Colder unsettled weather into Saturday with the potential for significant snow in the foothills and mountains. Snow possible across portions of the northern Sacramento Valley later this week. Gusty winds late this week. Additional rain and snow Sunday into early next week. Winter Storm Warning from 4 AM Thursday to 4 PM Friday.
Discussion
Colder air is filtering into region this morning. The north end of the valley indicates temperatures running around 10 degrees colder than this time yesterday and Blue Canyon is 25 degrees colder.
There is a lull in activity this morning but showers are expected to increase this afternoon with possible isolated thunderstorms. Profiles only indicate some weak instability today. Low snow levels will continue to be around 1000 feet into Friday and may dip to around 500 feet at times.
Western slopes are expected to increase with mainly snow showers today but are not expected to increasing in intensity until tonight when greater snow accumulations are expected. The valley may see a few snow showers tonight but precipitation continues to remain very limited for the valley, so no impacts are expected.
Later Thursday into Friday continues to be the period when the low moves southward and precipitation increases across the area. There is still a hint from the models that the initial main focus will be along the coastal range and into the Northeast foothills and adjacent mountains most of Thursday night.
Late Thursday night and Friday morning intensities increase over the Motherlode and western slopes with the heaviest totals south of US-50. Interestedly the models still keep a focus of heavier precipitation along the coastal range during this time period as well. This is the time period when the western side of the valley is vulnerable with the potential for some heavy snow accumulation.
What makes this a difficult forecast is the winds increase from the south starting Thursday afternoon and continue into Friday morning. This may help to bring some warmer air into the valley.
Already from a day ago forecasted low temperatures have increased but at this point models still have heavy amounts of snow forecast for the westside of the valley. Lower dewpoints on the westside may be what helps to keep snow in the forecast.
Wind advisory will likely be needed late Thursday afternoon into Friday morning throughout the valley.
Adding to the weather there will be the potential for some thunderstorms on Thursday. The storms will probably contain mostly small hail with all the cold air in place with some accumulating hail possible.
Wind shear looks really nice as well so a busy day looks to be in store. Friday evening the low moves south of the region along the coast. As it does precipitation will be winding down from north to south. Some wrap around moisture on Saturday may be able to continue to produce some light snow showers over the mountains mainly south of I-80.
Extended Discussion (Sunday through Wednesday)
Clusters and ensembles are consistent in showing a stormy pattern from early to the middle of next week. These systems originate from western Canada and while cold, they won’t be as cold as the ones this week.
This is due to the overwater trajectory, with marine modification to the airmass bringing milder temperatures. Because of this, snow levels are not projected to be quite as low, generally around 2,000-3,000 feet. This would bring accumulating snow over the mountains and potentially down into the upper foothills.
Moisture levels look higher with a better marine tap, so higher precipitation totals are anticipated, with the potential for several feet of moutain snow. Current probabilities for the northern Sierra for 3-day totals of 4 feet of snow are around 45% to 75%, which is quite high for this far out.
The setup begins Sunday as the cold upper low over Southern California exits to the east with just some light snow showers over the Sierra south of I-80. Morning lows should be cool but around 5 degrees higher than on Friday. A weak shortwave will gradually move southward into the region bringing increasing chances of precipitation through the day.
This is ahead of a stronger system Monday through Tuesday, with yet another cold system dropping down from western Canada. ECMWF ensemble run-to- runs are showing good consistency in a fairly strong, wet system Monday into Tuesday.
The current timing suggests the heaviest mountain snow Monday night into Tuesday morning. Winter travel is expected to be difficult. Some gusty southerly winds are also projected during that time period, bring additional impacts. Valley rainfall totals for Sunday through Tuesday are currently projected around 0.75″ to 1.50″, but this is just an early estimate with quite a bit of uncertainty in the details.