Long Island weather: Mostly sunny Saturday, a little dicey Sunday

Sunny but chilly describes the weather for the first part of this weekend, but Sunday could be more eventful.

Saturday should be mostly sunny, but with a slight chance of rain out east, the National Weather Service said. A touch of winter in the form of light snow might accompany downpours on Long Island Sunday, forecasters said.

It should all depart in time for the sunshine to return Monday, they said.

Daytime temperatures over the next week will cling to the low 40s, but the nights will turn increasingly cooler. After nighttime lows hovering around freezing on Saturday, thermometers will descend into the mid-20s, the weather service said.

Snow on Sunday?

The clear skies anticipated for the next two days will be caused by a high pressure system, in which falling air keeps vapors from condensing into raindrops.

The brisk northeast wind partly results from the difference in air pressure between that weather pattern and its opposite, a low pressure system in the Canadian Maritimes, the weather service explained.

On Saturday, it said, “Persistent northeast flow may allow for the development of some ocean-effect showers for the extreme eastern portions of the area, though the chance remains low.”

At a minimum, clouds will likely sail in, especially out east.

Sunday’s possible showers, with the odds at 50%, should arrive mainly after 1 p.m.

They stem from a shortwave trough, a disturbance in the upper atmosphere that can push air up ahead of it, in the Upper Midwest, which in turn “forces a weak low pressure [system] to develop over the Great Lakes Sunday morning,” the forecasters said.

And that pattern then drives a secondary low pressure system, in which rising air can condense into vapors, to develop off the coast by Sunday night.

While 1 to 3 inches of light snow may coat interior sections of the tristate area — especially in the lower Hudson Valley and southern Connecticut — coastal Long Island and New York City will probably only get rain, the weather service said.

“But there remains a chance that some snow mixes in with the rain at the tail end of the event as the precipitation pulls out of the area late Sunday night," the weather service said.

It added, "Areas closer to the immediate coast will likely not see accumulating snow, though a dusting of snow on cooler surfaces can't be entirely ruled out."

The offshore low pressure system should exit swiftly.

Still, early Monday, the weather service said, “There could be some lingering precipitation across Long Island and southeast Connecticut as shortwave energy passes offshore."

And then the sunshine should return Monday and last through Wednesday, as another cloud-vanishing high pressure system “builds down out of southeast Canada later Monday into Tuesday.”

However, the tristate area could start to cloud over Tuesday as the Canadian Maritimes gains another low pressure system, which “slowly meanders offshore” by the middle of the week, the forecasters said.

By Wednesday night, the odds of rain are 30%. They rise to 50% on Thursday.

Any downpours could result from low pressure systems that could sweep in from the Midwest.

“The forecast becomes highly uncertain at the end of the week,” the forecasters said, as it depends on when those systems reach the Eastern Seaboard.

With Robert Brodsky and Joan Gralla