January 8, 2018 – Widespread precipitation through Tuesday with accumulating snow mainly limited to the higher elevations.
Mainly light precipitation is spreading inland across NorCal early this morning as moisture and lift increase across the region from the southwest. Snow levels may start out near the pass levels this morning as evaporative cooling associated with the onset of precipitation briefly lowers them, but they will quickly rise above the passes by afternoon as the atmosphere saturates.
Models remain in good agreement with a wet event for the area into Tuesday. Light precipitation this morning will increase and become moderate to occasionally heavy tonight. Precip will decrease by Tuesday afternoon. Rain totals of 1-2″ are expected for the Valley with 2-5″ for mountains and foothills. Motorists are urged to allow extra time for travel with slippery conditions and ponding of water on roads. Some potential continues for minor debris flow across recent burn scars, but confidence is not high enough to warrant any flood watches at this point.
Snow levels will lower somewhat by Tuesday afternoon as the closed low pivots toward SoCal, but heavier accumulations will likely remain above the main northern Sierra passes (I-80 & US-50). A Winter Weather Advisory starts 4 am Tuesday and continues until 10 pm Tuesday evening. Snow amounts from 6000 to 7000 feet should be around 4 to 7 inches, with up to 1 to 2 feet possible over higher peaks. Travelers in the mountains should be prepared for delays and likely chain controls across the passes.
Breezy to locally gusty southerly winds are expected to peak tonight. The strongest winds will be over the northern San Joaquin Valley where gusts of around 30 mph are expected.
Drier weather returns for most of the region by mid-week, but a couple weak systems may bring some light precipitation to the northern mountains and northern Sierra Wednesday and Thursday.
Extended discussion (Friday through Monday)
High pressure ridging over the west coast will bring fair and dry conditions at the end of the week and through the weekend. Friday, Saturday and Sunday will see fair skies and daytime highs 10 or more degrees above normal. Upper west coast ridge begins to break down early next week but models not in good agreement. ECMWF brings Pacific frontal system into NorCal on Monday for precipitation over entire CWA while GFS moves a much weaker slower system in Monday evening for little or no precipitation. Have put in precipitation for northern CWA Monday but confidence remains low until models come into better agreement.