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July 22, 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.
Satellite imagery shows some mid and high clouds streaming northward along the eastern edge of the forecast area ahead of the low now located just offshore the Bay Area. The remainder of the area is clear other than some stratus beginning to work its way inland into the Central Valley south of KSUU. Profiler data indicate the marine layer has deepened a bit to just over 2k ft in depth at both Bodega Bay and Fort Ord, which may be enough to see a little early morning stratus around the Sacramento region.
Closed low forecast to drift slowly southeast into central California today before kicking out to the northeast on Thursday as its picked up by the next stronger short-wave trough moving through the PacNW. Weaker longwave trough then forecast to linger along the West Coast into the weekend. The result will be to maintain the seasonably warm weather across the region with highs at, or slightly below average for late July.
Thunderstorm activity today is expected to be similar to Tuesday with most storms just to the north and east of the forecast area. A few storms will be possible over the Coast Range to the west of the Sacramento Valley, and near Mt. Lassen. Beginning Thursday, thunderstorm chances are expected to retreat to the Sierra south of Lake Tahoe as deeper moisture/instability shifts east.
Extended Discussion (Sunday through Wednesday)
Weak negative tilt trough could bring a slight chance of afternoon and evening thunderstorms over the Sierra Crest south of I-80 and over Lassen Peak. By Monday the trough axis shifts inland, with convective activity shifting to the northeast of the area. Drier, more stable flow is expected over Norcal on Tuesday as a longer wave trough is forecast to develop over the Eastern Pacific off the West Coast.
Temperatures should trend a few degrees above normal next week. This will mean a return to triple digit temperatures for the northern Sacramento Valley. Continued onshore flow and Delta breezes will keep temperatures more moderated to the south in the Valley, Delta and lower foothills, with highs in the mid to upper 90s and overnight lows in the low 60s. Wednesday could cool down to around average across the area as the eastern Pacific upper level trough approaches.