January 5, 2020 – Light showers in the northern mountains today, with some fog in the Valley. Light precipitation possible Tuesday into Wednesday, mainly over the mountains. More widespread rain and mountain snow is possible with a cool system Friday into the weekend.
Discussion
Areas of dense fog have formed in the southern Sacramento and northern San Joaquin valleys early this morning and should dissipate by midmorning. Current satellite imagery shows a weak upper trough moving through NorCal. This produced light showers over the northern counties overnight with a chance of light showers continuing through the morning for the northern mountains and Shasta County. Otherwise, dry conditions are expected for the rest of the forecast area today.
Dry weather remains Monday before another low impact weather system moves into NorCal Tuesday. Precipitation is expected to spread south through NorCal throughout the day Tuesday into Tuesday night, with the best chances of precipitation in the mountains and northern Sacramento Valley. Low accumulations are expected with a few hundredths in the Valley and foothills up to a quarter of an inch of liquid precipitation forecast. Light snow showers possible above 4500 to 5000 feet with accumulation up to a couple of inches. Chances of light showers linger into Wednesday for the mountains and Shasta County with little to no accumulation expected.
Mild temperatures are expected through the short term forecast with Valley highs in the 50s and mountain highs in the 30s to 40s. Cold overnight lows continue with the Valley seeing widespread 30s and the teens to 30s in the mountains.
Extended Discussion (Thursday through Sunday)
From Thursday into the weekend, a pair of troughs will affect the region. Considering the initial system, little to no impacts are expected given the weak nature of the wave. Some light rain and snow showers will be possible the first half of Thursday, primarily over the Sierra with snow levels down to around 2,500 to 3,000 feet. Otherwise, conditions should improve through early Friday as a shortwave ridge decreases cloud cover over the region.
Global ensembles continue to advertise a discernible pattern change with a much more amplified trough impacting the western U.S. While such models agree on the pattern, they are not consistent with how far south of trough will dig. Knowing this aspect of the forecast will dictate impacts including precipitation amounts as well as snow levels. At this juncture not expecting a particularly wet system with the more concentrated precipitation confined to the northern Sacramento Valley into the mountains. Have kept widespread precipitation in the forecast as shown by ensembles with snow levels likely in the 3,000 to 4,000 foot range. Given lower heights over the West, expect a gradual cooling trend across the region. Moving into the following week, ensemble systems favor a continuation of this pattern which will keep cool, possibly wet conditions in place.