December 29, 2016 – Dry weather will continue through Friday with cool overnight temperatures and patchy morning fog in the Valley. A weak system will bring a chance for showers on Saturday with light snow accumulations in the mountains. Colder systems for next week with snow levels possibly dropping down into the foothills and northern Sac Valley.
Discussion
High pressure will continue to dominate the weather across the area for the next few days as closed upper low spins toward the SoCal coast. This will lead to mostly clear skies and light winds. Temperatures this morning will bottom out in the low to mid 30s with patchy frost again. Fog potential is limited and should remain very patchy across the Valley. Temperatures today will warm a few degrees from Wednesday’s highs with mid 50s to low 60s in the Valley and 40s-50s across higher elevations. Overnight lows should be several degrees warmer tonight.
Closed upper low will move inland across SoCal on Friday with a weak system dropping SE to our north. Models in general agreement with precipitation remaining either north or south of the region with another dry day on Friday. High clouds moving in though should cool temperatures a degree or two from today’s readings.
By Saturday, a weak system will drop south from the Pacific northwest and close off over NorCal. Latest model runs in general agreement with track of the low hugging the coast as it drops south. System is fairly moisture starved so expect precipitation to mainly limited to the mountains, although a few showers possible in the Valley as well. Snow accumulations will likely only be a few inches down to around 5000 feet. May be enough to cause a few headaches for holiday travelers during the day though. Temperatures will continue to cool into Sunday. Brief break in precipitation expected for New Years Eve into early New Years Day (Sunday) before next wave is progged to drop south into the area. Precipitation on Sunday looks mainly limited to the mountains, but colder air entrained in system may drop snow levels fairly rapidly to 1500-2500 feet by Sunday night. Again, precipitation amounts don’t look to be that significant but low snow levels will likely cause impacts across wide stretch of mountainous highways. Will be one to watch going forward as models come into better agreement.
Extended discussion (Monday through Thursday)
An inside slider will be dropping out of British Columbia and into our area early Sunday and into Monday. With it comes very cold air and at this point, it looks to be the coldest airmass of the season. By early Monday morning, 850 mb temperatures look to drop down to around -6 to -8C in the Northern Sacramento Valley which may be cold enough to support snow down to the valley floor for places like Redding. However, one big uncertainly is how much upper level support and moisture this system will have to work with. As mentioned in previous discussion, there has been poor run to run consistency in how much precipitation we may actually see.
While there are still many details to figure out, low elevation snow looks to be a possibility though at least Tuesday night before things begin clearing up Wednesday morning. When the clouds do eventually clear, our next concern will be the sub-freezing overnight temperatures. As this system kicks out, surface pressure gradients will tighten and bring a breezy, cold north wind to our area. This may limit how cold some places get Wednesday night but if the boundary layer happens to decouple overnight, valley locations will likely be looking at a hard freeze. Surface pressure gradients are expected to weaken further on Thursday and because of this, Thursday night may actually be more favorable for a widespread hard freeze. However, this is getting beyond our extended forecast but is something we will continue to watch.