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462 FXUS66 KSTO 182331 AFDSTO AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SACRAMENTO CA 331 PM PST Thu Dec 18 2014 .Synopsis... Fairly dry today for the region. Next frontal system moves through NORCAL Friday bringing rain and mountain snow. Drying south on Saturday with a few showers north especially over the mountains. All areas dry on Sunday except far northern Shasta county mountains. Dry Monday/Tuesday under high pressure ridge. Slight chance of mainly mountain showers on Wednesday. && .Discussion... Both Beale and the Davis radars are back up from maintenance earlier today. We did hear reports of light rain in parts of Shasta County when the radar was down and current radar imagery does verify light showers in that region. Otherwise, the rest of our CWA remains fairly dry. However, one more round of rain and snow is on its way. A storm offshore is already moving into the far northern CA coastline and will bring the next notable wave of rain and snow to interior NorCal on Friday. More widespread precipitation will move into the Coastal Range late tonight and then spread into the Central valley and Sierra on Friday. Forecast shows valley rain totals ranging 0.10" to 0.50" with the higher amounts in the Redding and Red Bluff vicinity. The Sacramento Metro region should get around a quarter of an inch and areas southward will be closer to a tenth. Primary concern with this system is snow impacting travel along the Sierra on Friday. Have issued a Winter Storm Advisory for snow above 6000 ft along the Western Sierra slopes from Friday 4 am to 10 pm. Sierra traffic tends to be heavier on Fridays and we`re expecting 3-6 inches of snow near pass levels which will likely cause traffic delays and chain restrictions. Mountain snow showers are expected to linger into Saturday, but any additional snow accumulation will be light. Ridging builds over the Eastern Pacific on Saturday and nudge into the west coast late Sat/early Sun. Lingering showers may continue to slide over the top of the ridge and impact far NorCal through the weekend, otherwise, most of our CWA will be dry. Clearer skies and a warming airmass under the ridge will allow daytime highs to warm to near normal on Saturday and to a few to several degrees above normal on Sunday. JBB .Extended Discussion (Monday through Thursday) Main concern in long term is around potential system for Christmas Eve/Christmas Day. Dry weather will continue Monday into early Wednesday as high pressure ridge remains in control across the region. Daytime temps will max out around 60 in the valley and upper 40s to 50s in the foothill/mountains. Only impact in this period will be areas of valley fog each morning with lingering surface moisture. High confidence for this period and it will lead to a few good travel days. The next system of interest is progged to move through late Christmas Eve into Christmas Day as it drops SE from the Pac NW into the Great Basin. Model solutions continue to be all over the place as previously advertised. 12Z run of the ECMWF...which was the strongest and wettest of the bunch and mirrored the latest GEM and parallel 13km GFS...has flip flopped from previous run and now mirrors the weaker, more progressive GFS solution. This would lead to a mainly dry holiday period, especially in most locations in the Valley. Of bigger note would be infiltration of colder air and lower snow levels. Snow levels could drop to 3000-3500 feet by Thursday with accumulating snow and travel headaches likely if a stronger system does materialize. Overnight temps late in the week could also drop into the 30s in some spots in the valley leading to frost. Confidence is low in details and chances for precipitation at this point so have not made major changes to the going forecast. Will need to be monitored as we get closer. CEO && .Aviation... Mainly VFR conditions across the area this afternoon, except local MVFR along the west side of the Valley. Next low pressure system is approaching California, with -RA developing along the Northern Valley between 09-12z, and Sacramento TAF sites by 15-18z. MVFR/IFR conditions possible in the Valley, with IFR/LIFR conditions in the mountains. Snow levels will be between 5000-6000 ft. Southeast winds will increase to 10-18 kt late tonight into Friday. Dang && .STO Watches/Warnings/Advisories... winter weather advisory from 4 am to 10 pm pst friday above 6000 feet in the west slope northern sierra nevada...western plumas county/lassen park. && $$