Cooling trend through this weekend. Smoke from area wildfires will persist over portions of interior NorCal, mainly foothill and mountain locations. Widespread shower chances return late Saturday into early next week.
Flat zonal upper level pattern will prevail for one last day, before heights begin to lower with troughing over the weekend as upper level low over British Columbia drops southward. Ensembles and clusters drop this low south off the the Pac NW coast Saturday and then just off the Northern California coast, west of Eureka Sunday. This will bring much cooler temperatures Saturday and especially Sunday, along with precipitation chances Saturday night into Sunday.
NBM has slowed the onset of precip a few hours back into Saturday evening over the northern Sac valley and coast range then spreading south through the night. Upper low will then drop to near the bay area Monday with disturbances rotating into NorCal.
Some clearing is possible behind front Monday afternoon which could enhance instability and this is reflected in increased thunderstorm probabilities showing up in NBM.
Heaviest rainfall amounts are expected from Sunday night through Monday afternoon. Storm total precip amounts(Sat night-Tue) continue to increase with current amounts now from 1 to 2 inches over the mountains and 1 to 1.5 inches for Valley locations.
This system does not appear to be particularly orographically driven with similar rain amounts in Valley locations and mountains. Valley high temperatures today will be generally in the 80’s, cooling into the 70’s Saturday and 60’s and 70’s Sunday.
A period of gusty ridge top winds are possible Saturday afternoon with southwest gusts from 30 to 40 mph, with a brief period fire weather concerns mainly over the ridgetops.
Snow levels will generally remain above 8,000 feet with some light accumulation possible on the highest peaks Sun-Monday night.
Extended Discussion (Tuesday through Friday)
Clusters leaning more to slower opening up and ejection of upper low through California Tuesday and this has lead to prolonged precip chances in NBM guidance with WPC showing 0.25-.50 inches of QPF tue/tue night. This uncertainty also extends into Wednesday with half of the clusters ejecting the low into the Great Basin and half lingering it back over California, will keep light shower chances going mainly over the mountains in line with NBM.
Low should finally move out Thursday and Friday with a drying trend and temps warming back to near or slightly below normal.