November 17, 2017 – A few Sierra crest showers this morning, otherwise dry today and Saturday with morning valley fog. Another weather system moves through Sunday night and Monday with another system coming through mid week. Each of these next systems will come with light to moderate precipitation and high snow levels at generally above pass levels.
Discussion
Upper level trough axis has now moved inland with lingering showers over the northern Sierra slowly ending. A shortwave trough rotating around the base of the main low now over the northern Sacramento valley is bringing a few light showers there but even those are now moving into the Sierra foothills as the main low shifts east. With minimal shower activity this morning, have allowed the winter storm warning for the Sierra/Cascade crest to expire. Clouds have begun to clear in some central valley locations so could see some areas of morning fog. After mid day today, skies should be mainly clear over the CWA.
Cool airmass will keep mountain temperatures the same or a little below Thursday readings while more sun in the valley will allow for a little warming in some areas there. Upper ridging pushes in over the state tonight and Saturday bringing mainly fair skies.
Main forecast issue over the weekend will be morning valley fog which will be likely due to the fair skies, generally light winds under the ridge and abundant surface moisture. This time of year it is likely that some of this fog will become locally dense.
Warming airmass and clearing skies will allow for a little daytime warming most areas with daytime highs pushing up close to normal for this time of year. NorCal remains under the ridge on Sunday for another dry day with valley fog during the late night and early morning hours.
Next Pacific storm on the horizon is forecast to move on to the north coast Sunday night. Model timing has slowed a bit so have made adjustments to precipitation onset time. By Monday morning, entire forecast area will be under a threat of precipitation. Warm advection will push up snow levels generally above pass levels by Monday morning so mountain travel impacts with this system should be fairly minimal. The main frontal band is forecast to push through the north state Monday afternoon. QPF with this system is not particularly impressive at this time as the main energy with this system is expected to move north of the forecast area as it rides over upper ridging over the southwest U.S.
Extended discussion (Tuesday through Friday)
Upper level ridge moves eastward as another disturbance slides into northern California with more chances for precipitation. Model forecasts are in general agreement to the atmospheric flow pattern with moisture streaming into the coast in an atmospheric river type pattern, but right now latest model runs have the brunt of the precipitation remaining well to the north of California. If this pattern holds true, light to moderate rain across the forecast area can be expected with snow levels well above pass level and light accumulations at high peaks.