Flash Flood Alerts as “Pineapple Express” Arrives in San Francisco Area

A flash flood warning was issued by the US National Weather Service (NWS) on Saturday 8th February for Sonoma County in the San Francisco Area, California, as storm systems pelt the drought-stricken region.

Sonoma county flood warnings
Flash flood warnings map from NWS.

According to forecasters, a flash flood warning means flooding is already happening or is imminent, and one was issued as Sonoma County and the North Bay Area were being drenched with heavy rain, causing flooding in creeks, streams, underpasses and other low-lying areas.

According to the NWS, the warning ended at 7:30pm local time on Saturday evening. A flash flood watch was also issued by the NWS for the North Bay area of San Francisco on Sunday 9th February, including Point Reyes National Seashore, interior mountains and valleys in the North Bay. The flood watch notice was to last until 3pm local time.

A flood watch is a notice to be on the lookout for floods, but differs from Saturday’s flood warming where flash floods were observed or imminent.

Urban flooding including street ponding is likely in many areas, and small landslides or debris flows can be expected in the vicinity of steep hills.

NWS officials have advised people not to try to cross swiftly flowing water or water of unknown depth and to be especially cautious at night when flooded areas are more difficult to see. They are advised to rather turn around instead of proceeding and risking being drowned, as water levels can rise rapidly.

If flash flooding occurs, people are advised to act quickly and move to higher ground to escape flood waters.

The downpours have caused power outages, damaged roads, and forced flight cancellations and delays. Over 2,100 San Francisco area customers were without power on Saturday afternoon, the intersection of Highway 121 and Highway 12 in Schellville was blocked by floodwaters, and 59 flights were cancelled and many more delayed at San Francisco Airport.

2 to 4 inches (50 – 100mm) of rain is forecast to fall in the San Francisco, East Bay and South Bay areas by Monday, with 4 to 6 inches (100 – 150mm) over the Santa Cruz Mountains and Marin County, and 7 to 9 inches (175 – 225mm) over parts of Sonoma and Mendocino Counties.

The storms are the result of a so-called “atmospheric river,” which transports large amounts of moisture from the tropics near Hawaii eastward to the US west coast. The ‘Pineapple Express’, as it is sometimes known, these ‘rivers of rain’ are long, narrow bands of highly concentrated moisture that are formed in the Pacific Ocean and move eastward until they hit land, where, sometimes carrying 15 times as much water as the Mississippi river, they bring downpours and often flooding.

Although this may be the heaviest rainfall in the Bay Area for the last four years, it will not necessarily end the drought in northern California. The region has only received between 2 and 3 inches (50 – 75mm) of rain since July 2012, about 25% of the normal rainfall for the period. If the weekend’s storm doubles the total, the bay area cities will still have received only 50% of their normal rainfall.

“We need a bunch of these. It’s a decent beginning, but we need more,” said meteorologist Jan Null, with Golden Gate Weather Services in Saratoga.
Another storm is forecast for Wednesday.

sources: ABC Local; CBS Local and also here; Mercury News