The first big winter storm of the season appears to be headed our way, but it won't be a direct hit, and there are still some variables that could affect how much rain we actually get.

You haven't heard the term "atmospheric river" on the local news since about February, but it's that time again. A bombogenesis or "bomb cyclone" is taking shape over the Pacific, and it stands to to bring an atmospheric river of rainfall to the Bay Area starting late Tuesday night or early Wednesday morning.

As the Chronicle's meteorology team explains, this storm is headed for the Pacific Northwest, but the outer bands of it are expected to lash Northern California with plenty of rain by Wednesday.

The National Weather Service confirms this, saying that confidence is growing "that heavy rain and gusty winds will impact portions of the Bay Area" this week. This could be mostly confined to the North Bay, however San Francisco is certain to get some rain — and there is still an unknown about how long the bomb cyclone will stall over the coast.

"The atmospheric river is expected to slowly drift back and forth between the Golden Gate and the Oregon border Wednesday through Friday, occasionally stalling completely," the Chronicle says.

It's that stalling that raises concern, as stalled atmospheric river storms can dump significant rain in a short span of time. The biggest such storm we've seen in the last couple of years was the New Year's Eve storm of 2022, when San Francisco saw 5.5 inches of rain in 24 hours.

Such deluges, when they continue for days — that New Year's Eve set us up for a parade of storms that week, as you may recall — have the potential to create damaging flooding in low-lying areas. (And forecasters have warned in recent years that California could see a megaflood one of these years on par with one that hapened in 1862, filling the Central Valley with feet of water, drowning livestock, and completely flooding the city of Sacramento.)

But let's not get ahead of ourselves. Between Tuesday and Thursday, the National Weather Service is forecasting 1 to 2 inches of rain in San Francisco and parts of the Peninsula, with 2 to 4 inches falling in parts of the North Bay. San Jose and much of the East Bay will likely be spared much heavy rainfall, but parts of the North Coast around Ukiah have a higher chance of catastrophic rainfall. And even Cloverdale has the potential to see 6 to 8 inches over 48 hours.

The forecasters themselves are saying that the forecast beyond Thursday remains pretty uncertain. However, the iPhone Weather app shows Thursday as partly sunny, and no rain arriving in SF until Friday. That forecast may change.

Update: New forecasts put SF in the 2-to-3-inch range for rainfall, and the Chronicle is now calling this an "extraordinary" bomb cyclone. Any slight shift in the storm's trajectory over the next 48 hours could mean significantly more rain for us as well.