Long Island weather: Sunny and warm weekend, then rain 

Saturday and Sunday will offer some fine early autumn weather, with abundant sun, light southwest breezes and surprising warmth, forecasters said.

High temperatures during the day should rise into the mid-70s, before easing into the upper 50s to low 60s at night, and likely will hover in those ranges for the next week, the National Weather Service said.

This is unusually warm: Sunday’s predicted daytime high of 76 degrees contrasts with the mean temperature of 54.3 for October in Islip, according to weather service records that date back to 1963.

Weekend beach-lovers be advised: Hurricane Sam, about 255 miles east of Bermuda and 1,035 miles south-southwest of Newfoundland’s Cape Race at 5 a.m. Saturday, will cause "life-threatening surf and rip current conditions" along the U.S. and Canadian East Coasts starting Saturday, the National Hurricane Center said.

On Long Island’s East End, wind gusts over the Atlantic Ocean could hit nearly 30 mph. There are expected to be 14- to 15-second pauses between the swells Sam will cause, which will rise to about four to five feet on Saturday afternoon and peak at five to seven feet Saturday night, before subsiding during the day Sunday, the weather service said.

"Localized minor beach flooding and dune erosion impacts are possible Sunday, and possible into Monday, due to a combination of increasing astronomical tides and rough surf," the weather service said.

Sam, now a Category 4 hurricane — the most powerful ranking — with sustained winds of 130 mph, should remain offshore as it heads up the Atlantic coast, dwindling by Tuesday or so into a tropical storm, the hurricane center added.

Rainy week ahead

A high pressure system sliding down from New England is delivering clear skies as its falling air dries and cools.

That pattern will, however, allow what the weather service calls a back door cold front —because it is approaching from the south and west — to swing in, possibly dampening skies as soon as Sunday night.

Sunday night through Tuesday night is expected to be at least somewhat rainy and stormy. And while skies should clear Wednesday, there will be only a one-day respite before rain may resume Thursday and Friday, the weather service said.

The cold front, which may allow downpours and thunderstorms to develop by Sunday night, is arriving from the Midwest, where it is drawing moisture up from the western Gulf of Mexico, the weather service’s Weather Prediction Center explained.

"As the front moves eastward, the showers and thunderstorms will also extend from Great Lakes to the Central Gulf Coast by Sunday afternoon," it said. "On Monday, the showers and thunderstorms will slowly expand into parts of the Mid-Atlantic/Southeast."

The odds of showers for Long Island on Sunday night are 50%. Monday’s odds are 40% and that night, they rise to 60% before slipping back to 40% for Tuesday, the weather service estimated.

"While unsettled weather will be possible Tuesday through Friday there may also be periods when the chances of precipitation are lower or the possibility that portions of the region are even dry as surface high pressure builds across the northeast through Wednesday then remains across northern New England," it said.

One of the factors complicating the forecast is whether a north-south trough, an elongated area of rain-causing low pressure, will switch its orientation so that its points toward the west instead of the east.

If it does slant towards California, it is called a negative-tilted trough, which can produce severe weather, the experts cautioned.

"This is because there is strong southerly surface wind with its warm air underneath the incoming cold air in the upper atmosphere creating unstable conditions," the weather service said.

The change in its direction can also help thunderstorms form.