November 8, 2021 – Dry and cool start to today. The next system approaches NorCal late today through Tuesday, and will bring with it widespread rain, high elevation mountain snow, and wind. Dry conditions will return by Thursday with a warming trend.
Discussion
A weak short wave is currently pushing through NorCal early this morning. This is bringing some cloud cover but we should see some clearing by daybreak. Any clearing we do see will be short lived as our next system will begin to push in later in the day Monday. This will be associated with a trough in the Gulf of Alaska and a low that will go through strong cyclogenesis over the next 36 or so hours. Now the main trough will remain to our north but a short wave trough currently near 150W and 45N will push east and create a secondary low center. This is what will bring us our active weather for later Monday into Tuesday.
A strong warm front which is very evident on model sounds with a saturated inversion will begin to push into CA Monday night with light showers pushing into the Coastal Range ahead of the front during the late afternoon. Showers will continue to overspread the area during the evening and overnight and will also pick up in intensity and become widespread as the front pushes east through the region. We do see some weak CAPE above that saturated inversion and that may lead to few rumbles of thunder over western portions of the southern Sac Valley and northern San Joaquin early Tuesday morning.
The cold front will push into the area during the morning and slowly work south and weaken into the afternoon. With the warm front through and the cold front weakening activity will become more showery in the afternoon except over the mountains where upslope flow will keep showers more widespread. The trough will push east late in the day Tuesday with things drying out. Snow levels will be a bit challenging with this one. Currently we have quite a bit of cold air in place and that will keep snow levels down initially in the 3000 to 4000 foot range over Shasta County and 6000 to 7000 foot range over the Sierra Monday evening. Snow levels will come up as the warm front pushes in and will rise above 8000 feet for much of the Sierra late Monday night and 6500 to 7500 feet over the Shasta County Mountains and southern Cascade. The main uncertainly will be on how quick we see the snow levels come up.
This system does tap into some deeper moisture with the IVT pushing to around 500 kg/ms for a time Monday night and PWATs 0.75 to 1.25″. This could bring locally heavy rain rates for a short time Monday night of 0.40 to 0.60″ per hour. Total QPF is looking to be 1.50 to 3″ over the Mountains, 1 to 2.25″ in the Foothills and 0.40 to 1.0 across much of the lower elevations except for up to 1.50″ in the northern Sac Valley. For snow we’re looking at 4-12 inches above 4000 feet in Shasta County, 10-24 inches around Lassen Volcanic National Park, 2-9 inches of wet snow in the Sierra between 6500-8000 feet, and 6-18 inches in the Sierra above 8000 feet.
With the strong deepening of the low this system will also become quite dynamic with a low level jet of 40 to 55 knots over our area for a time Tuesday morning. Now these strong winds will have a tough time mixing down given the strong inversion but we will still see gusty winds during the morning Tuesday with gusts 25 to 35 mph across the lower elevations and 30 to 55 mph over the Mountains with some stronger gusts possible over the highest peaks.
Quieter weather sets in for Wednesday but a trough to our NW will bring weak warm air advection to northern areas. This will keep light shower chances in the forecast with the best chances over the Shasta County mountains. Dry weather with upper level ridging builds in for Thursday.
Extended Discussion (Friday through Monday)
Dry weather expected through the extended forecast period as upper ridge progged over interior NorCal. Some night/morning fog could be an issue in portions of the Southern Sacramento/Northern San Joaquin Valleys. High temperatures expected to be near to slightly above normal.