December 5, 2023 – Rain and mountain snow chances Wednesday and Thursday, with gusty winds at times. Drier conditions return Friday with locally breezy north winds.
Discussion
GOES-West satellite imagery depicts some increasing cloud cover early this morning, generally north of Marysville. This could limit fog development across portions of the Sacramento Valley. Any fog should lift by mid to late morning. Otherwise, dry conditions are expected today with mild afternoon temperatures in the mid 60s across the Valley.
A pattern shift is anticipated on Wednesday as an upper-level trough and associated frontal boundary push into the region. This will bring precipitation chances to the area starting early Wednesday morning over the Coastal Range and northern Sacramento Valley, then spreading southward across interior NorCal the rest of the day. NBM probabilities show a 60-95% chance of exceeding 1″ over the mountains and the northern Sacramento Valley Wednesday through Thursday morning. Latest WPC guidance indicates 0.10-0.50″ for Red Bluff southward across the Valley, with 0.50-1.50″ across the mountains through early Thursday morning. Generally looking at 2-5″ of snow at Sierra pass levels, with up to 12″+ near Lassen National Park. NBM probabilities indicate a 10-40% chance of snow amounts exceeding 4 inches over the Sierra.
In addition, the NBM suggests a 10-15% probability of thunderstorm development Wednesday evening/night, including the northern Sacramento Valley and adjacent Sierra terrain.
Southerly winds will increase on Wednesday, mainly over the Sierra and the northern Sacramento Valley. Wind gusts up to 25-30 mph are expected across the northern Sacramento Valley, with gusts up to 30-45 mph across the Sierra. Another shortwave trough will bring another round of precipitation on Thursday.
Generally light precipitation is expected with this secondary wave, with best chances north of I-80. WPC guidance indicates less than a tenth of an inch across the Sacramento Valley, with 0.10-0.25″ over the mountains. Light snow accumulations are expected, with less than 1-3″ at pass levels.
Otherwise, a cooling trend is anticipated through the short term period, with Valley highs in the mid 50s to low 60s. Dry conditions return Friday as upper ridging begins to build in behind the departing system.
Locally breezy north winds may develop Friday afternoon, with gusts up to 15-20 mph along the west side of the Sacramento Valley.
Extended Discussion (Saturday through Tuesday)
Clusters are generally keeping a ridge over the area through the extended period although it does flatten out the the ridge slightly for Saturday and Sunday. The flatten of the ridge over the weekend matches up with the deterministic models that bring some precipitation over the northern areas mainly from Plumas County northward. The NBM ensemble brings precipitation further south so while there is agreement over the northern areas there is uncertainty on how far south any precipitation will extend.
Monday brief ridging looks like it should dry us out for the daytime but another weak system moves into the Great Basin late Monday night and Tuesday with the European model having the deeper trough and a little further west. Some showers would be possible over the western slopes for this pattern.