Cold Valley temperatures tonight into Tuesday morning, with low elevation frost possible. Precipitation impacts rapidly return to the region by midday Tuesday, with periods of moderate to heavy precipitation continuing into this weekend. Freeze Warning from 10 PM this evening to 10 AM PST Tuesday. Winter Storm Watch from Tuesday afternoon through Wednesday evening.
- Freeze Warning: 10 PM tonight through 10 AM Tuesday morning for elevations below 2000 feet.
- Cold Weather Advisory: 10 PM Monday night through 10 AM Tuesday morning for elevations above 6000 feet along the Sierra.
- Winter Storm Watch: 4 PM Tuesday through 10 PM Thursday for elevations above 3500 feet along the Sierra/ southern Cascades, Coastal Range, and Shasta County mountains.
Discussion
Scattered showers have mostly exited to the east, over the Sierra early this morning. Temperatures are quite chilly this morning, with Valley temperatures in the 40s while air temperatures in the mountains/foothills sit in the 20s and 30s. Low temperatures this morning are expected to dip into the mid to upper 30s in the Valley; 20s to 30s in the foothills; and into the teens to low 20s in the mountains.
Cold morning temperatures continue to be in the forecast for tomorrow morning when there is a 20 to 40% chance of temperatures less than 32 deg F in the Valley. As a result, there is increasing confidence in more widespread frost impacts at low elevations across interior NorCal. A freeze warning has been issued for locations below 2000 feet in elevation beginning tonight at 10 PM until 10 AM Tuesday. Additionally, cold temperatures 3 to 15 degrees are expected along the Sierra at elevations above 6000 feet as well.
A brief reprieve from precipitation impacts is expected through Tuesday morning. With breezy northerly winds ushering in cooler air as the system exits the region and clearing skies expected by the evening/overnight hours.
A separate trough is then expected to rapidly deepen as it digs southwestward from the Gulf of Alaska, introducing several periods of precipitation impacts to the region from midday Tuesday into next weekend.
Showers are expected to spread eastward into interior NorCal by Tuesday afternoon, with more appreciable precipitation impacts, including periods of moderate to heavy snow above 3500 feet, generally from Interstate 80 northward.
Snow levels will initially sit as low as 3000 feet in elevations in far north Shasta County, with snow levels elsewhere between 3500 to 4500. These are expected to increase to 4500 to 5500 feet by Wednesday afternoon.
The National Blend of Models (NBM) is indicating a 40 to 90% chance of 2.00″ or more of rain north of I-80, with probabilities highest over the northern Sacramento Valley, mountains, and foothills.
Additionally, the NBM indicates 60 to 90% chance of 12″ of snow or more at locations above 3500 feet and north of I-80. Snow showers will gradually taper off overnight Wednesday into Thursday as snow levels increase. An active weather pattern is expected to continue into the weekend.
Extended Discussion (Friday through Monday)
Latest ensemble guidance and cluster analysis indicate an active weather pattern continuing over the extended forecast period as an upper level trough set up off the coast of the Pacific Northwest brings along multiple rounds of precipitation through the end of the week and into the weekend.
Atmospheric river moisture will allow for the potential for moderate to heavy precipitation at times, and the GFS ensemble probability of Integrated Water Vapor Transport (IVT) greater than 250 kg/(ms) is projected to be around 80-100% for the second half of the week.
This ample moisture is reflected in the National Blend of Models (NBM) as well, which currently suggests a 35-65% probability of rainfall amounts greater than 1 inch for areas south of Interstate 80, and a 70-90% probability for areas north of Interstate 80 over 48 hours from Thursday through Friday night.
Snow levels early Thursday morning will be around 6000 feet, and then rise quickly to around 7000-8000 feet through the day as warmer air moves into the area.
As the weather system moves inland later this week (some uncertainty still with timing), an additional round of widespread precipitation impacts is anticipated across interior NorCal by the weekend.
Probabilities for 1 inch of precipitation or more from late Friday night through Sunday are around 60-90%. There is also a 10-20% probability of thunderstorms over the Friday/Saturday timeframe that will be dependent on the timing of the system moving onshore.
Periods of gusty southerly winds are also expected at times over the extended forecast period.
With the active weather in the forecast this week, be sure to stay up to date with the latest forecast at weather.gov/sto and check current road conditions at quickmap.dot.ca.gov before traveling!