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October 25, 2021 – A high-impact weather system continues to bring widespread showers to NorCal. This has lead to flooding and some debris flows. Heavy snow over the Sierra continues into tonight. Quieter and drier weather expected by the middle of the week.

Discussion

Record setting AR beginning to ease up across much of the forecast area early this morning, but widespread moderate to heavy precipitation continues to the south and east of Sacramento, and several additional inches of rain will be possible early this morning across portions of that area. The flash flood warning for the Caldor Fire burn scar has been extended until 9 AM.

Focus today shifts to heavy snow in the Sierra and threat for deep convection across the Sacramento Valley. We’re already seeing some lines of stronger storms developing to the east of Red Bluff. These are orienting parallel to the mean flow, so may pose a renewed flooding threat this morning due to the slow eastward movement of the lines.

This may transition to more isolated activity this afternoon as the upper trough moves overhead, and may pose more of a threat for small hail and perhaps a funnel cloud (low-level shear is quite impressive, but instability forecast to be pretty shallow).

All-time one day rain records were set in Sacramento and at Blue Canyon, and daily records (for Oct 24) were set at numerous locations.

A record rainfall of 10.40 inches was set at Blue Canyon yesterday. This breaks the all time one day record rainfall of 9.33 inches from December 22, 1964. The previous daily record for Oct 24 was 6.34 inches set in 2010.

A record rainfall of 4.57 inches fell at the Oroville Airport Yesterday. This breaks the old record of 1.42 set in 2010.

Lighter precipitation lingers across the northern third of the forecast area (mainly Coast Range, northern Sacramento Valley and southern Cascades) Tuesday before retreating further north on Wednesday. Dry and milder weather expected after mid-week.

Extended Discussion (Friday through Monday)

Upper ridge axis has shifted into the Intermountain West Friday as upstream Pacific frontal system approaches. Models continue to disagree with depth and progression of this short wave. Ensembles support a weaker system and thus forecast remains dry at this time, however will continue to monitor potential for some light precip over Shasta county over the weekend. Short wave upper ridge returns briefly for dry weather Monday but next Pacific front progged inland Mon night into Tue.