October 20, 2021 – A period of wet weather is expected across the region into next week as several Pacific weather systems affect NorCal. Moderate to heavy rain will be possible at times along with gusty winds, particularly Friday and Sunday. Heavy snow will be possible in the northern Sierra early next week.
Discussion
Satellite imagery shows the upper trough lifting to the north into the PacNW early this morning. Rainfall amounts since late Tuesday have been generally light, around 1/4 to 1/2 inch across the northern Sacramento Valley and surrounding terrain with only sprinkles or a few hundredths of an inch reported so far further south. Radar shows scattered light to very light precipitation over much of the region early this morning.
Current temperatures are considerably milder compared to 24 hours ago with many areas reporting readings that are 10-20 degrees warmer.
Southerly breezes are picking up across the northern half of the Sacramento Valley and over higher terrain as the surface gradient tightens. The SAC-MFR gradient is around 8 mbs while the SAC-RNO gradient is approaching 6 mbs. Gusts in the 20s to mid 30s mph are common across that area.
Minor disturbances embedded in the west-southwest flow, along with warm-advection, will keep mostly light precipitation going across the region today and Thursday. Some light snowfall accumulations (around an inch) will be possible this morning near the pass levels in the northern Sierra before snow levels rise further. Precipitation will likely pick up later tonight and Thursday across the orographically favored areas of the northern mountains and northern Sierra as flow backs ahead of Friday’s trough.
Moisture transport increases across NorCal on Friday as the next incoming trough draws 1 to 1.5 inch TPW northward. A relatively quick shot of moderate to heavy precipitation will move across the region Friday with 2-4 inches of rain possible in the northern mountains with 1-2 inches in the northern Sierra (mainly north of Highway 50). Lighter amounts are likely in the valley. Snow levels will be very high (above 9k ft) limiting accumulations to just the highest peaks.
A relative short break in precipitation is likely Friday night into Saturday before increasing again later Saturday ahead of Sunday’s stronger system.
Extended Discussion (Sunday through Wednesday)
High confidence continues in a moderately strong AR forecast to affect the region on Sunday as a wide TPW plume of 1.5 to 1.75 inches is drawn northward into NorCal. Strong upward vertical motion from the strong polar jet nosing into the region will support an extended period of moderate to heavy rain with 1-3 inches possible in the valley with 3-5 inches in the foothills and mountains. Significant impacts, including ash and debris flows, will be possible particularly in and near recent burn scars, though the details on where the heaviest precipitation will setup remain somewhat uncertain.
Light to moderate precipitation continues Monday as the trough swings through, then gradually tapers into the middle of next week as ridging from the eastern Pacific edges into NorCal.
Focus of more significant impacts will shift to the higher elevations of the northern Sierra early next week as snow levels lower below the pass levels bringing several feet of accumulation above about 6k ft, mainly Monday and Monday night.