The active weather pattern persists into next week. A warmer storm system will bring moderate to heavy rain with flooding impacts to rivers, creeks, streams, and roadways at elevations below 4,000 feet, along with higher elevation snow, strong winds and thunderstorms tonight and Friday. Unsettled weather will continue through the weekend before another strong storm affects the region early next week. Flood Watch from 1 PM PST this afternoon through Sunday morning. Wind Advisory from 4 PM this afternoon to 4 PM Friday.
Discussion
Short-lived break in the weather will last into this morning before precipitation begins to spread back across interior NorCal. Current temperatures are in the upper 30s to upper 40s across the Central Valley, and mostly in the upper teens to lower 30s over the mountains.
Satellite imagery along the West Coast is rather impressive early this morning with the cold trough lingering off the PacNW coast and increasing deep convection along the belt of tropical moisture moving up from the southwest.
These ingredients will come together later today into Friday as the polar jet (associated with the northern system) and subtropical jet merge over the region. Widespread light to moderate precipitation will spread inland by mid to late morning.
Heavier precipitation will develop tonight as deeper moisture along the strong atmospheric river arrives (IVT of 700-1000 kg/m/s) coincident with strong forcing.
Snow levels will rapidly rise tonight into Friday with strong warm-advection, reaching 6500 to 9000 ft before the cold front drops down Friday afternoon bringing slight lowering.
Current storm total precipitation amounts through Sunday night have increased some with around 2 to 3.5 inches in the Central Valley with 5 to 12 inches over the foothills and mountains.
Rain falling on the current snowpack over the foothills and lower mountains will cause snow melt and runoff, though snow melt will not occur immediately.
There are moderate flood concerns due to the heavy rain along with added runoff from snow melt, and a Flood Watch remains in effect for elevations below 4000 feet. Flooding of roadways, rivers, creeks, streams, and other flood prone areas will be possible, especially in areas that have poor drainage due to snow blocking drains and culverts.
For elevations above 4000 to 5000 feet, the snow pack should be able to absorb much of the rain, though localized flooding is still possible from heavy rain on surfaces without snow, especially if snow is blocking drains and culverts.
A Flood Watch remains in effect for elevations below 4000 feet for Thursday afternoon through Sunday morning. Rocks slides and local mud slides are possible in susceptible areas.
Burn scars should remain largely snow covered, at least for the next several days, so the debris flow danger is minimal.
In addition, heavy rain on accumulated snow on roofs will bring additional weight, bringing the potential for overloading. Heavy snow is expected at higher mountain elevations.
Snow levels will rise rapidly during the day over the Sierra. Before snow levels raise, the Coastal Range and Shasta County mountains above 3000 feet will receive 10 inches to 2 feet, mainly Thursday through Thursday night. Some Locally higher amounts are possible over peaks.
A Winter Storm Warning is in effect there from 7 am Thursday to 1 am Friday. Lighter snow amounts are possible down to around 1,800 feet for the Shasta County mountains.
Over the Sierra and mountains of western Plumas County snow will spread in by midday. For elevations above 5000 to 6000 feet, 2 to 4 feet is possible north of Interstate 80 and 4 to 8 feet is possible to the south. The heaviest snow will be above 7000 feet, with even higher totals possible above 8000 feet.
This snow will be more dense than the powdery snow that has been seen with recent storms, more of the ‘Sierra Cement’ variety. Travel at the passes could be difficult to impossible at times.
A Winter Storm Warning is in effect from 10 AM until 1 AM Friday. Strong winds will accompany this storm Thursday night and Friday. Southerly gusts of 45 to 55 mph are likely across the Valley.
A Wind Advisory is in effect through that period for the Valley, foothills and lower mountain elevations. Mountain ridges could see gusts up to 60 to 70 mph, locally up to 80 mph over higher ridges.
A few thunderstorms will be possible tonight and early Friday in the warm sector and will bring short bursts of heavier rain (rates exceeding 1/2 inch/hour). There will be a chance for post-frontal thunderstorms later in the day Friday. Best potential for some stronger, longer-lived, storms appears to be over the northern San Joaquin Valley where the best combo of shear and instability is likely.
Showery weather will continue over the weekend with a chance for mainly afternoon and early evening thunderstorms. Severe potential will have to be watched closely given strong forecast shear, but deeper instability will depend on seeing some clearing allowing stronger surface heating.
Extended Discussion (Monday through Thursday)
Overall pattern remains wet and unsettled during the extended period as clusters show broad troughing over the eastern Pacific with persistent moist flow. A storm system is forecast to impact the region early next week. While not as strong as this week’s system, it could still bring widespread moderate to heavy precipitation and renewed flooding concerns.
Latest QPF forecast indicates 1-2+” across the Valley, with 2-5″ over the foothills and mountains.
Heaviest precipitation is expected Monday night through Tuesday. Will continue to monitor given the potential for renewed flooding concerns. Heavy snow is also possible at the higher mountain elevations.
Snow levels for the extended forecast period start off between 5000 to 7000 feet Monday, lowering to 4000-6000 feet Tuesday. EFI also depicts breezy southerly winds developing early next week.
By mid- week, timing and strength differences remain, but ensemble guidance suggests additional rounds of precipitation are possible.