LOS ANGELES — When it came to forecasting L.A.'s biggest winter storm of the season, local meteorologists had a secret weapon: experience.

For sure, there was plenty of computer modeling available to indicate that Southern California was in for a severe — and potentially dangerous — soaking. But based on their expertise, forecasters at the National Weather Service in Oxnard correctly anticipated that even the machine-calculated, eye-popping rain totals were probably an underprediction.

When it comes to such a serious storm event, getting the forecast as close to correct as possible isn't just a matter of pride. Forecasters go to great lengths to assess a storm's strength so they can accurately inform the public about the dangers it may pose.

Forecasters will face another test in the coming days, when another storm is set for Presidents Day weekend. Showers are expected to move into western Los Angeles County on Saturday night before drying up Sunday morning. Then a second round of rain is forecast to begin Monday afternoon, ramping up during the evening and continuing through Tuesday night.

"We don't want to cry wolf and say, 'Oh, we're gonna get record amounts of rain, catastrophic flooding,' and then you get about half what you think. And people are like, 'That was no big deal,'" said Joe Sirard, a meteorologist at the National Weather Service office in Oxnard. "And then they'll tune us out. We don't want that to happen."

In this case, "We went a little bit above some of the models and, you know, we were right," Sirard said.

Sirard said the first indications of a potentially significant rain event emerged about 10 to 12 days ahead of the storm's actual arrival early this month.

To get an idea of a storm's possible strength, forecasters look at data generated by supercomputers that produce "ensemble forecasts" made from a series of model runs based on slightly tweaked initial conditions, Sirard said.

But the forecast is quite uncertain that far out.

Say you're trying to map out a forecast 10 days from now, when it looks like a storm is brewing. Half of the model runs might suggest 5 inches of rain will fall over a three-day period, but the other half could suggest less precipitation — sometimes significantly so.

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