Clear skies cover interior NorCal early this morning except for some areas of fog in the mountain valleys of Plumas and eastern Shasta counties. Current temperatures are cooler compared to 24 hours ago and range from the 20s in the mountain valleys to the mid 50s across milder portions of the Central Valley.
Temperatures will moderate slightly today as heights aloft and thickness increase as the trough edges a bit further to the east as upstream short-wave ridging approaches. Highs today are expected to be around 5-10 degrees warmer compared to Monday’s.
The short-wave ridging moves overhead on Wednesday providing several more degrees of warming, but another trough is forecast to move across the area on Thursday bringing a little cooling to much of the region. Temperatures will rebound a bit on Friday, but will still be several degrees below average for early October.
Extended Discussion (Saturday through Tuesday)
Building heights along the West Coast this weekend will support a period of mild temperatures accompanied by dry conditions. While a subtropical upper low may approach the state on Sunday, models have been trending toward a more sheared solution. Thus, outside of perhaps some high clouds, no impacts are anticipated with this feature. Valley high temperatures will sit in the mid 80s while the mountains can expect 50s and 60s. These readings are roughly a few degrees above early October climatology.
Looking to next week, a progressive mid-latitude pattern will usher the earlier mentioned ridge out toward the Rockies by Monday. A longwave trough is primed to reach the West Coast during the early/middle part of next week. Ensembles disagree on timing with the 00Z GEFS mean being nearly a day ahead of the 00Z ECMWF ensemble mean. At this point, the primary upper low appears to stay in the vicinity of the Gulf of Alaska with the attendant trough sweeping through the Pacific Northwest, possibly grazing northern California. Some chance for precipitation would loom for the middle of next week across Shasta County but confidence is low. Temperatures should drop a few degrees given the pattern shift.