Oh, oh.

The high hopes that Long Island sky watchers had for a clear viewing of Sunday night's rare combination of a supermoon with a total lunar eclipse are being put to the test.

Computer models for a potential storm system have been offering details that are "highly variable," the National Weather Service said in a Thursday forecast discussion.

The thinking as of early Thursday afternoon was for cloud cover of around 60 percent, with precipitation potential of around 21 percent, for the 10 p.m. to midnight time frame Sunday, said Tim Morrin, National Weather Service meteorologist based in Upton.

That covers the main-attraction time of total eclipse, when the supermoon, completely within the dark shadow of the Earth, appears to turn red -- resulting in its added moniker of "blood moon."

That seeming color change happens as sunlight moves through Earth's atmosphere, and its sunrises and sunsets get projected onto the moon, says Sue Rose of East Meadow, president of the Amateur Observers' Society of New York.

advertisement | advertise on newsday

She rattled off a steady stream of occasions when she and fellow enthusiasts have gone out on seemingly hopeless, cloud-filled nights, only to have clouds part, even partially, at just the right moment to reveal the celestial event of interest.

It's character-building, she said, to "be prepared for the worst," give it a shot anyway, and when you "defeat all the odds to say, 'Yes, I did it.'" Besides, photos that include traces of clouds can be so much more interesting than that perfect shot.

Sunday's eclipse is so special because it's of a supermoon, the name for full moons that appear supersized as a result of their coming closest to Earth in its elliptical orbit. Supermoons happen about once a year and can appear as much as 14 percent larger and 30 percent brighter than typical full moons, according to NASA.

The larger-than-usual bright, white moon will darken as it moves into Earth's shadow before turning a reddish hue at total eclipse. It will gradually darken again and revert to white.

The last time "planetary dynamics" lined a supermoon up with a lunar eclipse was in 1982, and the next one won't arrive until 2033, according to a Nasa.gov post.

@Newsday

And there's more: It's also a harvest moon, which is a full moon that comes nearest to the autumnal equinox, which was Wednesday, the first day of fall.

Rose said that regardless of the ultimate forecast, and cloud cover or not, she'll be out there Sunday with her eyes to the sky.

What's especially helpful about a total lunar eclipse is that it lasts for over an hour, as opposed to other events that can last only minutes.

Her advice for novice sky watchers is to go out on a clear night and identify the moon's position in the 9 p.m. - partial eclipse start time -- to midnight time frame. Then, even if it is cloudy Sunday, "go out every 10 to 15 minutes, as you never know."

Barring that, there's always the live stream option on NASA.gov starting at 8 p.m.

advertisement | advertise on newsday

Here's a viewing schedule provided by Rose:

Moon starts rising due east, Sunday, 6:34 p.m.

Partial eclipse begins with left side of moon starting to darken, 9:07 p.m.

Total eclipse begins with moon completely within dark shadow of Earth, 10:11 p.m.

Maximum eclipse, with moon, now red, in the center of the shadow, 10:47 p.m.

advertisement | advertise on newsday

Total eclipse ends, with moon starting to darken, 11:23 p.m.

Partial eclipse ends, moon back to white, 12:27 a.m., Monday.