You can get an accurate forecast at Weather.com.

Here are some general Weather Conditions in New York City:

January, February
Weather: Cold, wet, probable snow or mixed frozen precipitation. Temps can range from 10-40 degrees Fahrenheit. With a wind chill, it can get below zero.
What to Wear: Dress in layers because stores and restaurants will likely keep things warm. Bring an umbrella, warm coat, scarf, hat, gloves, and some good weather resistant shoes.

March, April
Weather: March is not as frigid as February, but you’ll still need a warm coat. March temps can range from 30-50 degrees Fahrenheit.
What to Wear: Bring an umbrella, warm coat, and maybe gloves. It probably won’t snow, but it has been known to snow in April every few years.

May, June
Weather: Spring in New York City is the best time to be in the city, without doubt. Spring usually means less humidity and temps between 50-80 degrees.
What to Wear: Bring an umbrella. In May, a light spring jacket or a mid-weight sweater is probably the heaviest thing you’ll need. If you are caught without an umbrella, buy a cheap one at the ubiquitous street vendors that pop up at the first drop.

July, August
Weather: Hot, humid, sticky, sometimes oppressive. And that’s outside, never mind the subways. Temps range from 70-95.
What to Wear: Wear light clothing: natural fibers are best, especially cotton and linen.

September, October
Weather: Starts off hot, humid, sticky, & uncomfortable., but ends a bit chilly. From Sep 1 – Sep 20, anticipate 70-80 degree weather. From Sep 21 – Oct 10, expect 70 degree highs. Beyond that, expect the highs to be in the 50-70′s. Halloween has been known to be in the upper 30′s or lower 60′s.
What to Wear: Bring summer clothes with an extra layer or two depending on how close to Halloween you come. If you’re coming in the back half of October, don’t expect to wear shorts, and you might not always need a jacket, but you should probably have one.

November, December
Weather: Expect highs in the 40-50 range in November and 30-40 range in December.
What to Wear: Bring a heavier jacket and consider a hat, scarf, and gloves.