An extended period of dangerous heat is expected for the Labor Day holiday weekend into next week. Above normal temperatures are expected to continue each day through the middle of next week, with the most significant heat forecast for Sunday through Tuesday.
Discussion
Warm temperatures are lingering over interior Northern California early this morning under high pressure. 2 AM temperatures are reading around 3 to 10 degrees warmer than this time yesterday with many Valley and foothill locations still in the 70s to low 80s. Lows this morning will only fall to the mid 60s to upper 70s for most Valley and foothill locations.
A trough will approach the West Coast today and move through the PacNW and northern California tonight into tomorrow. The main impact of this trough will be pushing the high pressure axis east over the Great Basin region and lowering heights over Northern California. This will dampen the heating and bring a slight increase in onshore flow, especially over the Sierra and through the Delta region.
Current ensemble guidance has continued to slow the progression of this trough slightly, which has lowered the temperature forecast on Saturday from previous forecasts. Temperatures today and Saturday will be similar to or a few degrees cooler than yesterday with Valley temperatures peaking around 100 to 106 degrees, foothills in the mid 90s to 103 degrees, and mountains in the low 70s to upper 90s.
Widespread moderate to patchy high to very high heat risk is forecast through Saturday, highest in the areas around and south of the Sacramento area and adjacent foothills. Because temperatures and heat risk have lowered on Saturday, the Excessive Heat Warning has been pushed back to begin on Sunday.
Ensemble guidance remains in good agreement that high pressure builds back in quickly over California by Sunday, with temperatures warming significantly Sunday into Monday. Highs Sunday will range between 104 to 109 in the Valley, 99 to 106 in the foothills, and upper 70s to low 100s in the mountains. Temperatures will warm even more on Monday with many Valley locations nearing 110 degrees.
Little relief is expected overnight with morning lows forecast in the upper 60s to lower 80s, warmest in the foothills. Widespread high to very high heat risk is forecast on Sunday and into the first half of next week.
An Excessive Heat Warning remains in effect Sunday through Tuesday. These temperatures increase the chance of heat-related illness for the general population, especially those who are sensitive to the heat, have outdoor plans, or do not have air conditioned homes.
Extended Discussion (Tuesday through Friday)
Strong 4 corners high continues to bring record heat to interior NorCal Tuesday with highs in the Central Valley around 105 to 110. Models indicate high will weaken over the area beyond midweek as strong short wave trough progged into the PacNW and northern portions of CA. High temperatures look to trend down Wednesday into Friday with mostly 90s expected in the Central Valley by the end of the week.