Severe thunderstorm watch in effect for Suffolk County until 9 p.m.

A severe thunderstorm watch is in effect for Suffolk County until 9 p.m. Friday, according to the National Weather Service in Upton.

The watch includes Long Island, New York City, portions of the lower Hudson Valley, northeast New Jersey and southern Connecticut.

The weather service said additional strong to severe thunderstorms are possible late this afternoon and this evening as the atmosphere becomes unstable with strong daytime heating. Forecasters said a more potent upper-level disturbance and an approaching cold front will provide a more consolidated trigger for storm development, though the front is expected to arrive later than usual — generally during the evening and overnight.

As a result, storms may be fairly isolated during the afternoon and become more scattered or widespread this evening. Any storms that develop could produce damaging wind gusts, the weather service said.

In Riverhead, the National Weather Service forecast calls for mostly sunny skies this afternoon with a high near 93 degrees and heat index values as high as 98. A slight chance of showers and thunderstorms is forecast between 8 to 11 p.m., with a low around 68 and a 20% chance of precipitation.

The severe thunderstorm watch comes as hot and humid conditions continue across the region. A heat advisory remains in effect for much of the area through 8 p.m., though coastal southeast Connecticut and eastern Long Island are expected to remain cooler and are not included in the advisory, forecasters said.

Residents should be prepared for rapidly changing conditions if storms develop, including sudden strong wind gusts and brief reductions in visibility in heavy rain.

In case of an emergency, dial 911.