Long Island weather: Partly sunny and mild

Wednesday’s sun will be partly eclipsed by clouds, as it will be on Thursday — and by late Friday night, rain is likely, forecasters said.

Temperatures will remain much as they have: fairly balmy for winter, with daytime highs in the mid to upper 40s and nighttime lows just around freezing, according to the National Weather Service’s Islip office.

The weekend is at least halfway promising, with sun expected after noon Saturday. And Monday’s holiday, Martin Luther King, Jr. Day, should be at least partly sunny, with the same mild temperatures.

Rain, then more sun

The odds of rain are 70% for both Friday night and Saturday before noon, but once the rain quits Long Island, the sun should return.

That should boost Saturday’s high back up to 47 degrees, just one degree cooler than Friday. Sunday will be a chillier 41 degrees under sunny skies.

Here is how the weather service describes the rain it predicts for Saturday, courtesy of a trough moving to the Great Lakes from the Northern Plains: "Precipitation ends late morning into the afternoon with a breezy west-northwest flow developing behind the low. The strength of the winds late Saturday and Saturday night will ultimately be determined by speed and intensity of the low as it lifts up into the Canadian Maritimes."

(A trough is an "elongated area of relatively low atmospheric pressure," the weather service says. The rising air of low pressure systems, which exert less force on the Earth’s surface, can cause vapor to condense, creating clouds and showers, scientists say. High pressure delivers fair weather as the air dries as it falls toward the ground.)

For boaters, the weather service warned: "Ocean seas are likely to continue to build through this period with potential for gusts on the ocean 25-30 knots." That would be a top wind speed of nearly 35 mph.

Daytime highs of 38 and 41 degrees, respectively, are forecast for Monday and Tuesday.