Active weather continues into next week with extended periods of moderate to heavy rain, mountain snow, and gusty southerly winds. Colder storms are expected to bring heavy snow and major travel impacts to the mountains next week.
Discussion
A series of 3 storms are expected with a wet atmospheric river pattern continuing through next week. The first storm is currently over the area and will continue through Sunday, with high snow levels limiting travel impacts to above 6500 feet. Significant rain is expected below those levels, especially in the foothills. The second system Monday through Tuesday will be colder with lower snow levels and feet of snow with major impacts to mountain travel, with moderate rain across the lower elevations. The third system is even colder with snow levels into the upper foothills and additional feet of mountain snow, with widespread but lesser amounts of light to moderate rain.
Radar at 3 am showed some scattered showers in the area, mainly over the mountains and foothills. Atmospheric profilers yesterday showed snow levels around 8500 feet in the early evening, but these levels have been lowering overnight to around Donner Pass and Echo Summit levels. Temperatures at Donner Pass (7200 feet) at 3 am were 33 degrees.
Caltrans cameras showed some accumulating snow on I-80, with some chain controls in effect. For current road conditions go to quickmap.ca.dot.gov. A Winter Weather Advisory remains in effect through 4 am Sunday for elevations above 6500 feet for the northern Sierra and southern Cascades, with additional snow accumulations of 4 to 8 inches above 7000 feet, up to 2 feet over the highest peaks. Most of the snow is expected this morning, with snow levels expected to begin rising again in the afternoon, so the Advisory may be ended early. Over the northern Coastal Range and Shasta County mountains snowfall should be limited to higher peaks, so travel impacts are not expected. Shasta Dam profiler at 3 am had snow levels at 6000 feet.
Rainfall totals have trended down some with the first system. The National Blend of Models (NBM) has a 40 to 90% chance of 1.5 inches or more of rain between today and Sunday night in the Sacramento Valley and Delta region, 10 to 20% in the northern San Joaquin Valley. Shasta County specifically, along with the foothills, and mountain areas below snow level have a 70 to 90% chance of 2 inches or more of rain. Moderate to heavy rain rates will bring additional 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 and mountains below 6500 feet, with accumulating snow at higher elevations. A Flood Watch remains in effect for areas up to 6500 ft through 10 pm Sunday.
A second storm system is expected Monday through Tuesday night. Snow levels start high (6500 to 7500 feet), but will drop significantly by Tuesday morning as colder air from the Gulf of Alaska drops into the region. By Tuesday night, snow levels are expected to fall to around 3000 to 4000 feet in the southern Cascades, and 4000 to 5000 feet in the Sierra. Snow totals for the Sierra continue to be projected by the NBM to be around 3 to 4 feet above 6500 feet, which would bring major travel impacts. Overnight Tuesday and into Wednesday, snow levels are expected to fall low enough to potentially impact higher elevations of I-5.
Travel impacts are likely across interior NorCal so be sure to check road conditions at quickmap.ca.dot.gov and weather conditions at weather.gov before heading out the door!
Extended Discussion (Wednesday through Saturday)
Cluster analysis and ensembles continue to show active weather continuing through much of the remainder of next week, with additional moderate rain and heavy mountain snow totals. The third storm of this atmospheric river system has trended wetter and will be much colder than the first two and currently is expected to continue through Friday.
While the latest snow levels have trended up slightly, they are still falling quite low at 1700 to 2500 feet in the southern Cascades, and 3000 feet in the Sierra by late Wednesday morning. This, combined with continued moderate precipitation rates will likely lead to significant travel impacts to mountain roads, including I-80 and I-5.
Snow totals over the Sierra could reach 4 to 5 feet of additional snow, though there remains some uncertainty in the exact trajectory of the main plume of moisture in this second wave of weather and thus the exact rain and snow totals. With the lower snow levels there is even a non-zero chance for Redding to see light snow, with a 9% probability of 1 inch of snow Wednesday and Thursday.
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. For the next weekend, ensembles currently suggest a drier pattern may return.