July 21, 2020 – Gradual cooling trend this week. A few late day thunderstorms are expected over the mountains. Keep an eye to the sky if in the mountains. Southwest breezes continue near the Delta with locally breezy conditions elsewhere.
Discussion
Satellite imagery shows clear skies across interior NorCal except for a few mid and high clouds moving over Shasta County from the southeast, remnants from Monday afternoon’s deep convection over the northern Sierra. Stratus remains extensive along the coast and IR difference imagery shows a little spreading inland into the Delta south of KSUU (profiler data indicate the marine layer depth remains around 2k ft). Most of the region is seeing current temps running slightly cooler compared to 24 hours ago.
Weak closed low off the coast moves closer to NorCal later today, then shifts slowly east into central California on Wednesday before lifting off to the east on Thursday as a deeper trough moves into the PacNW. Gradual cooling will continue the next few days with most areas seeing temperatures close to, or slightly below, average for mid/late July.
Isolated to scattered thunderstorms will be possible across the NorCal mountains today and Wednesday. Mid-level moisture increase is yielding some impressive instability on forecast soundings the next couple of days with MU CAPE of 1500-2000 j/kg. Relatively dry lower levels will allow for gusty outflows, but will limit overall precipitation outside of stronger storm cores. Deeper moisture instability axis is forecast to shift east on Thursday limiting diurnal convection to the Sierra south of Tahoe for late in the week.
Extended Discussion (Saturday through Tuesday)
Upper troughing/low remains near the CA coast through the extended forecast period. AMS remains relatively dry with only a slight chance of deep moist convection over higher elevations of the Sierra Nevada, south of Highway 50 on Saturday. Highs will be near normal Saturday, then about 3 to 6 degrees above normal Sunday into Tuesday.