Freeze Watch 1, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
335 AM PST Sun Dec 26 2021

.SYNOPSIS...
Periods of widespread Valley rain, mountain snow, and gusty winds
expected into early this week. Holiday travel will be impacted,
especially over mountain passes. Foothill snow possible at times
through Wednesday. Cold temperatures Monday night into next
weekend.

&&

.DISCUSSION...
Our unsettled weather pattern will continue and we will see
another wave of heavier precip later today into tomorrow with
weaker systems into the middle part of the week. Showers have
diminished in lower elevations for the most part and are becoming
less widespread over the mountains as the trough that brought
rain and snow Saturday pushes off to the east. Showers will
continue to become less widespread over the mountains during the
morning hours and the valley will remain mainly dry. The period of
drier weather will be short lived as our next short wave will dig
down the coast later Sunday into Monday. Showers will become more
widespread once again over the mountains and in northern areas
during the afternoon and evening Sunday continuing to work south
Sunday night into Monday with the trough axis and cold front. The
heaviest precip can be expected later Sunday evening into the
overnight. The trough axis and front will push south and east of
the area by the afternoon Monday with showers diminishing.

Snow levels are around 2500-3500 feet in the Sierra and 500-1500
feet over Shasta County. These snow levels will fall some yet this
morning and we should see snow levels between 1500 and 2500 feet
for Sunday into early Monday morning. Snow levels will then fall
from north to south Monday morning reaching 500 to 1500 feet by
the afternoon and down to a few 100 feet Monday night. Several
more feet of snow can be expected over the Sierra by Monday
afternoon and several inches to a few feet can be expected in the
upper foothills with some light accumulation down to 1000-1500
feet.

A few weak short waves will then push over the area on Tuesday and
Wednesday and that will bring light shower chances to the region.
Very cold air originating from Canada will be in place and this
will bring extremely low snow levels of only a few hundred feet.
These systems will have very little moisture with them and that
will limit accumulation overall but some snow flakes will be
possible at times with any of the showers across much of the area
including most of the Sacramento Valley and we could see a dusting
of snow down to a few hundred feet.

Cold temperature are also going to be a concern for early this
week as this is some of the coldest overnight lows we have seen
in December since 2015/2016 and coldest highs in December since
2011/2012. Overnight lows starting Monday night will be in the 20s
to low 30s in the valley with 20s over the foothills and single
digits to teens in the mountains. Clouds will keep things a few
degrees warmer Tuesday night but it will still be cold. Highs
Tuesday and Wednesday will only be in the upper 30s to lower 40s
in the valley, 30s in the foothills and teens and 20s in the
mountains. These highs will be within a few degrees of the daily
record lowest highs for these days.

-CJM

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...
The EFP continues the very cold wx pattern over Norcal for the
remainder of 2021, before some relief after the New Year. Issues
with the Day 8-10 Clusters this morning negated further discussion
of yesterday's apparent magnitude (depth) of the GOA/NErn Pac
trough, however we can say differences in the 26/00z models
continue this uncertainty.

NBM Forecast Records page does not indicate any record cold (either
low mins or low maxes, save for the low max on the 28th at RDD)
although the ECMWF EFI for min temps shows a trend toward the lower
end of the climatological mins for at least a portion of our CWA on
Thu and Sat.

Much less precip is expected during this EFP. The retrograding low
pressure system off the CA coast will track well to our SSW on Thu
with drier, but cold NNWly flow from the GOA/NErn Pac prevailing
over Norcal. Short wave energy dropping SSEwd from the Yukon and
across B.C could bring some light to modest amounts of snow to
the Nrn Mtns/Sierra on Fri before it digs a longer wave trough
across the Great Basin/Central and Srn Rockys on Sat. Clearing and
cold conditions behind this system are likely to bring another
shot of cold air into our region. Some moderation in temps
expected on Sun ahead of aforementioned large GOA/NErn Pac trough.
This system is likely to return some precip to our CWA by Mon.
The duration and magnitude of this system is uncertain at this
time. JHM

&&

.AVIATION...
Several hrs of VFR conditions in the Valley early this morning
before MVFR/IFR/LIFR conditions become widespread after 18-20z and
continue for about 24 hrs until 18-20z Mon. Area of showers
forecast to become more numerous from KMYV-KSCK this afternoon
along with gusty Sly winds 25-35 kts mainly KMYV northward to
KRBL/KRDD. Then more widespread light-moderate rain overspreading
the SFO Bay area into the Sac Vly mainly south of RBL after
midnight and continuing into Mon morning. Widespread heavy snow
and LIFR conditions improving slightly over the Sierra this
morning with more very heavy snow forecast over the Sierra
developing by midday and continuing until midday Mon before
improving conditions. New snow accumulations today thru Mon up to
7 feet over higher peaks. Over the Sierra SWly to Wly wind gusts
30-50 kts, locally higher over the Sierra crest.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 4 AM PST Tuesday for Burney Basin /
Eastern Shasta County-Motherlode-Mountains Southwestern Shasta
County to Western Colusa County-Northeast Foothills/Sacramento
Valley-Shasta Lake Area / Northern Shasta County-West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Freeze Watch from Monday evening through Tuesday morning for
Carquinez Strait and Delta-Central Sacramento Valley-Motherlode-
Mountains Southwestern Shasta County to Western Colusa County-
Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-
Northern San Joaquin Valley-Shasta Lake Area / Northern Shasta
County-Southern Sacramento Valley-West Slope Northern Sierra
Nevada.

&&