January 6, 2020 – Dry today with light precipitation possible Tuesday into Wednesday. More widespread rain and mountain snow is possible with a cool system Friday into the weekend.
Upper level ridging continues to build over the eastern Pacific today bringing dry weather to interior NorCal through most of Tuesday. Ensemble guidance depicts a quick moving upper level trough moving through the eastern Pacific Tuesday. Previous few model runs have slowed the onset of this system, so precipitation is not expected to impact interior NorCal until later Tuesday. Showers will spread south Tuesday evening and will dissipate over most of the area by early Wednesday. Precipitation is expected to remain mainly north of Calaveras and San Joaquin counties. Limited impacts expected with this system as QPF will remain fairly low. A hundredths to few tenths of an inch of liquid precip and one to two inches of snow above 5000 feet are forecast. Light showers could linger Wednesday for the mountains and Shasta County with only light accumulation expected.
Ensemble guidance continues to show a weak shortwave bringing another round of precipitation to NorCal Wednesday night through Thursday. This is expected to be another low impact system. Showers will spread throughout interior NorCal Wednesday night, moving out of the area by Thursday evening. Similar precipitation amounts to Tuesday’s system are expected.
Extended Discussion (Friday through Monday)
The train of shortwaves to affect the region continues into the extended period although these will be of higher amplitude. To start Friday, a transient ridge begins to exit the region in advance of lowering heights offshore. The approach of the longwave trough will increase the coverage of showers, especially north of I-80 and the higher terrain. A core of the stronger ascent will follow the trough which tracks from the Pacific Northwest toward the Central Great Basin. Thus, while stronger than preceding shortwaves, not expecting a particularly wet system this weekend. There is notable ensemble spread so any southward trends would increase precipitation coverage and amounts. Cool temperatures aloft will lower snow levels quite a bit, likely falling into the 3,000 to 4,000 foot range. There are indications snow could fall down to 2,500 feet over the Shasta County mountains into the southern Cascades. Travel impacts and delays are possible over the upcoming weekend.
Global ensembles show little change in the pattern with a broad negative height anomaly sitting over the western U.S. This would support enhanced cloud cover, cool temperatures, and additional precipitation chances. Considering global climate indices, the Pacific-North American (PNA) Pattern favors largely negative values into mid-January. This supports continued troughing as shown by the ensemble solutions.