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 on Thursday and Friday. Unsettled weather will continue over the weekend before another strong storm affects the region early next week. Flood Watch from Thursday afternoon through Sunday morning.
Discussion
Active pattern continues with another short-wave trough pivoting inland early this morning bringing renewed showers and gusty southerly winds to interior NorCal. Snow levels remain rather low (around 1500-2500 ft) and another foot or more of snow accumulation will be possible across the northern Sierra today.
Lighter additional accumulations are expected across the northern mountains and Coast Range early this morning before showers taper off.
A short break in the weather is expected later today into early Thursday before a wetter and warmer storm moves in Thursday afternoon.
Heavier precipitation is expected to develop by Thursday night as deeper moisture is drawn northward into NorCal and interacts with strong forcing.
Snow levels will rapid rise in with strong warm-advection reaching 6500-8000 ft or higher Thursday night and early Friday before the cold front drops down Friday afternoon bringing slight lowering.
A few thunderstorms will be possible Thursday night and early Friday given the strong forcing with the approach of the front. There will be a chance for post-frontal thunderstorms later Friday and again on Saturday. Severe potential will have to be watched closely given the strong forecast shear, but deeper instability will depend on seeing some clearing.
Current QPF for Thursday and Friday remains around 1.5 to 3 inches across the Valley with 3 to 7 inches over the foothills and mountains. Rain falling on the current snowpack over the foothills will cause snow melt and runoff, though snow melt will not occur immediately limiting the flash flood threat.
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.
Heavy snow is also possible at the higher mountain elevations. Before snow levels raise, the Coastal Range and Shasta County mountains will receive 8 inches to 2 feet, mainly Thursday through Thursday night.
Over the Sierra for elevations above 5000 to 6000 feet, 1 to 2 feet is possible along and north of Interstate 80 and 1 to 5 feet is possible south of Interstate 80. The heaviest snow will be above 7000 feet.
Strong winds will accompany this storm Thursday night and Friday. Southerly gusts of 40 to 50 mph are likely across the Valley with gusts up to 60 to 70 mph over the mountains, and a Wind Advisory has been posted for the valley, foothills and lower mountain elevations.
Extended Discussion (Sunday through Wednesday)
Widespread precipitation continues over the weekend in the form of Valley, foothill and mountain rain and high elevation snow. Precipitation amounts are not expected to be as heavy and snow levels will not be quite as high compared to Friday, however lingering flooding impacts are possible as a result of the continued rain and snow melt combination, mainly below 4000 feet.
At this point, flood impacts are mainly limited to urban, small stream, and roadway flooding, rises on mainstem rivers, with mud and rock slides possible.
Ensembles advertise the potential for another atmospheric river to impact the region Monday through Tuesday. This system may bring additional flooding impacts. Preliminary precipitation amounts suggest 1 to 2.50 inches in the Valley, and 2 to 5 inches in the foothills and mountains.
Heavy snow is also possible at the higher mountain elevations, with snow levels generally between 5000-7000 feet.