Storms were moving through Long Island, with a severe thunderstorm watch on for western Suffolk and southern Nassau until 8 p.m., the National Weather Service said Monday.

The service said the storms may produce heavy rainfall, isolated hail and gusty winds of up to 70 mph.

In a 4:59 p.m. statement, the service said the heavy rain was expected to produce flooding in areas such as Valley Stream, Long Beach and Glen Cove. Rainfall rates were reported at 1 to 2 inches per hour.

The flash flood warning lasts until 8 p.m. Monday, the service said.

About 8,000 customers had been without power Monday evening, officials said. The outages were “most likely caused by the lightning,” said Elizabeth Flagler, a spokeswoman for PSEG.

But that number went down as PSEG reported 5,638 customers without power as of 6:23 p.m., with the widest outages in the Town of Hempstead. Those included 2,606 in Valley Stream and 935 in Lynbrook, according to PSEG Long Island’s website.

advertisement | advertise on newsday

A thunderstorm watch means “conditions are favorable” for such severe weather, the service said, while a warning means storms could be imminent.

The service had earlier issued a heat advisory, in effect until 8 p.m., for Nassau County and for Suffolk County west of the William Floyd Parkway.

The Department of Environmental Conservation, citing high ozone levels, had declared an air quality advisory, in effect until 11 p.m., for all of Long Island.

The service defines a heat wave as three straight days or more of temperatures at or above 90 degrees. The high at the airport hit 96 Saturday and 90 degrees on Sunday.

The service said heat index values on Monday could reach as high as 100 in some parts of the metropolitan area. Heat index values for Long Island are expected to be around 95, the service said.

But the actual high temperatures did not hit 90 degrees on Long Island Monday.

The advisory on air quality means an increase in different air pollutants, which can have adverse health effects for those with respiratory issues including asthma, the state agency said.

Conditions clear by Tuesday morning with sunshine and heat dominating. Daily highs will be in the low to mid-90s, the service said.