Active weather continues through much of the week ahead as the last two systems in this series impact the region. Impacts from moderate to heavy rain and mountain snow and gusty winds are expected through midweek. Additional impacts from moderate rainfall, heavy mountain (and potentially upper foothills) snowfall, and breezy winds are then anticipated across the end of the week. Major mountain travel impacts are expected as a result.
Discussion
A shortwave trough with an atmospheric river will bring stormy weather to the area today into tonight. Radar and rain gauge observations show widespread moderate to locally heavy precipitation across the area. A heavier band of precipitation overnight extended through Butte County, gradually shifting southward to around the I-80 corridor by 3 am. Heavier amounts recorded are in the northeast foothills, just east of Chico, where 3 to over 4 inches have fallen over the past 24 hours. Hourly rain rates around 0.45″ to 0.65″ have been observed in that area, with some reports by the CHP of local road flooding.
A Flood Advisory was issued for minor flooding in low- lying and poor drainage areas due to excessive rainfall through 545 am this morning for much of Butte County as well as portions of Placer, Nevada, Plumas, Sierra and Yuba counties. Rainfall intensity should diminish some later this morning, except over Shasta County then increase across the area in the afternoon and evening with periods of moderate to heavy rain. There is also a slight chance of evening thunderstorms (10-20% probability) over the Motherlode.
Current forecast additional rainfall from early this morning through tonight totals are 2 to 3 inches in the Valley, 3 to 5 inches in the foothills, and 3 to 7 inches in the mountains below 5500 feet. A Flood Watch remains in effect to 4 am Wednesday, for below 3500 feet for the northern Coastal Range and Shasta County, for below 5500 feet for the rest of the area. Watch out for ponding of water on roads when traveling. In the mountains and foothills there remains the potential for mud and rockslides on roads.
Moderate to heavy snow has been falling in the mountains overnight, with chain controls on many mountain highways. Blue Canyon Airport automated station has been reporting some wet snow. Caltrans reported 11″ of new snow at Kingvale. Donner Summit and Castle Peak Caltrans webcams show snow covered roads.
A Winter Storm Warning remains in effect until 10 am Wednesday for the Sierra and southern Cascades/Lassen Park. Total snow accumulations of 1 to 3 feet are expected above 5500 feet, up to 4 feet over Sierra peaks, up to 5 feet in Lassen Park. Winds gusts could gust as high as 60 to 70 mph. There is a 20 to 50 percent chance of snowfall rates on I-80 around Donner Pass of one inch per hour through the morning, increasing to 80 to 100 percent chance in the afternoon and evening. There is a 30 to 60 percent chance of two inches per hour in the afternoon, 60 to 90 percent chance this evening. Snow rates this heavy combined with gusty winds could bring whiteout conditions at times. Additionally, snowfall totals of 4 to 8 inches and up to 1 foot over peaks will be possible across the Coast Range/Shasta County mountains.
A Winter Weather Advisory in place from 10 pm tonight through 10 pm Tuesday for the Coast Range/Shasta County Mountains above 3500 feet. While snow levels drop after midnight tonight below 2000 feet, precipitation will largely be over, but some light snow is possible down to those levels. Overall, be prepared for Major mountain travel impacts from this system through this evening.
In addition to the mountains, gusty southerly winds are also expected in the Valley, Delta and portions of the foothills. with southerly gusts to 40 to 50 mph. Forecast winds this evening have trended stronger, so the Wind Advisory has been extended to 10 pm tonight for portions of the northern and central Sacramento Valley, and goes into effect from 10 am to 10 pm today for the remainder of the Sacramento Valley and portions of the Delta and northern San Joaquin Valley.
As the trough shifts eastward, precipitation is expected to become less widespread and generally light for much of Wednesday. Gusty winds also look to become primarily breezy through the middle of the week as the system departs as well. Some portions of the southern Sacramento and northern San Joaquin Valley may even see a few rays of sunshine on Wednesday as well!
Depending on the timing of this potential clearing, some chilly morning temperatures will be possible throughout the Valley on Wednesday morning, with generally 30 to 50 percent probabilities of low temperatures less than 35F.
Despite the relative lull for much of Wednesday, precipitation chances are once again expected to increase late Wednesday night as the last of the series of 3 waves of precipitation begins to move toward interior NorCal.
This will be a cold longwave trough which will move into the area Thursday through Friday bringing 1 to 3 feet of additional mountain snow, locally up to 4 feet over peaks with major winter travel impacts expected. The cold air with this system could bring light snow down to the upper foothills. A reduced moisture tap will mean less rainfall for the Valley and Delta, with amounts around a half inch to an inch.
Extended Discussion (Saturday through Tuesday)
For the weekend into early next week, ensembles continue to show good confidence in a drier pattern. Expect breezy north winds at times as well as the area remains between the departing longwave trough and eastern Pacific ridging.
Morning temperatures are expected to be cool, near freezing for many Valley and Delta locations with widespread frost. The mountains could see lows in the teens and single digits.