The whole wide west is craving a wet winter, and the big El Niño brewing in the Pacific looks poised to satisfy. Today, the National Weather Service released its monthly update on the weather phenomenon, and the meteorologists are saying this year's is one of the strongest El Niños on record.

But that forecast comes laden with caveats. For one, it matters where and when that precipitation falls. If it falls too far south, it won't fill crucial reservoirs. Neither would it help much with the Pacific Northwest's wildfires. And California hasn't even entered its proper fire season, so if the rains come too late the Golden State could face (even more) epic conflagrations. Finally, no matter how much water the El Niño brings, it's unlikely it will be enough to sate the Golden State's four-year deficit.

Typically, the Pacific Ocean is warmest in the equatorial waters around southeast Asia. In El Niño years, the warm patch moves east, towards South America. "You get a huge redistribution of heat in the Pacific," says Bill Patzert, a NASA climatologist at the Jet Propulsion Laboratory in California. This flip causes all sorts of weird weather: hurricanes, droughts, flooding, and massive storms. "The equatorial stretch of the Pacific stretches a one third of the way across the Earth, and the ocean covers thirty percent of the planet," says Patzert. "So when that system kicks up, serious stuff goes down."

For the month of August, the National Weather Service measured temps that were at least 2 degrees above normal in the equatorial waters off South America. That makes this year's El Niño the third strongest on record, in terms of heat redistribution. And while effects are already being felt around the globe—India's crippling monsoon, for instance—El Niño means one thing to most Americans: rain.

"We're one month away from where we should see impacts this winter," says Mike Halpert, NOAA's deputy climate prediction director, who delivered the news. The only way to predict those impacts is by looking at the past. The graphic up top shows this year's El Niño compared to 1997's. That year was one of the largest weather events on record. "It doubled rainfall for most of California in the coast and lowlands, and in the Sierras it doubled snowpack," says Patzert. Snowpack is the important thing, because it's basically free water storage that trickles into the reservoirs throughout the year.

But even if this El Niño is particularly wet, it's still not likely to alleviate California's four-year drought. The state's wettest El Niño, in 1983, dropped nine times the annual average rainfall. The state would need at least that much to bring its reservoirs back to normal.

Beyond California, the implications are a mixed bag. "Generally, the canonical El Niño gives the southern tier of the US a good soaking, and the northern tier has a mild winter," says Patzert. Past El Niños have drenched and dusted the Rockies. Those mountains feed the Colorado River, the southwest's major watershed. But El Niños tend to leave northern states drier. This is bad news for the Pacific Northwest, which is suffering its own water shortages (not to mention wildfires).

But nothing is certain. "It's really hard to predict precipitation," says Amy Clement, an atmospheric scientist at the University of Miami. This is especially true in the midlatitudes, where equatorial and polar weather systems meet.

If the rains do come, it's critical that they come soon and last long. That's not just so the West can restock its water. While Washington, Oregon, and Alaska have had hot, burning summers, California's fire season doesn't start until late fall, when the hot, dry Santa Ana winds blow in from Nevada. "The interesting race is to see what will come first, the Santa Anas with their incendiary dry heat, or the El Niño," says Patzert. "We're so focused on February raindrops in San Francisco and Los Angeles, but that's only one part of the story."