July 25, 2020 – Temperatures trending to slightly above average this weekend, continuing into early next week. A few late day thunderstorms are possible over the Sierra Crest this afternoon, mainly south of Highway 50. Convection may become a bit more widespread across the Sierra by Sunday. Breezy southwesterly winds to continue near the Delta while spreading into the Valley.
Discussion
Overnight GOES-17 imagery and height fields show a weak area of cyclonic flow over central California. The resultant easterly flow aloft has ushered in a few high clouds. In terms of marine influences, the 10.3-3.9 micron channel shows any stratus remains back toward the Bay Area and into Napa and Sonoma counties. Per profiler data from Fort Ord and Bodega Bay, the marine layer is not particularly deep, generally running around 1,500 feet.
Early morning temperatures across interior northern California remain close to where they have been. Given breezy southwesterly Delta winds, areas in the vicinity and into the southern Sacramento Valley have dropped into the upper 50s. The usual mild spots are the northern Sacramento Valley and into the thermal belts with readings in the low 70s.
Cyclonic flow from a weak, but pesky, upper low will maintain Sierra thunderstorm chances for the weekend into early next week. Convection should be a bit more sparse in coverage today with activity confined to the high Sierra, mainly south of Highway 50.
By Sunday, Sierra thunderstorm activity is expected to become more widespread extending up into the southern Cascades. Given 0-6 km bulk wind shear around 15 to 20 knots, not expecting storms to be very organized. The 00Z HREF depicts an uptick in most-unstable CAPE values with areas of 1,000 to 1,500 J/kg possible.
Convection that does fire may attempt to drift west off the higher terrain given the easterly wind shear in place. Those with outdoor plans in the Sierra-Cascade range should plan for this increased thunderstorm potential.
Shower and thunderstorm chances will continue into Monday as the upper low pivots toward the Nevada/Oregon/California tri-state border. Activity should not be nearly as widespread as the previous day with drier weather expected for the Sierra by Tuesday. Regarding temperatures and winds, a warm-up will ensue over the weekend with mid/upper 90s across the lower Sacramento and northern San Joaquin Valleys.
Meanwhile, 100 to 106 degree readings are more likely over the northern Sacramento Valley with a few triple digit readings into the adjacent lower foothills. Temperatures cool by a few degrees into next week but still remain slightly above average.
Enhanced Delta breezes are likely, generally gusting between 25 and 30 knots with southerly flow extending into the Sacramento Valley. This will particularly be the case on Monday with 15 to 20 knot gusts possible.
Extended Discussion (Wednesday through Saturday)
Ensembles maintain dry southwest flow over NorCal later next week as the region is forecast to be in-between a trough over the eastern Pacific and ridging extending from the Desert Southwest northward into the Canadian Prairie Provinces.
Temperatures forecast to be close to average for late July.