October 23, 2021 – A significant weather system will bring heavy precipitation leading to flooding concerns Saturday evening into Monday. Strong southerly winds are also expected to develop. Heavy snow will be possible in the Sierra by Sunday night into early Tuesday. Flash Flood Watch issued Sunday morning through Sunday night.

Discussion

Radar imagery shows some light returns scattered throughout interior northern California this early morning. During the late evening hours, some embedded heavier rain showers impacted parts of the Shasta County area. These have since dissipated and not expecting any weather-related impacts in the immediate near-term. Beyond the next 12 to 18 hours, things will begin changing in a hurry.

The much-anticipated atmospheric river will materialize along a deepening trough in the North Pacific Ocean over the next 24 hours. With Integrated Vapor Transport values forecast to peak in the 1000 to 1250+ kg/m/s range, combined with strong upper level jet of 100 to 125+ kt, a powerful system will bring widespread heavy rain to much of northern California.

Onset of the precipitation will begin overnight Saturday and into Sunday morning with periods of the heaviest precipitation occurring during the day on Sunday and into Sunday night. The warm air associated with this system will result in high snow levels at the onset of the rain, starting in the 8,500 to 10,000 ft range early Sunday.

This means that nearly all pass levels will start off with heavy rain rather than snow. The colder air will gradually filter in from north to south late Sunday and Sunday night, resulting in a drop in snow levels to around 5,550 to 6,500 ft by mid-day Monday.

Strong south/southwest winds will be accompanied by the heavy precipitation; a Wind Advisory is in effect for the Delta, Sacramento Valley, and lower foothills early Sunday morning through Sunday evening.

Recent burn areas will be extraordinarily vulnerable to debris flows, mudslides, and rock slides, which has prompted the issuance of a Flash Flood Watch for 13 various burn scars from 2018 to 2021 that starts early Sunday and continues into early Monday. This includes the 2018 Camp, Carr, Delta, Hirz Fires; the 2020 August Complex, North Complex, LNU Complex, Zogg Fires; and the 2021 Caldor, Dixie, McFarland, River, and Salt Fires.

In our county warning area alone, this encompasses over 2.2 million acres of burn area under the Flash Flood Watch (with additional areas outside our CWA are also under a Flash Flood Watch). With storm total precipitation forecast in the 2 to 5 inch range in the Valley, and 4 to 12+ inches in the foothills and mountains, the Weather Prediction Center has issued a moderate and high risk of excessive rainfall over much of the region. The highest risk is focused between the Dixie and Caldor burn areas where the heaviest rainfall is forecast.

Nearly all of these burn areas have yet to be tested with such a magnitude of a storm, and the forecast rainfall is deeply concerning. Even if there were no burn areas to be concerned about, this is still far too much rainfall forecast in such little amount of time. For those living near and downstream of burn areas, have a plan in place to evacuate if necessary. Please stay safe and weather-aware during this impactful storm.

Precipitation will begin to taper off in intensity and coverage late Sunday night and into Monday morning. On/off precipitation will continue through most of the day on Monday, becoming more scattered in nature Monday night and into Tuesday. Winter weather- related impacts will begin to materialize then in the mountains with the lowering of snow levels. Presently a Winter Storm Watch remains in effect for the mountains above 6000 feet from Sunday evening through late Monday night.

Extended Discussion (Wednesday through Saturday)

EPAC upper level ridge builds inland over interior NorCal midweek resulting in drier and warmer weather. High temperatures in the Central Valley climb upwards of 10-12 degrees on Wednesday and inch up into the upper 60s to lower 70s Thu/Fri. Ridge axis shifts into the Intermountain West Friday as next upstream Pacific frontal system approaches. Forecast uncertainties with speed of progression of this system as significant model differences exist. Forecast remains dry at this time into Saturday.