Next storm moves in late tonight bringing periods of heavy snow over the mountains, widespread moderate to heavy rain at the lower elevations, isolated thunderstorms, and strong, gusty southerly winds through early next week.
Discussion
Afternoon radar imagery shows a few showers over the mountains. Otherwise, the next storm system will bring periods of moderate to heavy precipitation tonight through early next week as it taps into deep moisture. Hi-Res models and HREF show precipitation spreading northward tonight into Sunday.
NBM probabilities of exceeding 2 inches of precipitation range from 40-60% in the Valley, with 70-95% probabilities over the foothills and mountains. WPC guidance indicates 1.5-3 inches of precipitation across the Valley, and 2 to 4 inches in the foothills from tonight through Tuesday, leading to concerns for roadway ponding and flooding of urban and poor drainage areas.
Heaviest precipitation is still expected on Sunday. A Flood Watch remains in effect from Sunday morning through early Tuesday morning.
Isolated thunderstorms are possible Sunday and Monday across portions of the Valley, Delta, and foothills, with the latest NBM depicting a 15-30% probability of thunderstorm development. Any thunderstorms that develop could produce heavy rain, small hail, gusty winds, and lightning.
Heavy mountain snow is expected tonight, through early this week. Winter Storm Warnings remain in effect for portions of the Coast Range, Shasta County, Western Plumas/Lassen Park and northern Sierra Nevada, including I-5, I-80, Highway 50, and others. For the Shasta/Coast area, total snow amounts of 1 to 3 feet above 3500 feet are expected, with 2 to 5 inches in the lower elevations. Snow levels will be 2000 to 3000 feet Sunday morning, increasing to 3500 to 4500 feet by Monday morning.
For the Sierra, snow accumulations of 2 to 4 feet are expected, with up to 5 to 6 feet at the higher mountain peaks. Southerly winds gusting as high as 65 mph will be possible. Snow levels will be 3000 to 4000 feet initially, increasing to 4500 to 6500 feet by Sunday afternoon and 4000 to 5000 feet on Monday. HREF indicates snowfall rates up to 1-3″ per hour.
Difficult to impossible mountain travel with snow-covered roads, chain controls, whiteout conditions, and road closures will all be possible. Mountain travel is highly discouraged while the Warnings are in effect.
The Extreme Forecast Index (EFI) continues to highlight anomalous winds for much of the forecast area as pressure gradient tightens, particularly for areas near I-80 southward.
There is a 70-100% chance of wind gusts greater than 40 mph for most areas, and a 30-80% chance of wind gusts greater than 55 mph for areas near I-80 southward. Southeasterly wind gusts of 50 to 60 mph are expected, with local gusts up to 65-70 mph. Blown objects, downed trees, difficult driving conditions, and power outages are possible.
The Wind Advisory has been upgraded to a High WindWarning for the Delta, most of the Valley, and the Sierra foothills from Sunday morning through early Monday. People should remain weather aware this weekend, and always be checking the weather forecast at weather.gov, and road conditions at quickmap.dot.ca.gov before heading out the door.
Extended Discussion (Wednesday through Saturday)
Ensembles and cluster analysis continue to indicate that the aforementioned upper-level low will shift eastward by mid week next week.
Leaving in it’s wake, northwesterly flow, which will leave the door open for weak shortwaves to move into the PacNW and Northern CA; keeping the possibility of isolated to scattered showers in the forecast through late next week.
The first shortwave would move through sometime Wednesday with another slightly stronger one Thursday-Friday. Confidence in exactly where these shortwaves will dig and the exact timing is fairly low right now.
Regardless, the moisture associated with these shortwaves looks limited, so any impacts will likely be minimal, especially in comparison to those expected in the short term. This can be highlighted when looking at the NBM probabilities of >0.50″ from 12Z Thu-12Z Sat is less than 20% chance of precipitation in the Valley and a 30-50% chance in the foothills.
Additionally, there is a 20 to 40% chance of 4.00″ or more of snowfall above 3500 feet in the Sierra and Southern Cascades. Since we will continue to have northwesterly flow, snow levels will remain fairly through through the extended, fluctuating between 3000-5000 feet. As per usual, details will become more clear as we get closer in time.