January 19, 2021 – Strong, gusty north to east winds will continue into this evening. Less windy conditions expected by mid-week, then rain and snow chances return for the end of the week.
Second wave of the NorCal wind event is underway as strong northeast upper jet, associated with the closed low digging southwest through the Great Basin, moves over the region. Water vapor imagery is showing very strong subsidence increasing since late Monday evening, and a portion of the stronger winds aloft are making it to the surface over the higher elevations of the northern Sierra where gusts over 70 mph are occurring. Caltrans/CHP traffic info indicating multiple reports of trees down across the higher elevations from near Mt Lassen south across the northern Sierra.
Strong north-south and east-west surface pressure gradient continues across NorCal early this morning (RDD-FAT is over 16 mbs!). Strongest winds expected across the higher elevations this morning as highest NAEFS wind anomaly remains from 12Z-18Z, but winds will likely peak late this morning and this afternoon in the valley as stronger winds mix down to the valley floor.
Winds will start to decrease this evening. Locally gusty winds are expected to linger over the Sierra tonight, weakening significantly by Wednesday morning. Min temps will cool into the 30s by Wed and for the rest of the week. The dry air mass will preclude much widespread frost until the latter half of the week, although cloud cover, or the lack of it, will make it rather problematic for now.
Over the course of the week, temperatures will gradually cool with readings returning to around average by the end of the week. Precipitation chances return to the region beginning Friday.
Extended discussion (Saturday through Tuesday)
Ensembles continue to show a wave of energy dropping into NorCal early in the forecast period. Lingering showers are forecast over the Sierra along and south of I-80 over the Sierra through Saturday afternoon. Snow accumulations of 2 to 6 inches are possible at pass levels, leading to slick roads and mountain travel delays. Breezy north winds may develop Saturday, but winds are not anticipated to be as strong as the current offshore wind event. ECMWF ensemble suggests medium probabilities of wind gusts exceeding 20 knots. Dry conditions return Saturday night into Sunday afternoon as the system exits the region.
Potentially wetter conditions are forecast early next week as ensembles depict a stronger/colder system that could potentially bring snow down into the foothills. A rain/snow mix may be possible over far northern Sacramento Valley Tuesday morning. Active pattern may continue through the week. NBM guidance shows temperature readings trending downward through the extended.