December 14, 2018 – Dry weather this morning with areas of valley fog. Light rain spreading over the north state today and tonight with breezy south winds and light snow over the mountains. A moderate storm system moves through NorCal late Sunday into Monday with mountain travel impacts likely.
Upper trough off the coast with southerly flow aloft pulling high cloud cover over nearly all of the CWA this morning. Even with this cloud cover, valley fog has formed from about Oroville southward but so far nothing too dense. May get a few light sprinkles during the morning hours from the subtropical cloud cover but should not see anything significant.
Increasing gradients ahead of the through will bring breezy south winds especially over the highest terrain and a wind advisory remains in place above the 7000 foot level late this morning through the evening.
Pacific frontal band associated with the offshore trough is forecast to reach the north coast by mid day today then push inland this afternoon and overnight. Precipitation with this weak system is still expected to be fairly light with only light snow expected at the highest elevations. Therefore, travel impacts over the Sierra are expected to be minimal.
GFS and ECMWF have been consistent in bringing a secondary shortwave trough across NorCal on Saturday which may be strong enough to ring a few light showers out during the day but again precipitation amounts should remain quite light.
Next and stronger Pacific trough is forecast to approach the coast on Sunday. Pre-frontal moisture is again expected to bring light precipitation to the forecast area during the day but precipitation should remain fairly light during the daytime hours.
South winds will pick up again during the day Sunday but are currently not expected to be quite as strong as expected today. Main cold front is expected to push into the coast range late Sunday afternoon and then across NorCal during the night. This moister system will bring moderate precipitation to the forecast area. Snow levels with this system will be too high to have a significant travel impact on the coast range but low enough to impact the Sierra Cascade passes. GFS and ECMWF a bit more consistent with each other on latest runs bringing a slight split to the frontal band as is moves eastward across the state.
Latest snow amount forecast puts snowfall totals from early Monday through early Tuesday at several inches to a little over a foot or borderline between heavy snow warning and advisory criteria. Will put out a statement regarding mountain travel impacts but some winter hazard product will likely be needed once snow total estimates can be better fine tuned with successive model runs.
Frontal band moves quickly east of the forecast area by Monday afternoon with a transition to light showers during the day. Upper ridge begins rebuilding Monday night pushing any precipitation threat to the far northern forecast area.
Discussion (Tuesday through Friday)
Little change to the extended forecast this morning as models continue to depict uncertainty for the rest of the week. The ECMWF continues to be the more bullish of the two models and tries to push another frontal system into northwest California on Tuesday. If this solution were to pan out it would clip our most northern counties; however, the GFS supports ridging over the area. If the GFS pans out, we`d like see drier and warmer weather with the potential for some valley fog. The forecast right now is a blend of the two models for the middle of the week. As we head towards the weekend, the models continue to struggle to come into agreement. Both suggest that a low develops over the Pacific; however, they vastly differ on timing and strength.