March 21, 2020 – Few afternoon afternoon showers possible Saturday over the northern Sierra, otherwise dry weather is expected into the weekend with warming temperatures. Widespread rain and mountain snow will return later Sunday and continue into the middle of next week.
A weakening upper low will remain situated over the region today, with lingering afternoon showers over the northern Sierra and higher elevations of Western Plumas County. Some showers may even spread over the northern San Joaquin Valley early this morning. Shower activity ends Saturday evening as the upper low weakens and pushes east into Nevada. Dry weather continues elsewhere over interior NorCal with a warming trend continuing through the weekend. Valley highs will reach the upper 60s to lower 70s.
A pattern change is anticipated late this weekend as a Pacific upper low approaches from the west, with light precipitation moving into areas south of I-80 by Sunday afternoon. Precipitation will continue to spread northward by early Monday morning as the low opens to a trough and tracks inland. This Pacific low will be followed by a cold system dropping from the Gulf of Alaska into the region, bringing widespread precipitation.
Preliminary QPF amounts indicate 0.15 to around 0.50 inches in the Valley, and 1 to 2.25 inches of liquid precipitation in the mountains. Snow levels will start around 5,500 feet on Monday, then drop to 2500-4000 feet by Tuesday morning. Models indicate this system could bring 1 to 2 feet of snow in the northern Sierra and extreme Southern Cascades, mainly above 4500 feet.
Extended discussion (Wednesday through Saturday)
Ensemble mean and latest ops GFS/EC runs in reasonable agreement shifting the early week trough to the south and east by later Wednesday with decreasing POP/QPF by Thursday. Forecast confidence on details decreases late next week as impulses drop down in northwest flow and will lean forecast heavily toward NBM. Temperatures will be well below average mid-week moderating to close to average by next weekend.