Dry conditions except for a few showers and thunderstorms over the mountains today. Hot and drier weather returns Thursday and persists through the holiday weekend. Excessive Heat Watch in effect from Friday morning through Sunday evening. Major Risk for heat-related illnesses for pets, livestock, and much of the population, especially anyone without effective cooling and/or adequate hydration. Impacts likely in some health systems, heat-sensitive industries and infrastructure are possible.
Discussion
Skies have cleared out across interior NorCal early this morning. While profiler data indicate the marine layer is shrinking in depth (down to around 1-1.5k ft north of the Golden Gate), onshore surface pressure gradient persists from the coast into the valley with a robust Delta Breeze continuing at Travis AFB (current gusts over 30 mph). Current temperatures are mostly a little milder compared to 24 hours ago and are mainly in the mid 40s to around 60 in the mountains, and upper 50s to mid 70s across the Central Valley.
One more day of thunderstorm chances over the mountains is expected today before ridging and subsidence lead to drying and more stable conditions. As the lingering trough gradually shifts to the south and east, the primary thunderstorm risk this afternoon and evening will be in the Sierra and southern Cascades from the Mt Lassen area southeastward. Occasional lightning, brief heavy rain, small hail and gusty winds will accompany stronger storms, though HREF probabilities indicate a low chance (<30%) of rainfall rates exceeding 1″/hour over the Sierra south of Tahoe (Alpine & Tuolumne counties) in the late afternoon.
Thereafter, heat will be the primary focus for the remainder of the week as ridging builds in from the eastern Pacific warming the airmass and further suppressing the marine layer. After 90s for highs today, triple digits will be common through the Central Valley on Thursday.
The heat will continue to build heading into the weekend when highs are forecast to top out around 105 to 110 degrees through most of the valley on Saturday. Overnight lows will also be very mild across the valley and foothills, in the mid 60s to mid 70s.
The result will be widespread major heat risk for the valley and foothills by the weekend, and an Excessive Heat Watch remains in effect for Friday thru Sunday.
Extended Discussion (Sunday through Wednesday)
Upper ridging centered over the Great Basin Sunday progresses into Rockies Monday as upper troughing in the EPAC moves into NorCal. Troughing lingers over the area through midweek.
Dry weather will result with a cooling trend through the extended. High temperatures return to near to slightly above normal by Wednesday.