Atmospheric river brings flash flood watch for SF [Updated]

Update: A flash flood watch is now in effect in San Francisco and the entire Bay Area too, effective Wednesday through Thursday morning, according to SF 72.

“Be cautious of flood-prone areas,” the SF Department of Emergency Management warned Wednesday, advising commuters to “drive slowly and carefully” and to “be especially cautious at night” when flood dangers may be more difficult to spot. “Never drive your car into water of unknown depth,” the warning adds.”

The “onslaught of precipitation” forecast for this week has thus far treated San Francisco fairly gently, but that may change as potentially hazardous weather convenes on the city over the next few days.

SF 72 adds that a High Surf Warning will follow “Thursday morning through Friday evening along all coastlines,” meaning “dangerous, battering waves are expected to pound the shoreline, bringing potentially damaging and life-threatening conditions.”

An atmospheric river, a long concentrated corridor of moisture in the atmosphere, will bring heavy rains and winds starting Wednesday. According to the National Weather Service [NWS], winds of up to 60 miles per hour will buffet the Bay Area starting tomorrow:

The National Weather Service in San Francisco has issued a High Wind Watch, which is in effect from Wednesday afternoon through late Wednesday night.

Winds [...] with gusts 50 to 60 miles per hour from early Wednesday afternoon through lates Wednesday night, as a strong cold front moves through the Bay Area. Damaging winds will blow down trees and power lines.

Widespread power outages are possible. Travel will be difficult, especially for high profile vehicles.

The warning anticipates “sustained winds of at least 40 miles per hour” during the highlighted period, over an area extending from Point Reyes in the north down to Monterey in the south.

NWS issues a High Wind Warning whenever it expects “sustained winds of 40 mph or higher for one hour or more.”

At the same time, the hazardous weather warnings also note that, while this “strong Pacific storm” is not likely to cause flooding, it’s still possible:

Wet weather moving across the region today and Tuesday will increase soil saturation, and therefore rapid rises of creeks and rivers will be possible. [...] Mainstem rivers, from the Russian and Napa Rivers in the North Bay to the Carmel and Big Sur Rivers farther south, are forecast to see significant and rapid rises Wednesday night into Thursday.

Although present projections keep them within their banks throughout, they could well approach or even exceed monitor stage. Smaller rivers and streams could flood, especially if heavy rain rates persist across a particular watershed. Flooding would be most likely in the more typically flood prone low-lying areas.

A flash flood watch has been issued for the entire Bay Area and Central Coast of California.

Even worse, areas across Northern California with recent fire damage, where “scarring” may make it difficult for the soil to absorb water, run additional risks of mudslides and flash floods.

“Latest data from USGS research instrumentation in the San Francisco Bay Area indicates that shallow soils on steep hill slopes are approaching saturation,” the forecast warns.

Rain is expected to continue but diminish in intensity through Thursday and likely break on Friday, though another set of storms may arrive over the weekend.

• Onslaught of Rain Predicted [Curbed SF]
• High Wind Warning [NWS]
• Hydrologic Outlook [NWS]
• SF Forecast [NWS]