Near normal temperatures through early this week. Hot conditions with high to very high heat risk will return later this week.
Clear skies prevail overhead this afternoon for most communities across northern California. The exception to this is extreme northwest California where a very late-season atmospheric river is bringing localized rainfall. As the afternoon progresses, can’t rule out a stray rain shower in portions of Shasta and Tehama Counties, but any precipitation that falls will remain very light. The National Blend of Models gives Redding a 21 percent chance of measurable rainfall (greater than/equal to 0.01 inches). Onshore gradients are ramping up this afternoon — present surface pressure gradient between San Francisco and Sacramento is 2.5 millibars and climbing. As a result, expect breezy conditions to develop and continue into the evening hours around the Delta, as well as for portions of the Sacramento Valley along the I-5 corridor. Forecast highs will peak in the upper 80s to low 90s, or within a few degrees of where we should be for this time of year.
The main synoptic player over the coming days with be a strong and broad upper level ridge centered over the Desert Southwest. Deterministic models and ensemble means have this ridge building westward through much of the week, and will result in day-to-day warming after Tuesday. Expect Valley temperatures will warm to the upper 90s to around 105 on Wednesday afternoon with additional warming (and dangerous temperatures!) expected later in the week. More details on the late-week heatwave can be found in the extended discussion.
Extended discussion (Thursday through Sunday)
The Desert Southwest ridge will expand west late week, resulting in dangerously hot temperatures across a vast portion of interior California. Temperatures at 850 millibars (or roughly 5000 ft above sea level) will drastically warm from the present 14-15 deg C up to 28-29 deg C (possibly even 30 deg C) by Thursday/Friday. This translates to around 108-112 deg F at the surface, locally hotter. Additionally, temperatures overnight will be very warm — particularly in the northern Sacramento Valley and the lower foothills — where temperatures may struggle to drop any lower than the middle 70s, possibly even 80 deg F. Consequently, an Excessive Heat Watch has been issued for the Sacramento Valley, northern San Joaquin Valley, adjacent foothills starting noon PDT Thursday through Saturday evening.
Daily record highs will be jeopardized during the peak of this heatwave on Thursday and Friday. Corresponding heat risks will land in the high to very high category, impacting the entire population. National Blend of Models is forecasting 111-112 deg F for Redding on Thursday and Friday, and 107-108 deg F for Sacramento. Residents are urged to prepare for this dangerous heat event, especially considering the rapid warming from the below normal temperatures just a few days ago. Temperatures will gradually cool (but still remain hot) for Saturday and Sunday. Astronomical summer begins on Sunday, June 20 at 7:57 PM PDT, so spring 2021 will undoubtedly go out hot.