Warming temperatures with light winds and dry conditions into the weekend. Cooler temperatures return next week. Opportunity for precipitation toward the middle to end of next week.


Weak upper trof and cool front will be sagging slowly SEwd across NorCal today and tonight with the front undergoing frontolysis. Not expecting a significant change in temps today given little change in 850 mbs temps and only a slight dip in thickness values. Given the weak pressure gradients and hence light surface winds, NorCal will not see the adiabatic warming effects from Nly winds or the potential cooling effects from onshore winds (Delta Breeze) today as the marine layer is well mixed-out. So max temps will likely be within a couple of degrees of adiabatic descent from 850 mbs. Patch of high cloudiness north of I-80 at press time associated with the upper trof and will be eroding/dissipating as it sags SEwd today.

As the trof splits this weekend, a weak closed low is forecast to form in the vicinity of 30N/130W, to be dealt with during the EFP, per all of the 5H Clusters. Until then, high pressure will prevail over our CWA with warm and dry conditions through the weekend and into Mon.

The NEW Water Year (WY) begins today, and like any new year is approached with cautious optimism. The CPC 6-10 day and 8-14 day precip outlooks suggest that above normal precip is likely for much of NorCal. This should end the record-breaking run for the number of consecutive days without measurable rainfall (>= 0.01″) at Downtown Sac sometime during the 6-10 and 8-14 day periods. As of 9/30/2021, the number is 195 days and counting. The old record was 194 days ending 11/22/1880.

Extended Discussion (Tuesday through Friday)

Pacific frontal system approaches NorCal Tuesday resulting in cooling with increased onshore flow. Front progged into the NW portion of the CWA Wednesday as baroclinic zone weakens and associated offshore upper trough digs.

Models/Ensembles differ with progression of the upper trough late in the week which leads to forecast uncertainty with POPs. Cooling trend expected through the extended period with high temperatures in the Central Valley in the low to mid 80s Tuesday, cooling to the low to mid 70s Thursday into Friday.