That means hazardous driving conditions, with drivers advised to slow down, use low-beams and allow for added distance between cars, the weather service said.
The fog is created as warmer air arrives over the cold, snow-covered ground, said Mike Layer, weather service meteorologist in Upton. As of midafternoon Monday, the temperature at Islip was 44 degrees, up from an overnight low of 15, he said.
Layer also cautioned Long Island drivers to beware of "ponding" on roadways, a result of Monday's rain, combined with melting of compacted layers of snow and ice along roads.
Temperatures are expected to fall to a low of 35 degrees Monday night with a west wind of 9 to 14 mph, the service said.