December 9, 2018 – Series of weak storms will pass over NorCal over the next several days but impacts will be minimal with each of them. Valley fog will be possible between systems.
Fog in the Valley has lifted into a layer of low cloud over most of the area, so the Dense Fog Advisory was cancelled earlier this morning. Mist will likely linger much of the day, with visibility around 5 miles or less in many locations. In addition to low clouds, there will be high and mid clouds spreading across the area ahead of an approaching trough. These clouds should act to limit widespread dense fog formation tonight, with areas of mist and some patchy fog.
We are seeing widespread dense fog from near Winters south through the Sacramento and San Joaquin Valleys. The fog will continue to drift north as the night goes on but is not expected to be nearly as dense as it reaches the northern part of the Sac Valley due to light north winds. The majority of the airports are reporting vis 1/4 or less from Sacramento south and a Dense Fog Advisory will remain in effect for the central Sac Valley south into the San Joaquin Valley until 11 am. We will begin to see warm air advection during the morning hours and that is going to bring increasing clouds on top of the fog with mainly cloudy skies by mid morning. This will slow the erosion of the fog and we will likely have to deal with patchy fog as we head into the afternoon Sacramento south.
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Our next trough and associated cold front will push into NorCal during the early evening with light showers spreading over the coastal range. Light showers will continue to spread over the remainder of NorCal overnight as the cold front sweeps through the area. This system is very unimpressive and rain totals are expected to remain light. The highest totals are expected over the coastal range where around .25″ is expected. The Sierra and southern Cascades should see less than .25″ and many valley locations will remain below .10″. Snow levels will be around 5000 feet with most locations seeing an inch or less of snow but totals could reach up to 3″ in the Lassen area. A short wave ridge builds in for later Monday into Tuesday. North winds and some lingering cloud cover will keep fog chances Monday night low.
A trough will drop south into the PacNW late in the day on Tuesday and this will bring the chance for light showers to portions of the Sierra and southern Cascades. The forcing and better moisture will remain well north of us keeping our rain totals very light. Snow levels with this system look to be between 4500 and 5500 feet but we are not expecting much snow accumulation.
Extended Discussion (Thursday through Sunday)
Dry weather expected to through Thursday as upper ridging holds along the West Coast. Precipitation chances return Friday and increase over the weekend as the ridge shifts east and a series of systems move through NorCal.