Quiet, dry, and seasonable weather prevails through the early week time frame. A shift to more active weather then arrives by midweek and is expected to continue into the weekend. Periods of widespread Valley/foothills rain, gusty winds, and mountain snow (likely heavy at times) will be possible from Wednesday through at least next Monday.
Discussion
As of early this afternoon, quiet weather has settled in over interior NorCal with a few stray areas of high clouds being the only disruption. Temperatures are beginning to rise as a result and look to reach the upper 50s to low 60s across the Valley and foothills, with 40s to low 50s still at higher elevations.
Although the ridge aloft is expected to flatten moving into Tuesday, generally stagnant weather will persist. As a result, another round of morning Valley fog will be possible on Tuesday. 70 to 80% probabilities of visibility lower than 1/2 mile are forecast for the northern San Joaquin Valley, with 40 to 60% probabilities across the southern Sacramento Valley.
As the ridge continues to break down into Tuesday, a shortwave forecast to move across northern California and the Pacific Northwest will begin to impact the region early Wednesday. Given the further northward trajectory of the wave, the general anticipation is for heaviest precipitation to occur generally from the Highway 50 corridor northward. Forecasts continue to trend wetter and windier with this system as well.
Precipitation generally looks to begin by early Wednesday morning in the northern Sacramento Valley and spread south and eastward throughout the day, with the heaviest precipitation anticipated Wednesday evening into Thursday. Latest probabilities of QPF exceeding 0.5″ are in the 30 to 50% range at Valley and foothills locations and in the 60 to 80% range across the higher terrain.
With snow levels around 5000 to 6000 feet through the event, resultant probabilities of exceeding 8″ of snow are in the 40 to 60% range for areas above 6000 feet. Overall, expecting 0.5 to 1″ of rain in the Valley and foothills with snowfall totals up to a foot above 6000 feet.
Additionally, gusty south- southeasterly winds look to increase late Wednesday into Thursday, with occasional gusts nearing 40 mph across portions of the northern Sacramento Valley and adjacent foothills locations as well as along the Sierra.
Extended Discussion (Friday through Monday)
Ensembles and Cluster Analysis continue to indicate an active weather pattern overtaking NorCal as we move into the weekend. Upper level ridging is expected to break down and move off towards the east on Friday, which will allow upper level winds to transition into a more westerly flow on Saturday as a trough approaches the area.
Precipitation is expected to begin late Friday over the Coastal Range and in the far northern counties of the Sacramento Valley before becoming more widespread as we move throughout the day on Saturday. Periods of moderate to locally heavy rain, gusty winds, and heavy mountain snowfall are all possible.
The National Blend of Models (NBM) currently has around a 60-95% probability of exceeding one inch of rainfall in a 72-hour period ending at 4AM Tuesday for the Valley, with the higher probabilities further north of I-80 and in the Foothills and northern Sierra/Southern Cascades.
Probabilities of exceeding two inches of rainfall are around 50-90% in the Foothills/Mountains and in Shasta County including Redding. Snowfall levels look to be around 6000-7000 feet on Saturday morning, then dropping slightly to 5000-6000 feet on Sunday and Monday.
Heaviest totals will be at elevations greater than 6000 feet, with NBM probabilities of 12″ inches or more of snowfall around 40-80% through early Tuesday morning for areas including I-80 and HWY-50.
Along with rainfall and high elevation snowfall, gusty winds are expected to accompany this weather system. Currently, we are not expecting a repeat of last week’s winds although confidence is still low on exact impacts, the NBM depicts around a 35-60% chance of wind gusts greater than 40 mph northern and central Sacramento Valley, adjacent Foothills, and mountains Saturday through Monday.
Much uncertainty regarding the exact precip amounts, onset of precip, and potential wind threats still exist. As we move throughout the week, forecast details will become more clear so stay tuned for updates, especially if you have any travel plans this upcoming Holiday weekend.