December 12, 2019 – Light rain at times today, heaviest in the mountains. Heavier snowfall returns to the Sierra Friday through Saturday, creating mountain travel issues. Drier weather Sunday and early next week.
Upper level flow is mainly zonal over California this morning. Radar shows lingering showers remain over portions of the Valley and mountains as a weak Pacific moisture plume continues to stream into NorCal. Scattered showers will continue throughout the day, heaviest in the mountains and foothills. Snow levels are high, generally above 7500 to 8000 feet, with little to no accumulation expected at pass level.
Precipitation continues Friday through Saturday as another slightly stronger weather system moves into NorCal. Another influx of Pacific moisture will produce liquid precipitation amounts of around half an inch to one and a half inches in the foothills and mountains. Precipitation will be lighter in the Valley with around a quarter to a half of an inch in the northern Sacramento Valley, tapering significantly south of I-80. Heaviest precipitation expected Friday afternoon through early Saturday. This will be a colder system with snow levels dropping to around 6500 feet Thursday night through Friday. Snow amounts up to 5 to 10 inches, locally up to 15 inches, are forecast in the Sierra through early Saturday for elevations above 6500 feet. A Winter Storm Advisory has been issued. Travel is discouraged during this time as gusty winds and periods of heavy snow will create hazardous travel conditions over Sierra passes.
Precipitation turns showery Saturday with additional snow amounts of a few inches forecast Saturday through Sunday morning. Snow levels are expected to fall to around 4000 to 45000 feet behind the frontal passage. Light snow accumulations under an inch are possible down to the foothills. Current ensembles indicate another brief increase in precipitation is possible Saturday evening producing a quick few inches of additional snowfall over mountain passes. Precipitation will taper off overnight Saturday, clearing out by Sunday morning.
Upper level ridging builds over the area Sunday bringing dry weather for the beginning of the work week.
Extended Discussion (Monday through Thursday)
The start of next week will be dry as models build heights across the region. Forecast 1000-500 mb thicknesses should remain on the low side in the wake of the earlier frontal passage. In spite of the ridging aloft, this will keep Monday’s temperatures close to climatology, roughly in the mid 50s over the Valley with 30s into the mountains.
While skies will be mostly sunny to start the week, a projected pattern shift is likely by sometime mid-week. A somewhat complicated pattern offshore remains unresolved by the available guidance. Multi-day ensemble trends have shown a slowing of this system, generally arriving on Wednesday. It appears some form of closed low will pinch off from the prevailing westerlies which frequently offers low predictability. Recent runs of the GFS and ECMWF differ in timing and placement, with the former bringing the better threat for precipitation.
At a minimum, expect an increase in cloud cover Tuesday into the subsequent days. Looking farther out, the GEFS Atmospheric River ensemble probabilities show a signal for wet conditions likely during the Dec 21/22 timeframe. Such values are higher than usual this number of days out, so this bears watching as well.