A significant storm will bring rain, wind and very heavy mountain snow through Tuesday. Mountain travel will be hazardous through Tuesday with chain controls and possible road closures. Another system will bring additional rain and mountain snow Wednesday into Thursday with mountain travel impacts likely.
Discussion
Widespread hazardous travel through Tuesday. Travel will be dangerous, especially for the mountain passes, and is discouraged. Roads through the passes are likely to experience chain controls and possible closures. Whiteout conditions, downed trees, and power outages are all possible.
A mid-level trough will sharpen off the Pacific coast today and Tuesday, finally ejecting eastward Tuesday evening. This trough will then amplify into a negative tilt by Wednesday across the Great Basin before closing off and lifting into the Plains.
Increasingly confluent mid-level flow and warm air advection ahead of surface fronts moving eastward within the flow will combine with a robust Pacific jet streak to drive ample moisture onshore noted by PWs reaching +1 to +2 standard deviations above the climo mean.
This setup will supports a prolonged atmospheric River event for the region with latest GEFS AR Scale maintaining the AR 2 rating of Moderate for the area. The digging shortwave will drop south through the period amplifying in nature and increasing low level flow.
Gusty winds will continue through the day with gusts 35 to 45 mph in the Valley and as highs as 60 mph over the mountains. Winds will decrease behind the front Monday evening into the overnight.
Monday into early Tuesday very heavy snow is expected over the Sierra Nevada where the most intense ascent will produce snowfall rates that may reach 3″/hr, due to the nearly orthogonal flow within the AR driving intense upslope into that range.
Ensemble probabilities are high for 18 or more inches across most of the Sierra, with locally in excess of 5 ft likely above 6000 ft.
Forecast remains generally on track with slight higher snow levels at around 5500 feet, expected to drop through the day down to 4000 feet by late afternoon.
Rain totals will range from 1 to 3 inches in the Valley and 3 to 7 inches in the foothills and lower mountains.
By Wednesday the primary trough shifts into the Plains, an impressive closed low will dive along the British Columbia coast towards Washington before opening as it shifts eastward.
This will be accompanied by another surge of moisture into Northern California, with precipitation diving back into the Sierra and northern CA ranges late Wednesday afternoon after only a brief respite.
Even colder air will be in place with this system with lower snow levels. Initial snow level could be down to 1,000 feet Wednesday morning over Shasta county with some light accumulation possible in Redding. Snow levels may start around 2,500 feet over the Sierra with a few inches over some foothill communities before snow levels rise during the day.
Ensemble probabilities are high for 8 inches or more across the northern Sierra, and southern Cascades. 3-day total snowfall across the Sierra may reach 8 feet over the highest peaks.
Extended Discussion (Friday through Monday)
Drier weather expected Friday and Saturday as EPAC ridge builds inland. Some gusty north to east wind possible Friday into early Saturday. Model discrepancies exist Sunday with GFS swinging short wave trough through interior NorCal while EC maintains ridging.
Forecast uncertainties leads to following NBM attm with a slight chance of showers over the Shasta mountains Sunday. Models in better agreement with stronger system impacting the CWA Monday with another round of widespread precipitation possible.