Active weather returns today and continues into next week with extended periods of moderate to heavy rain, mountain snow, and breezy southerly winds. A colder storm could bring heavy snow and major travel impacts to the mountains next week.
Discussion
A major pattern change is in the works as the first in a series of 3 storms with an atmospheric river brings widespread rain and mountain snow. The first storm goes from today through Sunday evening and will be a fairly warm system with snow impacts above 6500 feet. Radar shows light to moderate precipitation spreading through the northern Coastal Range into the northern Sacramento Valley early this morning. Models show this spreading across the rest of the Valley and mountains by mid-day.
The National Blend of Models (NBM) has show a bit of shift northward with precipitation for this first system, with the main change. Highest precipitation amounts are still expected to be centered around the I-80 corridor, with a forecast total of around 10 inches for Blue Canyon from today through Sunday evening. The National Blend of Models (NBM) has 65 to 90% probabilities of 1.5 inches or more of rainfall totals through Sunday evening in the Sacramento Valley and Delta, 20 to 40% for the northern San Joaquin Valley.
For the foothills and 70 to 90% of 2 inches or more of rain in Shasta County, the foothills, and the mountains below snow levels. Moderate to heavy rain rates will bring rises in area streams and rivers, along with nuisance flooding in low lying roads. No debris flow impacts are expected over burn scars. There is the potential for mud and rock slides in the foothills. A Flood Watch continues for areas below 3000 feet from this afternoon through Sunday evening. Heaviest rain is expected from Friday afternoon through Saturday afternoon.
This system remains fairly warm, with snow levels rising from around 5000 feet early this morning, to 6000 to 7000+ feet by late this afternoon. This will bring high mountain travel impacts, with chain controls and delays possible.
A Winter Weather Advisory remains in effect for areas above 6500 feet in the Sierra and southern Cascades through 4 AM Sunday. The latest NBM has a 60 to 90% chance of snow totals of 6 inches or more above 7000 feet, with a 30 to 80% chance of 12 inches or more. Storm totals greater than 3 feet are possible over the highest peaks. Highest snow rates with this first system will occur tonight and into Saturday, around an inch an hour at times. Some high snow totals (a foot or more) are possible over the higher peaks of Shasta County, but should there should be limited impacts to travel.
Gusty winds are expected in the mountains, generally up to 40 mph in most locations above 6500 feet, and up to 60 mph over the highest peaks. These could bring visibility issues at times, with blowing snow.
A second storm system is expected Monday through Tuesday night. Snow levels start high (6500 to 7500 feet), but will drop significantly Tuesday-Tuesday night as colder air from the Gulf of Alaska drops into the region. Snow totals for the Sierra are currently are projected by the NBM to be around 3 to 4 feet above 6500 feet, which would bring major travel problems. Snow levels drop low enough for impacts to reach I-5 by Tuesday night.
Travel impacts are likely across interior NorCal on Friday and over the weekend, so be sure to check road conditions at quickmap.ca.dot.gov and weather conditions at weather.gov before heading out the door!
There is the potential for thunderstorms Monday afternoon, mainly for the east side of the Sacramento Valley and northeast foothills. Instability looks limited at this time, so just expecting a few isolated storms.
Extended Discussion (Wednesday through Friday)
Cluster analysis and ensembles continue to show active weather continuing through much of next week, with additional moderate rain and heavy mountain snow totals. The third in a series of storms will be different in that it has cold air with low snow levels for mid to late week. One to three feet of additional high elevation of snow is possible, while light amounts are possible down into the upper foothills. This storm will likely bring significant travel impacts to mountain roads, including I-80 and I-5.
Even Redding has a potential for seeing light snow, with a 20% probability of 1 inch of snow. There is still some uncertainty in the exact trajectory of the main plume of moisture in this second wave of weather though, so exact rain and snow totals, as well as the timing of heaviest precipitation remain uncertain. Be sure to prepare now for the continued wet and wintry weather, especially if you plan to travel in the mountains! In additions to precipitation, periods of gusty southerly winds are also expected to continue.