back at a livable setting.