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June 22, 2020 – A prolonged period of hot weather is forecast this week. Moderate to very high heat risk is expected in the valley and foothills, with little overnight relief. Slight chance of thunderstorms over the mountains on Tuesday and Wednesday.

Discussion

Clear skies across interior NorCal early this morning beneath the building West Coast ridge. The marine layer remains around 1000-1500 ft deep and a robust Delta Breeze continues at Travis AFB where the current temperature is down slightly compared to 24 hours ago. However, the cooling doesn’t extend much further inland as readings elsewhere are generally up slightly compared to early Sunday morning. Current temperatures range from the mid to upper 40s in the colder mountain valleys, to the upper 70s across warmer portions of the Central Valley.

Strengthening high pressure will bring a prolonged period of hot weather to the region this week. Most areas will see highs around 10-15 degrees above average with the northern half of the Sacramento Valley mostly in the 105-110 degree range. For the southern Sacramento and northern San Joaquin Valleys, highs will range from 99-105. The mountains will see temperatures in the upper 70s to 90s.

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Unfortunately, not much relief from the heat is expected overnight for much of the area. Low temperatures for most of the valley and foothills will only fall to the mid 60s to mid 70s. Marine influenced areas near the Delta may experience slightly cooler overnight lows, but will remain hot during the day. Both the above normal daytime temperatures and warm overnight temperatures will cause moderate to high heat risk across the Valley and foothills, with much of the mountains in moderate heat risk as well. As a result of the prolonged heat event, a Heat Advisory is in effect until 8PM Saturday for the Central Valley (does not include the Delta) and foothills.

Aside from the heat, ensembles show the upper ridge axis shifting over Nevada Tuesday with a weak upper trough moving through the PacNW into Wednesday. GEFS and NAM indicate the potential of enough moisture and instability for a slight chance of late day thunderstorms over the mountains Tuesday and Wednesday. Ridge restrengthens Thursday which will likely cap off any deep convection and bring renewed warming.

Extended Discussion (Friday through Monday)

Strong upper ridging persists over interior NorCal Friday into Saturday with continued well above normal temperatures. Forecast highs will generally be near to below record values Friday. Slight cooling expected Saturday as ridging begins to weaken. Heights lower significantly over NorCal Sunday and Monday as upper low drops into the PacNW. High temperatures cool to below the century mark Sunday in the Central Valley and into the low to mid 90s Monday.