San Francisco weather: Rain is coming

Weather forecasts call for significant rain in the Bay Area on Thursday night or Friday, possibly complicating conditions in recent fire zones.

The National Weather Service, an outlet operated by the National Oceanic and Atmospheric Administration, predicts a “slight chance of rain in the evening, then chance of rain after midnight” on Thursday, with “rain likely” on Friday and then possibly continuing as long as Sunday.

The Weather Channel concurs, also calling for Friday rain and then likely again next Thursday. Weather Underground predicts a 50 percent chance showers will start as soon as late tonight or Friday morning.

Early November showers would be good news for those fretting about the possibility of the state falling back into drought. This as meteorologists struggle to anticipate what the winter might bring.

“We’re kind of shaking our heads a little,” Deputy Director of NOAA’s Climate Prediction Center said in October. “There’s no way to say if you’ll be on the wet side or on the dry side in California.”

All of the uncertainty stems from the La Nina effect, which NOAA defines as “unusually cold ocean temperatures in the Equatorial Pacific.”

This usually means that winter temperatures will be “warmer than normal in the Southeast, and cooler than normal in the Northwest,” but in this case the precise effect on California’s rain patterns isn’t clear yet.

Photo by Justin Sullivan/Getty Images

Complicating things further, heavy rain during the winter could pose an extra hazard in fire areas. NWS warns that flash floods may occur within recently burned zones. The agency cautioned in 2012:

Rainfall that would normally be absorbed will run off extremely quickly after a wildfire, as burned soil can be as water repellant as pavement. As a result much less rainfall is required to produce a flash flood. A good rule of thumb is: If you can look uphill from where you are and see a burnt-out area, you are at risk.

Wired magazine this week noted the possibility that heavy rain will mix with fire ash in burnt areas to produce limestone, a seemingly freak event that essentially “paves over” wooded places.

And of course, weather watchers at outlets like Bloomberg pointed the finger at last winter’s rains for leading to a huge vegetation bloom that then dried out in the hot summer and served as ideal fire stuff, a cycle which can repeat itself in other areas.

Forecasts also predict rain in Santa Rosa and other parts of the north state where some fires still burn, so nearby residents should use caution.

• NWS Forecast
• Weather Channel Forecast
• Weather Underground Forecast
• La Nina Or Neither [SJ Mercury]
• What Is La Nina? [NOAA]
• Fire May Contain Toxic Ash [Curbed SF]
• Rain, Relief On the Way [Curbed SF]