November 23, 2019 – Quiet weather with mild temperatures are expected through the weekend. Low pressure passing to the east will bring a brief period of dry, northerly winds to the Valley on Monday, which will bring short lived elevated fire weather concerns. A pattern change will be seen Tuesday as a strong Pacific system reaches the West Coast. Rainfall may begin as early as Tuesday afternoon, with heavier precipitation seen by Tuesday night. Snow levels drop overnight and heavy snow is forecast in the mountains, with accumulations seen down into the upper foothills. Rain and snow continue Wednesday into Thursday with a chance for isolated storms being possible in the southern Sacramento Valley. Valley rainfall may be impactful given it has been several months since the region saw widespread rain. The weather becomes more showery on Thanksgiving Day, possibly into Friday, with below average temperatures and low snow levels.
Discussion
Weak high pressure is building into northern California tonight and remain through the weekend. Quiet, mild weather looks to be on tap for Saturday and Sunday with Valley highs in the upper 60s and 70s, with mountain temperatures in the 40s and 50s. Low temperatures will be around average values for late November.
A pattern change is still expected to begin Monday as ensemble models in alignment with depicting a weak system dropping into the Central Great Basin Monday, bringing breezy north winds to the Sacramento Valley and northern mountains. Wind gusts from 20 to 30 mph are expected mainly Monday afternoon, with locally higher values in wind prone areas. Elevated fire concerns are anticipated as fuels are extremely dry and and low relative humidity values will be seen in the afternoon. Winds are expected to decrease Monday night.
A deep, unseasonably cold upper level trough begins to impact NorCal Tuesday which will bring the first significant winter storm of the season to NorCal that could severely impact holiday travel. Ensemble models in better agreement today showing the low dropping down from Canada and along the western US coast Tuesday, deepening significantly over NorCal by Tuesday night. Light precipitation will begin Tuesday, becoming widespread by Tuesday afternoon to evening. Snow levels will be very low with this system with mountain snow accumulation expected above 2500 feet, and lighter snow is possible at even lower elevations, especially for the Sierra foothills and northern mountains north of I-80. Daytime temperatures will drop substantially Tuesday with the Valley in the low to mid 50s and mountains and foothills in the 20s to 40s, around a 10 degree drop from Monday`s temperatures.
This winter storm will last through Thanksgiving and possibly into Friday, with the heaviest snow currently expected to fall overnight Tuesday into Wednesday. A Winter Storm Watch has been issued for Tuesday through late Thursday for elevations above 2500 feet. If you have mountain travel plans leading up to the holiday, now is the time to consider altering those plans as mountain travel Tuesday night through Wednesday will be significantly impacted.
Extended Discussion (Wednesday through Saturday)
Cool, wet weather continues through the Thanksgiving holiday, potentially through Friday, with accumulating mountain and upper foothill snow causing travel problems.
Confidence is increasing that Wednesday, the day before Thanksgiving and traditionally the busiest travel day of the year will see a period of moderate to at times heavy snow, most likely in the morning hours. Snow levels with this rather cold system are currently expected to drop to around 2500 feet and possibly lower for the northern Sierra, below 2000 feet for the southern Cascades and northern Coastal Range. Winds at the Sierra crest could gust to around 50 mph, with blowing snow an issue for visibility. Travel over major interstates such as I-80 and I-5 will likely be significantly impacted. Drivers are urged to avoid mountain travel.
More showery snow continues through Wednesday afternoon and through Thanksgiving day, with additional snow accumulations expected. Afternoon/early evening thunderstorms are possible both days in the Valley and Delta from instability from the cold core aloft and possibly some sun breaking through the clouds. The area with the best potential is expected to be from around southern Lake County southward.
Latest models indicate unsettled conditions may linger through Friday, especially over the mountains. The Winter Storm Watch has been extended to 4 am Friday. While snow intensity should be decreasing, this is when the lowest snow levels are expected, around 1500 feet over Shasta County mountains to 2000 feet over the northern Sierra. Lighter, more scattered snow showers are expected through the day with snow levels rising about 1000 feet.
Valley highs are forecast to be in the upper 40s to low 50s while overnight temperatures range from the mid 20s to mid 30s. Participants in outdoor events like Thanksgiving runs and parades should dress for cool, wet weather. In the mountains, conditions will be much more wintry in nature. Highs in the 20s and 30s with lows in the teens are expected.
The weekend looks drier, but there could be some light showers, especially in the mountains. Temperatures look to continue to be cool.