Flash flood watch in SF through Wednesday

Yet another storm is on its way to San Francisco beginning Monday—and once again, it will bring the threat of potentially dangerous flooding

The National Weather Service (NWS) has issued a flash flood watch for the entire Bay Area, in effect from February 25 through February 27.

According to the forecast:

Rainfall totals during this period of time range from one to two inches in the Santa Clara Valley, two to five inches near the coast and Bay shoreline, and six to 10 inches in the Santa Cruz Mountains.

Prolonged moderate to heavy rain on already saturated soil will result in rapid rises on small rivers and creeks. Flash flooding is possible from late Monday through Tuesday night. Urban flooding in low lying areas is likely. Widespread shallow landslides, rockslides, and debris flows are likely in steep terrain. Commutes Monday evening through Wednesday morning will be adversely impacted.

There’s also a wind advisory in effect for the same period, anticipating gales of “20 to 30 MPH with gusts up to 50 MPH.”

NWS defines a flash flood as “a flood caused by heavy or excessive rainfall in a short period of time, generally less than six hours.”

The weather agency warns that enough rain under the right conditions can create “raging torrents” that “rip through river beds, urban streets, or mountain canyons sweeping everything before them” and can happen “within minutes or a few hours of excessive rainfall.”

Most floods are not as dramatic, and a flood watch is not necessarily an indicator of imminent danger. (As opposed to a flood warning, which means that “flooding has been reported or is imminent.”)

However, there is always some risk associated with large storms. The National Severe Storms Laboratory warns that “in the U.S. floods kill more people each year than tornadoes, hurricanes, or lightning” combined, and that “densely populated areas” are particular flood risks:

The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential.

Sometimes, streams through cities and towns are routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking garages, and basements can become death traps.

The National Disaster Management Authority warns not to attempt to walk or drive through flood waters, particularly moving water, as it can be very difficult to judge how deep water in flooded areas may be or how fast it’s moving—or will be moving in a few seconds.

“Don’t drive into flooded areas. If floodwaters rise around your car, abandon the car and move to higher ground,” flood protocols advise.

In other weather news, all of this stormy weather has helped eradicate drought conditions across the state, at least according to U.S. Drought Monitor.

A drought map issued last week for the state of California determined that only 3.77 percent of the state is currently suffering from “moderate drought” or worse, and only a third of California is classified as “abnormally dry.”

Note that the drought map also represents the judgment of the researcher who compiled it, and other qualified commentators may disagree. Nevertheless, it’s a dramatic turnaround from just seven weeks ago, which saw more than 75 percent of the state in drought.

Drought conditions persist only in the very northernmost regions of the state, mostly in Siskiyou County.

• Flash Flood Watch [NWS]
• Floods & Flash Floods [NWS]
• Severe Weather 101 [NSSL]
• Flood Advisory [NDMA]
• California Drought Map