A series of strong storms will move across the region into next week bringing moderate to heavy rain, high elevation snow, and gusty winds. Periods of heavy precipitation will keep flooding threats elevated and mountain travel impacts at the forefront.
Discussion
Short-wave ridge moving in has brought a break in precipitation to much of the forecast area overnight, though light precipitation associated with the approaching warm front is beginning to spread back inland across areas mainly to the north of Interstate 80.
The next frontal system will begin to move into northwest California this afternoon bringing an increase in QPF to the Coast Range and Shasta County mountains where some areas could see around an inch of QPF by sunset. This front will slow tonight and early Saturday as the parent short-wave lifts north, but widespread light to moderate precipitation will only gradually spread south.
Another short wave trough will be right on the heels of Saturday’s. Warm air advection precip develops ahead of it later Saturday with another cold front moving through Sunday. Both of these systems will have plenty of moisture to work with and periods of moderate to heavy rain can be expected. 1′ to 2″ over the Valley, 1-3″ over the foothills, and 2-5″ over the mountains (especially the Shasta Co mountains and Feather basin where southerly orographic enhancement will be maximized.
Snow levels with the weekend systems will be 4500-5500 feet with the majority of the snow accumulation above 5000 feet. Snow totals are looking to be 1-2 feet with some of the higher elevations seeing 3 feet or more leading to significant travel impacts.
We will also see increased southerly winds over the weekend with wind gusts 30-45 mph over the Valley and up to 55 mph over the mountains as each front moves through, though winds will likely be less than the last couple of systems.
The rainfall over the weekend will bring renewed rises to local streams, creeks, and rivers. We’re still looking at Monday as the beginning of another potentially “big, or bigger” event as a stronger system moves ashore bringing widespread moderate to heavy rain and strong southerly winds.
Snow levels will rise to near the higher pass levels, but travel will likely remain dicey with strong winds, lots of slush with a mix of rain and snow at the pass levels.
This will be a longer duration strong AR system, and the trajectory will be more favorable for very heavy rain in the foothills and northern Sierra from orographic enhancement. Flood impacts will likely become more widespread Monday as the cumulative effects from all the recent rain come together. Pay close attention to the weather and have a plan if you live in or near a flood prone area.
Extended Discussion (Tuesday through Friday)
Moderate to heavy rain from Monday’s AR will likely be continuing during the day on Tuesday as a strong negative-tilt upper trough swings in bringing the potential for bands of heavy showers and embedded thunderstorms to put a finishing touch on early week precipitation.
GEFS plume averages for QPF for Sacramento are about 2 inches for the 36 hour period from early Monday through Tuesday afternoon, though there’s a large number of members that are up around 3-4 inches for that period. Modesto’s numbers look pretty similar. Up at Redding, the average is about the same at 2 inches, but the higher end numbers are around 4 inches or more.
Given the strength of the AR, the official forecast is tracking toward these higher end numbers.
Other than Monday, Tuesday is probably the day where you’ll likely need to keep a really close eye on the weather as the potential for widespread flooding of rivers, creeks, streams and roadway and urban flooding will be at its highest during the next week as all the runoff and heavy precipitation comes together resulting in a mess.
Winter weather impacts will also be increasing in the Sierra on Tuesday as snow levels lower and heavy precipitation continues. We may see a brief break during the middle of next week before additional rain arrives late next week.