Warm and dry weather prevails today, our next weather system arrives tomorrow and brings gusty southerly winds, periods of moderate to heavy rain and mountain snow, and isolated thunderstorms. Unsettled weather pattern remains in place as we move into the weekend as another system moves through interior NorCal.
Discussion
Quiet and dry weather prevail across interior NorCal this afternoon. The morning fog that was evident on GOES-18 Satellite imagery in the northern San Joaquin and southern Sacramento Valley has lifted and visibility conditions have improved.
Our next system begins to move into the area overnight and will bring widespread precipitation chances for NorCal beginning tomorrow morning. Upper level analysis shows the ridge currently responsible for our dry weather and warm daytime temperatures is breaking down and moving off to the east today. Our next weather system is a trough located in the Pacific Northwest and it is beginning its southward decent into our area and will change our upper levelflow to a more southwesterly component.
Some Hi-Res models have the onset of precipitation in Shasta County and Coastal Range beginning late tonight, then as we move through Wednesday morning widespread precipitation overtakes interior NorCal.
The heaviest rainfall is expected to occur late Wednesday morning through early Thursday morning. Afternoon model runs have trimmed back some of the rainfall totals, especially for areas south of Red Bluff. Latest NBM rainfall totals are around 1-2″ inches for the Central/Southern Sacramento Valley and northern San Joaquin Valley, and 2-5 inches for areas in the northeast Foothills and areas north of Chico.
Latest National Blend of Models (NBM) probabilities for exceeding two inches of rainfall from this storm are highest in the Foothills, the Southern Cascades, the Sierra, and Shasta County, with a general range of 60-95%.
For the Central and southern Sacramento Valley and northern San Joaquin Valley, probabilities of exceeding one inch of rainfall are around 70-90%.
A FloodWatch remains in effect from early Wednesday morning through late Thursday night for the aforementioned areas. Potential impacts include urban and small streams may flood from prolonged heavy rain. Moderate rise are likely along the Upper Sacramento River System and tributaries, however at this time all points look remain in Monitor Stage, except for Tehama Bridge which is forecast to reach Flood Stage Thursday night.
Periods of moderate to heavy mountain snow are likely, with the heaviest snowfall expected Wednesday afternoon through Thursday. Snow levels will start at around 6000-7000 feet on Wednesday, then lowering to around 4000-5000 feet on Thursday, and any lingering snow showers on Friday will have snow levels around 3000-4000 feet.
NBM probabilities for exceeding one foot of snow for elevations of 6000 feet, including locations on I-80 and HWY-50, are generally at 70-100%. Forecast totals range from 1-2 feet for locations at 6000 feet or higher, with locally 3 feet or more at mountain peaks.
Gusty winds of 65 mph are possible with this system as well, combined with snowfall rates of 1-2″ inches per hour, white-out conditions may occur.
Our Winter Storm Watch has been upgraded to a Winter Storm Warning beginning 10AM PST Wednesday to 4 AM PST Saturday. If you can, avoid travel during this timeframe as traveling conditions will be hazardous with snow covered roads and chain controls, road closures, and possible downed trees and telephone poles with the gusty winds.
Gusty southerly winds are also in the forecast for the Valley, adjacent Foothills, and the Delta Wednesday morning through early Thursday morning as the frontal system moves through the area. The strongest winds are forecast for the aforementioned areas in the Wednesday night timeframe.
Latest NBM probabilities of exceeding 40 mph wind gusts are around 70-100% for the areas mentioned above, with a 50-95% chance of exceeding 50 mph for areas north of Chico and south of Redding.
A Wind Advisory is in effect from 10 AM PST Wednesday through 6 AM PST Thursday, with potential impacts including difficult driving conditions, tree limbs blown down, and power outages. Now is a good time to secure any loose objects in your yard.
Additionally, There is a slight (10-25%) chance of thunderstorm development behind the initial front passage on Thursday and Friday in the Valley and adjacent Foothills. NBM CAPE values are not overly impressive, with a modest 100-200 J/kg of CAPE, however any clearing will aid in adding instability, so an isolated thunderstorm with heavy rain, dangerous lightning, gusty winds, and small hail cannot be ruled out.
EXTENDED DISCUSSION (Saturday THROUGH Tuesday)
Ensembles and Cluster Analysis continue to indicate that active weather will remain through the extended forecast, with another upper- level low developing off the southwest CA coast. This will bring southwest flow across the region beginning Sunday.
This low will tap into another AR, leading to additional widespread rain, mountain snow, and breezy winds. The overall moisture associated with this AR system across is not as impressive as the AR in the short term across our area, but because cold air will remain in place from the mid week front, snow levels will be fairly low for the late weekend- early next week system.
Snow levels will be lowest initially Sunday morning, with levels down to around 2500-3500 feet, increasing to around 4000-4500 feet. This will allow for the primary impacts to be snow, as snow ratios will be higher, allowing for larger snow totals.
There’s still some discrepancies as to exactly how much precipitation we may see with the late weekend-early next week system, so exact forecast totals right now are not that confident. The overall message is, that a colder system will bring more widespread rain and mountain snow to the region.