Warm and dry weather with patchy morning Valley fog continues into Tuesday. A more active weather pattern then arrives mid to late week with gusty southerly winds, periods of moderate to heavy rain and mountain snow, and possible isolated thunderstorms.
FloodWatch from late Tuesday night through late Thursday night. Wind Advisory from 10 AM Wednesday to 10 AM Thursday. Winter Storm Watch from Wednesday morning through Friday morning for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.
Discussion
Overall quiet weather is being observed across interior NorCal early this afternoon, with a few small areas of mid level clouds throughout the Valley. Temperatures have rebounded into the mid to upper 60s for most Valley and foothills locations and upper 50s to low 60s at higher elevations. By the end of the day, little change is expected, although a few low 70s will be possible for portions of the southern Sacramento and northern San Joaquin Valleys.
More extensive cloud cover just off the coast is already being observed via latest GOES-West satellite imagery as an upper trough over the Pacific approaches. This area of cloud cover will gradually move inland this evening into Tuesday, somewhat limiting radiational cooling potential overnight.
A somewhat complicated forecast for Tuesday morning Valley fog is unfolding as a result. While overall lower potential, there is still a 20 to 50% chance of visibilities dropping below 1/2 mile across the southern Sacramento and northern San Joaquin Valleys, with highest probabilities for northern San Joaquin Valley locations.
Following any morning fog development, a mostly quiet and slightly cooler Tuesday is in store.
By Tuesday evening, a pattern change is expected to become more pronounced as upper level ridging pushes eastward toward the central Plains and the aforementioned trough nears the coast. Resultant precipitation chances begin to increase for the northern Sacramento Valley early Wednesday morning, before becoming more widespread for all of interior NorCal by late Wednesday morning.
There is still some variability in ensemble guidance regarding the progression of the trough that may push precipitation onset to slightly later in the day on Wednesday, but nonetheless, the impacts will remain the same.
Periods of heaviest precipitation and mountain snowfall are still expected to occur from midday Wednesday through Thursday.
Gusty southerly winds are expected to accompany this system from midday Wednesday through midday Thursday as well. Widespread gusts to 40 mph will be possible for much of the Valley, with occasional gusts of 50+ mph at higher elevations.
Additionally, given the warm, moist flow into the region, some weak instability on the order of a few hundred J/kg will be possible in the Valley and adjacent foothills ahead of a crashing cold front. This will spark a 10 to 20% chance for some isolated thunderstorm development on Thursday. Any thunderstorms that do develop will have the potential for gusty winds, dangerous lightning, brief heavy rainfall, and small accumulating hail.
As far as rainfall goes, the general anticipation is for 1 to 3 inches of QPF through the central and southern Sacramento and northern San Joaquin Valleys, with 3 to 6 inches possible across the northern Sacramento Valley, foothills, and mountains.
Resultant probabilities of exceeding 1″ of QPF are 70 to 90% in the Valley, and 95 to 100% across the northern Sacramento Valley, foothills, and mountains. In contrast though, probabilities of exceeding 2″ of QPF are only in the 15 to 40% range in the Valley, with still 50 to 95% probabilities across the northern Sacramento Valley, foothills, and mountains.
Snowfall with this system remains somewhat dynamic as snow levels are expected to gradually lower throughout the event. Initially, snow levels of 6000 to 7000 feet are expected on Wednesday, lowering to the 4000 to 5000 foot range by Thursday and even lower into Friday.
Heaviest snowfall is still expected from midday Friday through Thursday. As a result, probabilities of exceeding 1 foot of snow are in the 70 to 90% range at elevations above 6000 feet, with still a 15 to 50% chance of snowfall exceeding 2 feet above 6000 feet.
At lower mountain elevations, some snowfall accumulations are still expected, with a 60 to 90% chance of exceeding 4″ between 5000 and 6000 feet. Overall, looking at 1 to 2 feet possible at pass level above 6000 feet and locally up to 3 feet at mountain peaks, 4 to 8 inches between 5000 and 6000 feet, and up to 1 to 3 inches possible at some 4000 foot locations.
Extended Discussion (Friday through Monday)
Lingering showers are expected on Friday as the storm system progresses inland and moves out of the area. Snow levels will continue to lower as colder air moves in. Snow levels look to be around 2500 to 4000 feet Friday. The National Blend of Models (NBM) suggests a 30 to 70 percent probability of an additional 4 inches of snow over the mountains on Friday.
The Extreme Forecast Index (EFI) also indicates some CAPE anomalies on Friday, and the NBM suggests a 10 to 20 percent probability of thunderstorms mainly in the Delta, Valley and foothills on Friday.
Showery weather may continue on Saturday over the mountains, and then another wave of precipitation will be possible Sunday into early next week, however there is still quite a bit of uncertainty with this second storm.
Even so, unsettled weather looks to continue over the extended forecast period.