January 17, 2018 – Active weather pattern continues, with a stronger and colder storm system arriving Thursday-Friday. A similarly cold and wet storm system is expected Sunday-Monday.
Brief ridging over the west coast bringing stable enough conditions for valley fog again this morning. Several locations in the Sacramento area have reported less than one quarter mile visibility. Bands of high clouds will be rotating over the top of the ridge this morning but breaks in between will allow for more periods of local dense fog. Therefore, have issued dense fog advisory through the morning hours. For the most part, today will be dry under the ridge with above normal temperatures. This ridge will be shifting eastward today as a Pacific frontal system pushes towards the coast.
Precipitation should begin over the coast range during the nighttime hours then spread over the northstate during the day on Thursday. This looks to be a moderately wet system with precipitable water values feeding it forecast at just over an inch.
The main frontal band moves into the Sierra Thursday evening with snow levels expected to take a significant drop behind the front Thursday night. Snow levels starting out at or above 7000 feet Thursday afternoon will quickly drop to between 3000 and 4000 feet by Friday morning.
Upper trough drops through northern California on Friday keeping showers going through the day although precipitation rates will be on the decrease. Overall, should be enough snowfall below pass levels to warrant a high end snow advisory or minimum heavy snow warning. GFS instability progs indicate marginal instability mainly over the northern portion of the state Friday. There may be some afternoon clearing over the valley and combined with the instability could be enough to spark a few isolated thunderstorms over the northern Sacramento valley Friday afternoon. Daytime highs Friday should come in at or a little below normal in the cool air mass.
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Upper trough shifts east of the CWA by Saturday morning with an upper ridge pushing in off the Pacific during the day. Could see a few showers over the northern forecast area mountains but the remainder of the CWA should be dry Saturday with daytime highs warming back up to near normal and winds shifting to northerly.
Extended discussion (Sunday through Wednesday)
Short-lived upper ridging will be displaced on Sunday as an upper trough pushes onshore into the Pacific NW. The associated frontal boundary is expected to bring precipitation over the Coastal Range by Sunday afternoon. Precipitation will then spread into the interior through early Monday. Early precipitation estimates suggest 0.50-1 inches across the Valley, and around 1-3″ inches over the mountains. Snow levels are expected to be around 3500-5000 feet. Thus, mountain travel impacts could be possible. Breezy winds are expected late Sunday into early Monday as pressure gradient tightens.
Lingering shower activity will continue over the mountains through Monday afternoon, otherwise drier weather is expected as the trough pushes eastward. Stable conditions are anticipated on Tuesday before the next Pacific storm moves in by midweek. However, there are still model differences in timing and precipitation amounts. Stay tuned.