Update, 12:55 p.m.: Developments on Thursday's mostly North Bay storm:

• The National Weather Service has issued a flash flood warning for areas along Willow Brook and Lichau Creeks, in southern Sonoma County between Petaluma and Rohnert Park.
• The California-Nevada River Forecast Center says the Russian River will reach flood stage late Friday night and crest nearly 3 feet above flood stage late Saturday morning.

Rainfall totals reflect the dramatic disparity in precipitation, with locations in the North Bay getting steady rain with occasional downpours since just after 3 a.m. and areas south of San Francisco and throughout the East Bay getting virtually nothing. Here's what rain gauges showed through 1 p.m. Thursday.

Original post: We've been hearing for several days about how the Bay Area is about to be walloped with another round of heavy rain on the last two days of our workweek. The storm would be fed by an atmospheric river of moisture flowing from Hawaii and beyond.

Thursday dawned bright and beautiful in most of the region south of the Golden Gate. Where's the rain?

If you live in northern Sonoma County, you know the answer: The atmospheric river is in place, and rain began falling in the predawn hours.

Venado, the superwet location in the Russian River watershed west of Healdsburg, recorded 2.48 inches of rain in the seven hours that ended at 8 a.m. Santa Rosa is seeing heavy rain, too, with 1.20 inches falling by 8 a.m. at Charles M. Schulz-Sonoma County Airport just north of town.

The North Bay downpours will feed a rapid rise on the Russian River, which is forecast to reach flood stage at Guerneville late Friday night and crest about 3 feet above flood stage late Saturday morning. Inundation is forecast for some areas of Guerneville and nearby Rio Nido.

Farther south, very little rain has been recorded from the Thursday storm -- yet. The Middle Peak weather station atop Mount Tamalpais, generally a fabulously wet spot, has gotten just a hundredth of an inch -- .01 -- as of 8 a.m.

And from San Francisco south -- nada.

Weather models are showing that steady rain could be falling in San Francisco by 9 a.m. -- with a rather showery pattern for the rest of the day and into the evening.

South of San Francisco NOAA's High-Resolution Rapid Response models suggest a big disparity in rainfall from Marin and Sonoma counties, where most locations are forecast to receive 1 to 2 inches by this evening, to San Francisco, Contra Costa and northern Alameda County -- .25 to .50 of an inch -- to the South Bay, where San Jose is forecast to get a few hundredths of an inch.

More abundant rain is forecast for the entire Bay Area, including the Santa Cruz Mountains, before the storm system exits Friday evening.