Valley rain showers and accumulating mountain snow return to the region today, along with breezy winds and much colder temperatures. Additional rain and mountain snow chances for the second half of the week.
Discussion
The dry, northerly winds are a thing of the past for most of northern California. Late Sunday afternoon/evening, the north/northwest winds switched and become more west/southwest, and even southeasterly for some locations at this hour. Accompanying this wind shift is more surface moisture. Sacramento International Airport’s dewpoint at 1 AM Sunday was a mere 14 deg F, now it is 39 deg F.
The much-discussed incoming system is right on our doorstep. As of the publication of this forecast discussion, the precipitation band is nearly into Shasta County, and will gradually advance south/east through the rest of the region today.
Winds will increase through the day, initially southerly, then switch to become west/northwest. This wind shift will reintroduce strong gusts for some areas, notably for the mountains as well as locations below west-wind-prone gaps in the northwestern Sacramento Valley adjacent to the Coastal Range.
Gusts are expected to remain below Wind Advisory criteria, but can’t rule out a periods with gusts around 35 mph in portions of Shasta, Tehama, Glenn, and Colusa Counties. Winds in the higher elevations will be even stronger, possibly approaching/exceeding 60 mph at the exposed peaks.
Best precipitation chances with highest QPF forecast will be in the mountains where enhanced orographic lift will be realized. Radar may end up looking more impressive than reality for the Valley this morning as the atmospheric layer just above the surface between the ground and cloud may be a tad too dry support stratiform precipitation (i.e., virga).
Best potential for precipitation the Valley will be from scattered convective showers and isolated thunderstorms. SPC Day 1 outlook paints nearly the entire region under general thunder risk. Convective-allowing models suggest around 100-250 J/kg surface-based CAPE which would support some isolated thunderstorms. Main threat would be small hail and heavy downpours. As is with scattered convection, rain totals will range from nothing to around 0.10″ in the Valley, locally higher in stronger showers/thunderstorms.
Mountain snow levels will start out around 4000 to 5000 ft with this storm, then lower in the afternoon. Expect around 3 to 6 inches in the Coastal Range and Shasta County, increasing to 6 to 14 inches in the southern Cascades and northern Sierra, and localized 18 inch totals south of I-80 at peak-level.
Latest WPC guidance has been a bit more bullish compared to the operational National Blend of Models. Official forecast follows the WPC guidance with snow totals generally falling on the higher end of the above ranges.
Precipitation chances drop off quickly after sunset today with clearing skies overnight.
Tuesday morning will start off cold with some areas in the Valley approaching the freezing mark or lower. Coldest Valley communities around sunrise will be around Redding where 30-32 deg F is a possibility. National Blend of Models gives Redding Airport a 70% chance of hitting 32 deg F and a 60 percent chance of hitting 30 deg F.
Taking into consideration the recent weeks’ warm weather and the growing season upon us, will issue a Freeze Watch for the northern Sacramento Valley. Valley locations south of the Freeze Watch will also be cold with widespread middle/upper 30s likely, even for Sacramento.
Cool and mostly dry conditions will prevail through mid-week. Any precipitation chances through Wednesday will be generally limited to the northern quarter of California, including parts of Shasta and the southern Cascades.
The next system that will bring renewed precipitation chances to most of the region will be on Thursday as a subtropical moisture tap arrives to the coast. Main impacts will again be in the form of mountain snowfall as the Valley appears to be on track for generally light rain amounts. Precipitation chances will continue on Friday/Saturday; more details in the extended discussion.
Extended Discussion (Friday through Monday)
Wet weather looks to continue Friday into Saturday as another Pacific frontal system moves through. Snow levels are expected around 4500 to 5500 feet Friday lowering to 3500 to 4500 feet Friday night into Saturday. Additional significant amounts of mountain snow expected with travel impacts possible.
Drier weather forecast Sunday into Monday with high temperatures warming to above normal as EPAC ridging builds inland.