The pineapple express is a tropical moisture plume that runs from near Hawaii to the West Coast — one of the biggest rainmakers California gets all winter.

Meteorologists call it an atmospheric river. Firehose is more like it.

(IN DEPTH: What's an atmospheric river?)

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The blueprint

The setup is straightforward: a low parked in the trough of the jet stream offshore, high pressure over the Four Corners, and the two working together to steer Pacific moisture straight into California, Oregon or Washington. It moves fast like a conveyor belt.

What isn't straightforward is what happens when a Super El Niño gets involved — and there have only been two of those on record.

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Super El Niño adds fuel

El Niño tilts the odds toward a wetter, more active Pacific storm track. What it doesn't do is decide where the water lands.

(MORE: Your complete guide to El Niño)

Look at the strong El Niño winters on record and the spread is striking. In 1982/83 and 1997/98, California ran far above average — deep green across nearly the whole state. In 2015/16, an event that ranked among the strongest ever measured, the map came out patchy and Southern California never got the winter it was bracing for.

(MORE: Is a record Super El Niño inevitable?)

The difference between those winters wasn't strength. It was position — where the warmest water sat, and where that pushed the jet. This is why the first place forecasters look is the sea surface temperatures.

Right now those temperatures are running above average across the equatorial Pacific — the kind of warmth that loads the atmosphere with moisture and tilts the West toward a wetter winter.

(MORE: How wildfire season fuels winter flooding)

Before the first storm

The pineapple express will show up this winter and most of the regions that will face flood impacts were struck with wildfire disasters this summer. The compounded disasters make them vulnerable this winter.

(MORE: What is debris flow?)

So whether it shows up as a rainmaker or a flooding disaster depends partly on where it lands — and partly on who was ready.

Preparation is the one part of this forecast you control.

• Check your flood insurance now — standard homeowners policies don't cover flood
• Map two routes out if you live in a canyon, foothill, or floodplain, in case the first one is cut off.
• Sign up for county emergency alerts, which often go out before federal wireless alerts do.
• Clear your gutters and storm drains before the first plume arrives.
• Build a go-bag you can grab in under five minutes.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her master's in Earth and Atmospheric Sciences from the Georgia Institute of Technology.