'We don't want to cry wolf:' How forecasters predict LA's next huge rain storm

The weather service also made late adjustments to what the computer models were showing. Over a four-day period, models said to expect 8 to 10 inches of rain in the San Gabriel Mountains and 4 to 5 inches of rain in downtown L.A.

Meteorologists thought the computer models were underpredicting the projected rainfall totals, so they added a couple of inches to that forecast, Sirard said.

Their instincts proved correct. The weather service's final forecast was for 8 to 14 inches of rain in the mountains and foothills through Feb. 6. And that was very accurate — the highest rainfall amount recorded in the San Gabriel Mountains over that period was 13.86 inches.

"A lot of us have been here for 25 years. So we know the weather patterns of what can cause the maximum amount of rainfall here," Sirard said. "You get the high amount of water vapor in the atmosphere, you get the strong jetstream aloft nearby, you have the strong southeast to south low-level flow — all that moisture throughout the atmosphere, from the ground to 20,000 feet or more — all gets squeezed up into the mountains."

In some areas, the storm proved to be a rainmaker more prolific than even experienced meteorologists had anticipated.

Before the storm began, the weather service had forecast 6.37 inches of rain would fall over a four-day period in downtown Los Angeles. Some people might've been hard-pressed to believe such an astonishing amount: On average, downtown gets 14.25 inches of rain in an entire year.

For the four-day period ending at 9 p.m. Feb. 6, 8.66 inches of rain fell on downtown L.A.

Still, the range of the forecast totals helped accurately guide the kinds of warnings that needed to be issued. Once forecast totals in lower-lying cities reach "5, 6, 7, 8 inches, the impacts are pretty much the same" in terms of flooding and landslide risk, Kittell said.

That messaging helped fuel substantial storm preparedness, so officials and residents were not caught completely off guard when land began sliding in a number of hillside communities across L.A. County, including north of Hollywood, Beverly Hills and Westwood, as well as in Baldwin Hills and Hacienda Heights.

Pre-storm warnings also let residents know to stock and stack sandbags. And officials readied response teams like swift-water rescue crews that were needed across Southern California.