By Jack Lee, San Francisco Chronicle The Tribune Content Agency
A bomb cyclone and strong atmospheric river are expected to fuel downpours across Northern California this week, with flood watches and wind warnings in effect starting Tuesday night.
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But whether parts of the North Bay face over 8 inches of rain from Wednesday through Friday or collect just a few inches is still up in the air.
Forecasts from Chronicle meteorologists provide a range of possibilities for how the storm may play out. Weather apps don't. Instead, they produce forecasts that predict what the exact temperature will be at a particular time or anticipate when specific amounts of rain will fall.
"That's ridiculous," said Andrew Oliphant, a meteorologist and climatologist at San Francisco State University's School of the Environment. This misses much of the nuance found in forecasts made by meteorologists, experts say.
"The biggest flaw, I think, in all of these weather apps is the way in which they tell you what's going to happen, as though it's the truth, with a capital 'T,'" Oliphant said.
Global weather models predict the future state of the atmosphere using complex mathematical equations. These models are updated every few hours, using the output from an earlier run and the latest weather measurements from a worldwide observation network.
"It's amazingly high quality," said Zan Stine, a climate scientist at San Francisco State University's School of the Environment.
These models are run multiple times, with slight tweaks in initial conditions to reflect uncertainty, resulting in an ensemble of predictions. The National Oceanic and Atmospheric Administration runs the American ensemble model four times a day.
If out of 20 model runs, 12% call for rain, that would point to a 12% chance of rain at a certain time, Stine explained.
Meteorologists consider such probabilities when making forecasts. This uncertainty is reflected in National Weather Service area forecast discussions, which are released multiple times throughout the day. Local forecasters, who have detailed knowledge about their areas, explain the reasoning for the latest forecast in these reports; that's not the case with the latest forecast in weather apps.
"The apps don't tell you any of the uncertainties or any of the variables or how the models are actually differing from each other," Oliphant said.
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Weather apps did a poor job of forecasting conditions in San Francisco during an October heat wave. One reason is these apps may just rely on individual runs for weather models, rather than ensembles, explained UCLA climate scientist Daniel Swain in an online presentation.
"They're not using the full suite of information," said Swain, who has often recommended directly relying on the National Weather Service website for local weather forecasts.
While weather apps may be precise - providing extremely detailed forecasts for specific times - that doesn't mean they're accurate, Oliphant added.
Some climate experts are concerned that the National Oceanic and Atmospheric Administration could face sweeping changes following the election of Donald Trump.
"I really think that if people are concerned about accuracy of forecasts, they need to make sure the government keeps funding NOAA," Oliphant said. "It is the government funding of that agency that produces the accuracy that we have."
Reach Jack Lee: [email protected]