Dry weather and cold morning lows continue through Thursday. Unsettled weather returns Friday and continues into next week with periods of light to moderate rain, high elevation mountain snow showers, and breezy southerly winds.
Discussion
Clear skies and light winds are being observed across interior NorCal early this morning. Temperatures are falling into the mid to upper 30s across the region, but most locations are still above freezing at the time of this writing. The area will wake to low to mid 30s this morning, with areas of frost and the potential for areas of isolated patchy fog as weak Delta Breeze winds push moist air into the Valley.
The upper level pattern features ridging over the PacNW and a closed low now making its way towards the Four Corners Region. These two synoptic features will continue to move eastward the next few days, allowing our area to remain dry with cold morning temperatures through Thursday. Continue to keep protective measures in place for the cold mornings for people, pets, and plants. Friday, our weather pattern is going to change and introduce precipitation chances for the area through next week. For the short term discussion, we will look at what to expect Friday and Saturday regarding precipitation and potential impacts.
On Friday, an upper level trough originating from a parent low pressure in the Gulf of Alaska will begin to move into the PacNW. This will allow our heights to flatten out, and the movement of decent moisture plumes to move into the area. Precipitation chances will begin Friday morning in the far north and northwestern sections of the warning area, before spreading south later in the day. While the entire region will have chances of precipitation, the better chances and stronger moisture content will likely remain north of I-80 in the Valley, so the northern Sacramento Valley, southern Cascades, northern Sierra and adjacent foothills stand the best chances of seeing precipitation. The upper level pattern will remain flat on Saturday, as the aforementioned low pressure digs slightly further south off the coast of British Columbia, which will allow further moisture to move into the area. Current National Blend of Models probabilities of 1 inch of rainfall or more Friday-Saturday are around 50-90% in the northern Sac Valley, mountains, and adjacent foothills. The rest of the region will see some accumulating rain, but chances of getting 0.50″ inches are around 20-50%.
Next, we’ll talk about the snow briefly. This first system, while it has trended higher in terms of total QPF, will be quite warm in nature. Snow levels will begin at around 5000 feet on Friday, then climb to 7500+ feet Saturday, so impacts appear to be minimal at this time. Roughly 1-3 inches, locally higher along the peaks is anticipated, so the need for an advisory is not currently warranted. Lassen NF will likely see the bulk of the snow with this first system. Again, the forecast has fluctuated several times over the past few days, so be sure to check the latest forecast at weather.gov/sto for updates and check our social media accounts.
Extended Discussion (Sunday through Wednesday)
Now we’ll get into the expected weather for next week. Ensembles and Cluster Analysis continue to show the trough continuing to influence our weather next week. Recent ensemble runs have brought the trough slightly further south, which has led to an increase in potential snow totals for the northern Sierra, southern Cascades, and the mountains of Shasta County. The total QPF footprint has also slightly increased across the region, with rain totals now reaching an inch or more into the Modesto area. Another factor that is increasing confidence of an active pattern continuing is the Extreme Forecast Index (EFI) showing contours and some shift of tails over the northern Sacramento Valley and mountains for QPF through Tuesday of next week. Snow levels look to remain at around 7000 feet on Sunday, then begin to drop into the 5500-6500 feet range Monday and Tuesday, falling further to around 5000 feet on Wednesday.
The NBM currently advertises around a 50-80% chance of 2 inches of rain or more in the northern Sacramento Valley, northeast foothills of the Sierra, and in the Sierra north of I-80 Sunday through Wednesday AM. Snow probabilities for 8 inches of snow or more currently are around 40-60% for elevations above 6000 feet for the same timeframe, however with the recent southerly trek of the trough shown within ensembles, these totals are likely to continue to change. Periods of breezy southerly winds are also in the forecast during the extended forecast period. Again, be sure to check back for the latest forecast information as we move through the rest of the week and details become more clear.