A brief lull today in significant accumulations ahead of a colder weather system bringing widespread rain and major mountain snow travel impacts with gusty winds. Chances for cold mornings return to the area this weekend into early next week.
Discussion
Looking to Wednesday, forecasts show a brief break in inclement weather with precipitation quietening across much of the area. With near zonal flow and minor height rises, this should put a pause in the active weather we have seen but keep some general breezy winds over area around 15-30 MPH. There is a chance for snow accumulation across the Shasta County mountains above 2500 feet but there is still some uncertainty regarding if enough moisture and forcing will be able to produce accumulating snow amounts further down in elevation. Current NBM forecasts only show a 25-50% chance of 4 inches or more for Northern Shasta County with current percentiles only showing amounts approaching impactful accumulations at the 75th and 90th percentiles. Given the unsupportive synoptic pattern and low probabilities, chances for any impactful accumulation along the lower elevations looks unlikely at this time. Another cool morning is in store however, with Valley lows in the mid to upper 30s.
Our next wave of weather begins on Thursday early morning as our low pressure system in the Gulf of Alaska slides further south and swings another weather system through Northern CA. This will bring another round of widespread rain, mountain snow, and gusty winds. Forecast amounts from Thursday morning through Friday night show 0.25-0.75″ in the Valley south of I-80, 0.50-1.50″ north of I-80 in the Valley and adjacent foothills, and 1.25-3″ over the mountains and Northern Sac. Valley, highest north of I-80 over the Sierra. Lastly, with the rain there will be slight chances for thunderstorms Thursday (5-15%) and Friday (10-20%) over the mountains and adjacent foothill areas.
Snow levels start relatively low with levels with 2000-3000 feet over the Shasta County mtns. and 2000-4000 feet over the Sierra. Late morning Thursday will see warm air push in from the south and raise snow levels to roughly 5000-7000 feet over the Sierra by Thursday night with cold air retaining over the Coast Range. As the system begins to wane, another dump of colder air moves in on Friday dropping snow levels again over the Sierra to around 3000-5000 feet and colder air remaining in place over the Coast Range. Forecasts amounts will be 8 to 24 inches with up to 3-4 feet along the highest peaks in the Sierra for elevations above 4000 feet. For the northern Shasta mountains, 6 to 12 inches is forecasted up to 2 feet for elevations above 2500 feet. Additionally, probabilistic Winter Storm Severity Index shows high chances of moderate impacts (80-100%) over the Sierra with a 40-70% chance over the Shasta mountains. Chances for Major impacts remain relatively over the Sierra but still lies at 40-70% chance showing positive signs for a moderate to major winter system Thursday and heading into Friday morning. Slippery and snow- covered roads, chain controls, and significant travel delays are all expected so please exercise caution if planning any mountainous travel.
Thursday will have the brunt of precipitation with things tapering off by Friday afternoon and a dry forecast for Saturday outside of some very slight chances for light showers over the Sierra. Temperatures are forecasted to nosedive early Saturday morning with lows near or below freezing in the Valley with even 27 degrees forecasted as a morning low for Redding. Cold mornings look to carry throughout the weekend and possibly into next week.
Extended Discussion (Sunday through Wednesday)
While interior NorCal will still remain underneath the broad influence of the departing trough, drier weather is expected through the weekend and into early next week, with breezy northerly winds and cold temperatures being the primary weather impacts expected into the extended. Slightly below normal high temperatures in the low to mid 50s are anticipated for the Delta, Valley, and foothills, with 40s to low 50s at higher elevations. Resultant cold weather impacts from brisk morning low temperatures across the extended are expected as well. Sunday and Monday’s chances show the biggest areal extent of cold weather chances with Monday being the stronger of the two. Current probabilities of low temperatures less than 30F for lower elevations sit around 40 to 70 percent from Sunday morning through Tuesday morning, although up to 80-90% across the Northern Sac. Valley. Take time now to prepare for expected colder temperatures and be sure to protect pets, plants, and people, especially those more susceptible cold weather impacts.