One last day of onshore flow bringing near normal temperatures and isolated showers across the Northern Sierra and Northern Sacramento Valley. Hot weather develops Tuesday through late week with periods of breezy north winds in the Valley. An Excessive Heat Warning is in effect for the Valley and foothills Tuesday through Thursday.

Heat Timeline

Discussion at a Glance

  • Cooler today with periods of increased onshore flow and Delta Breeze.
  • An Excessive Heat Warning goes into effect from Tuesday morning through Thursday evening across the Valley and adjacent foothills.
  • Areas of Major HeatRisk Tuesday for the Valley, become widespread Wednesday for the Valley & Foothills with widespread triple digit high temperatures. Prepare now for elevated daytime highs and overnight lows and reconsider outdoor activities during the hottest part of the day (3:00-7:00 PM).
  • Areas of Major HeatRisk continue on Thursday, with primarily Moderate HeatRisk by Friday; Triple digits in the Northern and Central Sacramento Valley and adjacent Valley foothills with mid to upper 90s in the Delta and Sacramento metro area.

Short-Term Discussion (Today through Thursday)

As of early this morning, satellite imagery shows clouds and virga moving into the area from the northwest as a weather system moves through the Pacific NW. Radar shows virga moving through the Southern Cascades and portions of the Northern Sacramento Valley. Impacts will be minimal from this weather passage but isolated showers mainly over the Northern Sierra/Southern Cascades and Northern Sacramento Valley are expected this morning into the early afternoon. Latest WPC amounts show 0.01-0.30″ of total rain accumulations with highest amount in the Lassen National Park area.

By this afternoon, the weather system exits eastward from the Pacific NW replaced by ridging over Northern CA by Tuesday. This will result in a rapid warm up to well above normal temperatures from Tuesday through the end of the week. Resultant Valley high temperatures look to rise firmly into the upper 90s to low triple digits on Tuesday, with widespread triple digits in the Valley anticipated on Wednesday. Even at higher elevations, high temperatures into the 80s to 90s are expected. Overnight low temperature recovery will remain limited as well, with 60s to low 70s in the Valley and foothills and 50s to 60s across the mountains. This will lead to areas of major HeatRisk throughout much of the Valley and foothills, with moderate HeatRisk elsewhere.

Excessive Heat Warning

As a result, an Excessive Heat Warning goes into effect for much of the Valley and foothills at 11 am PDT Tuesday, June 4 and continues through 8 pm PDT Thursday June 6.

Additionally, as the ridge builds in on Tuesday and Wednesday, surface winds are expected to shift to a north-northwesterly direction. Strongest winds are expected to occur Tuesday late morning through the afternoon across the Sacramento and northern San Joaquin Valleys, with gusts of 20 to 25 mph possible at times. This dry northerly wind combined with hot temperatures and forecast relative humidities into the upper teens to low 20s will likely result in some elevated fire weather conditions midweek. Lowest RHs will be over the Northern Sacramento Valley and northeastern foothills with minimum RH values of 15 to 25 Wednesday and Thursday.

Elevated fire weather conditions

Remember, area waterways continue to run fast and cold from snowmelt, creating dangerous conditions for those seeking relief in rivers and lakes next week. Stay tuned for updates as we get closer and plan ahead now to practice heat and cold water safety!

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Major heat risk

Lastly, the aforementioned ridge axis shifts as it sinks into the Southwest US to allow a shortwave trough to move through Northern CA. This will see weak onshore flow move through the Delta and portions of the Sacramento Valley, seeing temperatures cool several degrees. With this there is a chance for afternoon and evening showers and thunderstorms over the Sierra, mainly south of I-80. With the forecasted flow pattern, storms will form over the crest then exit our forecast area to the east.

Extended Discussion (Friday through Monday)

Upper ridge axis extends from the Desert SW northwestward over NorCal early Friday. Ridge axis shifts east Friday as upstream upper troughing in EPAC progresses. Triple digit heat continues Friday over northern and central portions of the Sacramento Valley. Delta breeze will help to taper the heat some in the S Sac/Northern San Joaquin Valleys where 90s are forecast. Upper troughing in EPAC progged to weaken Sat/Sun as it approaches the West Coast and encounters strong downstream upper ridge. EPAC ridge then builds Sun into Mon as upper trough forms closed low. Models differ with placement of synoptic features late in the extended period. NBM advertising 90s in the Central Valley Sat into Mon with dry weather.