March 2, 2021 – Dry and mild weather through Friday. Rain and mountain snow return over the weekend and early next week.
Discussion
Sly flow ahead of the closed low near 35N/130W is spreading some moisture Nwd over Norcal with satellite imagery showing some enhancement in the diffluent flow aloft. This moisture will gradually thicken over much of Norcal through Wed as the closed low moves into Socal. As the upper low tracks across the Srn half of CA on Wed/Wed nite, there is at least a chance of moisture/instability in the NE quadrant of the upper low reaching into our zone 69, mainly near the Sierra Crest where we have continued the low PoPs.
Mild Pacific air will remain over Norcal through Wed before cooling during the latter half of the week. Max temps will trend about 5-10 degrees above normal through Wed before trending cooler on Thu and into the weekend. Increasing onshore flow ahead of the deepening trof should begin the cooling trend on Thu with a possible intrusion of marine low clouds into the Srn Sac Vly in the morning.
5H cluster analysis for Day 3 (Fri) would indicate the potential for a deep trof to develop over the Ern Pac resulting in a change to a cooler/wetter weather pattern this weekend. IVT plume forecasts indicate a weak AR making landfall this weekend. Some uncertainty with the timing of the onset of precip and precip amounts in our CWA on Fri due to the ECMWF forming a second closed low (per cluster 4) and tracking it across Socal, while the GFS is more progressive. The former would be a drier scenario for our CWA, while the latter would be wetter.
Extended discussion (Saturday through Tuesday)
Negatively tilted short wave trough progresses through CA Saturday bringing widespread precipitation to the CWA. Snow levels begin around 4500 to 6000 feet, lowering to 3500 to 4500 feet Saturday night as precip turns showery. Periods of precipitation continue as another short wave trough moves through Sunday afternoon into night, followed by additional waves Monday and Tuesday.