January 13, 2017 – Lingering showers over the Sierra this morning. Then mainly dry conditions expected through Monday except a slight chance of light showers coast range on Sunday. Next chance of precipitation next Tuesday with heavy precipitation possible during the middle of next week with renewed flooding concerns.
Discussion
Upper low pressure system now centered along the Socal coast and continuing southward. Dry northerly flow is currently developing north of this low over the forecast area with cloud free skies most areas. Still a few scattered clouds over the northern Sierra as southeast flow aloft brings lingering moisture northward and there still may be a few light showers there this morning. Other weather concern this morning is fog forming in the central valley.
Light northerly breezes have kept fog from forming in the northern Sacramento valley but farther south, abundant ground moisture and mostly clear skies have allowed fog to form and areas of dense fog appear likely.
Upper ridging moving in across the north state will bring fair skies and a slight warming this afternoon with daytime highs near or slightly above normal. Fog will likely make a repeat performance on Saturday morning under similar stable conditions under the ridge. Models fairly consistent with each other in dropping a weak low southward along the coast on Sunday.
Looks like only precipitation threat will be along the coast range but ECMWF does bring precipitation farther inland. No matter which solution pans out, precipitation will be quite light. Cloud cover associated with this weak system might hinder fog formation a bit Sunday morning but some valley fog still likely.
Despite the weak system Sunday, overall airmass continues to warm with daytime highs expected to warm to a few to several degrees above normal. Upper ridging keeps dry weather over the north state Monday with another round of morning valley fog.
Extended discussion (Tuesday through Friday)
Models still showing that another atmospheric river could be aimed for the West Coast next week. Both the GFS and ECMWF indicate that the moisture plume should start impacting NorCal sometime Tuesday.
Heavier and more widespread precipitation is forecast to begin by Wednesday morning and continue through Thursday. Looks like weather will become more showery on Friday, but another wave of widespread precipitation would arrive Friday night into Saturday.
Snow levels would initially be high, above 7000 ft, then lower on Thursday and Friday ranging 3000-4000 ft. Given the already high river/creek stream levels, flooding will continue to be a concern.