Patchy fog remains possible through the morning hours before a system arrives bringing widespread light rain and light mountain snow to the region throughout the day today. Quick-moving system will bring widespread rain, breezy southerly winds, and light mountain snow Today-Thursday. Isolated thunderstorms possible today in the Delta, Valley and foothills; no debris flow concerns expected for burn scars. Another round of precipitation arrives Friday into Saturday before a quieter weather period sets up late weekend into early next week.
Discussion
Partly to mostly cloudy skies are evident across interior NorCal on latest GOES-West imagery as of early this morning. A few areas of patchy fog are also being reported across the southern Sacramento and northern San Joaquin Valleys as well. The cloud cover is working to keep temperatures mild at this time, with overnight lows only expected to drop down into the low to mid 40s in the Valley and foothills and 30s to low 40s at higher elevations.
Moving into the day today, a shift toward a more active weather pattern is anticipated. Upper troughing will near the Pacific Northwest throughout the day with a southwesterly fetch aloft over interior NorCal providing moisture and lift to the region.
This will result in widespread light rain across much of the forecast area and light mountain snow at elevations of 5500 feet and higher along the Sierra. Probabilities of exceeding 0.25″ of QPF are in the 80 to 100% range from the I-80 corridor northward, with a 40 to 70% chance for points southward.
Furthermore, snowfall probabilities of exceeding 1″ along the Sierra are only in the 20 to 40% range at this time. Despite the overall light nature of the precipitation, given the lower comparative snow levels, be prepared for the potential for some minor travel impacts at pass level, especially during the evening commute.
Breezy winds are also anticipated today with southerly gusts 35 to 45 mph at times over the Sierra.
The upper trough is expected to retrograde westward back over the Pacific as it evolves and as a result, precipitation chances are expected to dwindle into the day on Thursday.
With this brief reprieve in precipitation and increasing southerly flow as the next trough moves in off the coast of California, a slight warming trend is anticipated through the latter portions of the week. High temperatures on Thursday look to reach the upper 50s to low 60s in the Valley, with more widespread low 60s to mid 60s on Friday.
As the aforementioned trough off the coast begins to travel inland, precipitation chances will also begin to increase on Friday and linger into Saturday. Given the trajectory of this second trough, this looks to be like a wetter system compared to the midweek one.
Probabilities of exceeding 0.25″ of QPF from this system are in the 80 to 95% range across the entire forecast area, with probabilities of exceeding 1″ in the 50 to 70% range over the mountains and in the 20 to 40% range in the Valley.
Snow levels with this system look to be higher, generally in the 6000 to 7000 foot range, with probabilities of exceeding 1″ in the 70 to 90% range and probabilities of exceeding 4″ in the 40 to 70% range along the Sierra.
Extended Discussion (Sunday through Wednesday)
As precipitation chances begin to taper off throughout the day on Saturday, overall quieter weather is then anticipated across the late weekend into early next week. A shortwave does look to move through southern California late Sunday into Monday, but precipitation impacts across interior NorCal will be limited due to the location.
Otherwise, the new year looks be off to a quiet and seasonable start for much of the region. Cluster analysis does hint at a potential pattern change nearing mid next week, but details remain limited at this time.