Widespread Valley/Foothill rainfall, mountain snow continues today through Saturday. A brief break in precipitation expected this afternoon/evening, then a second, wetter wave of precipitation arrives Friday evening. Breezy northerly winds develop on Sunday and into Monday.
Discussion
Key Points:
- First notable precipitation event of the Water Year will bring TWO WAVES of widespread measurable rainfall and mountain snow accumulations continuing today thru Saturday.
- First Wave (Through this afternoon/evening), Second Wave (Friday PM-Saturday)
- Following this system, north winds increase through the Valley on Sunday & Monday
- A Winter Weather Advisory has been issued for elevations greater than 6000 feet in the northern Sierra until 5 PM Today. Snow levels will start around 5000 feet this morning before rising to around 6000 feet throughout the day
The first wave of our first notable precipitation of the Water Year has begun across NorCal. Currently seeing light rainfall returns on RADAR across the Valley and Foothills, with some light snow showers in the Sierra. Total rainfall accumulations at the time of this writing range from a trace to around 0.15″, with the higher totals in the northern Sacramento Valley, specifically at Redding AP. This wave of precipitation is expected to continue to move off to the east-northeast through the morning hours, then a brief break in precipitation will occur this afternoon/evening, with some lingering rain showers possible in the northern Sac Valley. Some lingering upslope mountain snow showers across the Sierra will also be possible during this “break.”
On Friday, another wave of precipitation will rotate around a Gulf of Alaska low pressure and eject another trough into the region. This second wave still has some uncertainty regarding track; mainly if the second wave takes a more easterly trek we would likely see less precipitation. The second wave of precipitation may also bring some isolated thunderstorms across the Valley and Foothills, with the NBM showing around a 10% chance for the aforementioned locations. Any t-storms that do form will have dangerous lightning, heavy rainfall, gusty erratic winds.
Overall rainfall and snowfall totals have trended slightly down this morning, although potential impacts remain the same. Snowfall totals in the Sierra are forecast to be around 6-12″ inches through Saturday, on top of what has already fallen. Rainfall totals will range from an additional 0.50″ inches to 2.00″ inches, with the lower totals over the Central and Southern Sacramento and Northern San Joaquin Valleys. Snowfall totals across the Sierra are still trending at around 4-12″ inches, with the higher totals at the peaks. Snow levels look to fluctuate around 5000-6000 feet when precipitation is forecast.
In terms of products, the Winter Weather Advisory until 11PM tonight is still active, however with the “break” in widespread precipitation likely occurring later this morning/afternoon, we may end the advisory early. With the second wave of moisture moving in Friday PM, we are considering another advisory to capture that event. Stay tuned to our social media accounts and visit weather.gov/sto for forecast updates.
Finally, on Sunday as the trough ejects further east, gusty northerly winds will develop across the area. Northerly wind gusts of 30 mph will be possible, with the strongest winds along the I-5 corridor and Delta. Latest NBM probabilities of 30mph gusts or more for the mentioned areas are around 40-80%. Fire weather risks should be lowered thanks to the widespread rainfall in the forecast, but we will monitor conditions as we move through the rest of the week.
Extended Discussion (Monday through Thursday)
EPAC upper ridging builds into NorCal next week as upper troughing digs into the Great Basin. This pattern will result in dry weather with gusty north to east next Monday into early Thursday. Depending on amount of precip this week, locally elevated fire weather conditions might be a concern later next week. High temperatures gradually warm through the extended period returning to near normal next Thursday.