Sea­son’s first ‘at­mo­spheric river’ takes aim at Bay Area

Sys­tem could de­liver as much as 1 to 4 inches of rain to the re­gion

The Mercury News - - Local News - By Mark Gomez mgomez@ba­yare­anews­group.com

North­ern Cal­i­for­nia is off to a slow start to the 2017-18 rainy sea­son, but the ex­pected ar­rival Wednesday of the first “at­mo­spheric river” storm of the sea­son could quickly make up those early deficits.

The storm is on track to soak the Bay Area with wide­spread rain­fall, in­clud­ing the po­ten­tial for down­pours dur­ing the evening com­mute, ac­cord­ing to the Na­tional Weather Ser­vice.

Fueled by an “at­mo­spheric river,” a type of storm that gets its name from the plume of mois­ture com­ing from Hawaii into Cal­i­for­nia, the Sierra Nevada will also see a mix­ture of rain and snow, ac­cord­ing to the weather ser­vice. The storm will con­tain warmer mois­ture, which means most of the snow­fall will oc­cur at higher el­e­va­tions.

The rain is ex­pected to be­gin fall­ing late Wednesday morn­ing in the North Bay and then spread south of the Golden Gate, said Scott Rowe, a me­te­o­rol­o­gist with the weather ser­vice. Rain­fall to­tals are ex­pected to range from 2 to 4 inches in the Santa Cruz Moun­tains, 3 inches in

the North Bay moun­tains, and 1 to 2 inches in most cities.

The weather ser­vice has is­sued a flash flood watch for the North Bay, be­gin­ning Wednesday at 4 p.m, warn­ing that heavy rain will bring an in­creased risk of de­bris flows, mud­slides and flash flood­ing, in­clud­ing to the ar­eas burned in the deadly Wine Coun­try wild­fires.

While fore­cast­ers are still track­ing the tim­ing of the sys­tem, the brunt of the rain is ex­pected to hit most of the Bay Area af­ter sun­set, Rowe said. The weather ser­vice also is­sued a wind ad­vi­sory for Wednesday, warn­ing many lo­ca­tions in the Bay Area will ex­pe­ri­ence gusts be­tween 20 to 30 mph, with higher el­e­va­tions reach­ing up to 50 mph.

Com­ing off one of the wettest rain­fall sea­sons on record, North­ern Cal­i­for­nia so far is lack­ing in the pre­cip­i­ta­tion depart­ment. Rain­fall to­tals through­out the Bay Area are all be­low-av­er­age for the sea­son, which be­gan on Oct. 1, in­clud­ing Santa Rosa (3.04 inches, 78 per­cent of nor­mal). Oth­ers in­clude San Fran­cisco at .93 inches (41 per­cent), Oak­land at .69 inches (32 per­cent) and San Jose at .54 inches (39 per­cent).

This sea­son’s cur­rent to­tals are a far cry from this

time last year when a se­ries of Oc­to­ber storms kicked off what be­came one of the wettest wa­ter sea­sons on record in Cal­i­for­nia. For ex­am­ple, at this time last year, Santa Rosa had recorded 7.66 inches of rain, more than twice its cur­rent to­tal (3.04 inches).

The pre­cip­i­ta­tion amount so far this wa­ter sea­son in the 8-sta­tion North­ern Sierra In­dex, a key col­lec­tion of rain gauges from Lake Ta­hoe to the Mount Shasta area in the wa­ter­sheds above many of Cal­i­for­nia’s largest reser­voirs,

stood at 79 per­cent of nor­mal Wednesday morn­ing. So far this sea­son, 4.6 inches of pre­cip­i­ta­tion has been recorded; last sea­son was record-set­ting, with 94.7 inches dur­ing the en­tire sea­son.

In a typ­i­cal year, Cal­i­for­nia has be­tween 10 and 15 “at­mo­spheric river” storms which ac­count for up to 50 per­cent of the state’s rain­fall. The ar­rival Wednesday of the first “at­mo­spheric river” storm of this sea­son could make up most of the early short­fall.

“This sys­tem will def­i­nitely present it­self to make up some short-term deficits,” Rowe said.

A third storm could bring more rain to the Bay Area this week­end, ac­cord­ing to the weather ser­vice. Me­te­o­rol­o­gists ex­pect to is­sue a more de­tailed fore­cast in the com­ing days.

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