Cold temperatures this morning with low elevation frost. Storm system arrives late afternoon/evening, with periods of moderate to heavy rain, mountain snow, and gusty winds then expected through the end of the week and into the weekend.
Moderate to strong atmospheric river storm will bring prolonged periods of gusty southerly winds, rain, and mountain snow with rising snow levels and isolated thunderstorms tonight – Saturday AM. Heaviest precipitation expected north of Interstate 80. Additional lighter rain and mountain snow expected Sunday into Monday with isolated thunderstorm potential. Widespread major flooding impacts not expected given dry soils but enough rain to cause sharp rises on creeks, streams, and rivers and issues during commutes.
- Winter Storm Warning in effect from 4 PM today through 10 PM Wednesday
- Flood Watch in effect from 10 PM tonight through 4 AM Saturday
- Wind Advisory in effect from 10 PM tonight through 4 PM Wednesday
Discussion
Current GOES-West satellite imagery shows mid to high level clouds spreading inland across interior northern California early on this Tuesday morning. Current temperatures observations are trending approximately 5 to 15 degrees cooler than this time yesterday morning, with temperatures in the 30s to around 40 in the Valley and foothills, and in the teens and 20s in the mountains, valid at 345 AM PST. As temperatures continue to cool as the morning progresses, frost development will be possible throughout much of the Valley and foothills. The National Blend of Models (NBM) suggests a 40-80% probability of minimum temperatures less than 36 degrees this morning across elevations less than 2000 feet. For the mountains, lows in the single digits to teens are forecast, generally above 6000 feet. As a result, the Freeze Warning and Cold Weather Advisory both remain in effect through 10 AM PST this morning.
Ensembles continue to indicate the trough digging southward from the Gulf of Alaska mid to late week, allowing for widespread moderate to heavy rain and mountain snow beginning Tuesday afternoon and continuing through the end of the week. Precipitation is expected to arrive to northwestern California this afternoon and spread inland overnight, continuing over the next several days as the trough stalls and provides for additional rounds of precipitation.
Storm total precipitation amounts from this afternoon through Friday morning are around 3 to 6 inches in the Valley and lower foothills north of Interstate 80, locally up to 8 inches in some of the upper foothill locations. South of Interstate 80, storm total precipitation amounts are generally in the 0.50 to 2.5 inch range. Latest guidance shows the trough moving inland sometime over the Friday into Saturday timeframe, which will bring additional precipitation chances to the area that will continue into the extended forecast period.The Flood Watch remains in effect from 10 PM PST this evening through 4 AM PST Saturday.
Moderate to heavy mountain snow will also be produced by this storm system over the next few days. Snowfall is expected to first arrive this afternoon to the Coast Range and northern mountains before spreading inland moreso on Wednesday. The majority of snowfall with this wave will be over the Coast Range, Shasta County Mountains, Southern Cascades, and Sierra north of Interstate 80.
The NBM offers up a 65-95% probability of snow amounts of 12 inches or greater, with best chances above 4500 feet in those aforementioned locations. Snow levels initially start out around 3000-4000 feet, before rising on Wednesday to around 4000-5500 feet, and with continued rise to around 7000-8000 feet on Thursday. Motorists should plan on difficult to impossible mountain travel in those areas due to snow covered roads, chain controls, travel delays and gusty winds.
The Winter Storm Warning remains in effect from 4 PM PST this afternoon through 10 PM PST Wednesday.
Periods of gusty southerly winds are also anticipated with this storm system. Strongest winds are currently expected to be with the initial wave, late Tuesday into Wednesday. The NBM offers a 60-90% probability of wind gusts of 30 mph or greater, with best chances in the northern/central Sacramento Valley, adjacent foothills and over the mountains where a Wind Advisory has been issued from 10 PM this evening through 4 PM Wednesday. Gusty southerly winds continue at times through the end of the week, although tonight through Wednesday still looks like the strongest period. Isolated thunderstorms will also be possible beginning late Friday, as the NBM offers a 10-20% probability.
Extended Discussion (Saturday through Tuesday)
An active weather pattern is expected to continue into the weekend, with renewed chances for precipitation across interior NorCal. Ensemble guidance and cluster analysis depict the deepening trough offshore slowly progressing eastward toward the California coast. This will result in prolonging the window for additional precipitation impacts through the weekend and potentially into early next week.
As the trough becomes more centered over NorCal, snow levels look to fall to around 4000 to 5500 feet over the weekend as a colder airmass moves in. While exact details regarding precipitation totals from Friday into early next week remain somewhat uncertain, current probabilities of precipitation exceeding 2 inches linger around 40% to 70% for the Delta, central/southern Sacramento Valley, and northern San Joaquin Valley, jumping to 70% to 90% probabilities across the northern Sacramento Valley as well as along the Sierra. Given lowering snow levels through the weekend, additional mountain travel impacts will be possible along the Sierra, with current probabilities of exceeding 1 foot of snow around 60% to 80% along the Sierra above 5000` through Monday. Additionally, chances for isolated thunderstorms appear on Saturday (15 to 20%) and Sunday (10-15%) during the afternoon and evening hours, with Saturday being more favorable for convection. As the trough progresses further towards the coast on Saturday, southerly winds over the Sierra are forecast to periodically gust 40 to 50 mph Saturday morning with gusts 20 to 25 mph in the Valley.
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!