A strong atmospheric river brings periods of moderate to heavy rain and mountain snow, isolated thunderstorms, and gusty southerly winds to interior NorCal through the end of the week, over the weekend and into early next week.
- Winter Storm Watch: From 4 PM Friday through 4 PM Tuesday for elevations above 5500 feet along the Sierra/southern Cascades including Interstate 80 and Highway 50.
- Flood Watch: Through 4 AM Saturday for Valley/foothills locations generally north Interstate 80 and below 3000 feet.
Discussion
As of early this morning, current RADAR illustrates widespread precipitation spreading across interior Northern California early on this Thursday morning. Caltrans and CHP have reported slick roads and ponding water on roadways in the Valley, and snow covered roads up in the mountains including Interstate 80. The strong atmospheric river continues to bring periods of moderate to heavy precipitation to the region today and over the next several days, before moving onshore over the weekend and bringing additional widespread precipitation to the area.
Some lingering snow showers will be possible through the morning up in the Sierra Nevada, and the current Winter Storm Warning was extended through 8 AM, as snow is still falling and mountain travel impacts are still being observed primarily along I-80. Snow levels are expected to continue to rise through the day today, starting out initially around 6000-7000 feet, and rising to around 7000-8000 feet later this morning.
This will allow for rainfall to be the primary precipitation across interior NorCal today into Friday. Snow levels then start to come down Friday evening and over the weekend, lowering to around 4000-6000 feet Saturday and Sunday (lowest in Shasta County). Periods of moderate to heavy snow are currently expected to continue over the weekend and into next week, and a Winter Storm Watch has been issued from 4 PM Friday through 4 PM Tuesday for elevations in the Sierra and southern Cascades above 5500 feet.
Periods of moderate to heavy rain will continue over the next several days. Rain amounts of 1-5 inches in the Valley with 3-12 inches in the foothills are in the forecast through Saturday morning, with an additional amounts of 0.25-2 inches from Saturday into Sunday as the system moves inland. The Flood Watch remains in effect through 4 AM PST Saturday. Slick roads, ponding of water on roadways, and sharp rises on rivers, creeks and streams continues to be possible.
Periods of gusty southerly winds will also continue over the next several days. Winds are still gusting to around 25 to 40 mph in portions of the northern and central Sacramento Valley and adjacent foothills, and the Wind Advisory is in effect through 7 AM PST this morning. Another round of gusty southerly winds will develop later tonight into Friday. Isolated thunderstorms will be possible Friday afternoon into Saturday, and the National Blend of Models (NBM) suggests a 10-20% probability of thunderstorm development over this timeframe. Isolated thunderstorms may produce small hail, heavy rain, lightning, and gusty winds.
Extended Discussion (Monday through Thursday)
By the start of the work week, rain and mountain showers will continue to impact interior Northern California through Tuesday as the aforementioned trough pushes inland. Snow levels are forecast to be between 5000-6000 feet Monday through Tuesday, with additional forecast snow accumulations of 6 to 12 inches and up to 2 feet along the highest peaks of the Sierra. Probabilities of exceeding 12 inches are between 50% and 70%. Across the Valley, there is a 40 to 60% chance of rainfall accumulations of 1.00 inch or more through Wednesday morning. Lower elevation precipitation is expected to persist as well, with totals of 0.5 to 1.5 inches possible across the Valley and foothills and 1 to 3 inches possible across the mountains through Tuesday.
Heading into the midweek timeframe, there is some uncertainty whether this period of active weather may slowly taper off or persist. Ensemble solutions diverge slightly. The favored solution shows the potential for an additional few inches of mountain snow. The National Blend of Models projects an a 40 to 50% chance of 2.00 inches of snow or more over the Sierra, Wednesday into Thursday with snow levels around 5000 to 6000 feet. Rainfall amounts in the Valley and mountains during this time will be far less compared to previous active weather periods, with a 20 to 40% chance of 0.5 inch or more across the Valley and foothills.
Be sure to stay up to with the latest forecast at weather.gov/sto if you have outdoor holiday plans and check current road conditions at quickmap.dot.ca.gov before traveling!