529
FXUS66 KSTO 241135 CCA
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
329 AM PST Tue Dec 24 2024

.SYNOPSIS...
Widespread rain, mountain snow, and breezy south wind gusts
expected through tonight (Christmas Eve) as our unsettled weather
pattern continues into weekend bringing minor to moderate travel
impacts.

.DISCUSSION...
Rain has moved over the far northwestern CA coast and will make
its way over the Coastal Range early this morning. This system
will be colder and wetter than the previous weekend system,
tapering off Wednesday morning with a Winter Weather Advisory in
effect from 7 AM this morning to 7 PM this evening over the
Northern Sierra and Southern Cascades areas in our forecast area.

With this, there is a 10-20% chance of isolated thunderstorms
from this morning through the evening with best chances this
morning over the Northern Sac. Valley and northeastern foothills.
Looking at hi-res models. there is a line of convection ahead of
the front this morning from (3 AM - 7 AM) affecting Valley
location north of I-80 and strongest as you move northward. With
any thunderstorm brings the threat of brief heavy rain showers,
lightning, breezy winds, and small hail.

Regarding the totals of this weather system (Now through 10 PM
Tonight), the Valley can expect 0.25-0.75" with 0.50-2.00" over
the Sierra and adjacent foothills and the Northern Sacramento
Valley, up to 2.50" near the Feather River Valley area. The
highest totals will be over the mountains north of I-80 with the
heaviest precipitation late this morning into the early afternoon.

Snow with this passage starts over the Cascades this morning
spreading throughout the Sierra throughout the morning. Snow
levels are fairly high at the start of this morning but falls to
5000 to 6000 feet by this afternoon becoming 4000 to 5000 feet by
tonight. A Winter Weather Advisory from 7 AM today to 7 PM tonight
is in effect Sierra Nevada and Lassen Park counties for
elevations above 6000 feet for snow accumulations of 5 to 10
inches, locally higher over peaks. Also, winds will be gusting as
high as 45 MPH.

Dry conditions will prevail by Christmas morning with a brief lull
in active weather through the early afternoon. The next wave of
active weather begins Wednesday night into Friday afternoon
bringing another round of mountain snow and widespread rain.
Highest amounts will be over Shasta County with accumulated
precipitation up to 2-3 inches. Snow impacts will be similar to
today`s system with a 30-60% chance of amounts greater than 6
inches above 5500 feet. Snow levels as the snow begins in full on
Thursday will be around 4500 to 5500 feet but increasing to 6500
to 7500 feet by early Friday AM.

The next wave develops Friday evening through the weekend. With
the increased snow levels on Friday up to 7000 to 8500 feet, this
should hinder any moderate snow from developing and be more rain
impacts through the beginning of the weekend.

Please continue to check weather.gov/sto for the latest updates
and check quickmap.dot.ca.gov before you go this holiday season!

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...

Models showing overrunning precip potential Saturday/Saturday
night from I-80 northward. Main shortwave trough then progged
through interior NorCal Sunday. GFS Ensemble IVT probability
showing Cat 2 AR (> 500 kg/(ms))associated with this wave
suggesting significant precip potential. WPC QPF/model guidance
showing liquid totals of 2-5 inches in the foothills and mountain
north of I-80. Snow levels look to be initially above 7500 feet
early Sunday, lowering to around 4000-5500 feet Sunday night.
Windy conditions likely late Saturday into Sunday as wave
progresses through.

Forecast uncertainty increases early next week as models diverge
on solutions. NBM leans towards drier EC with upper ridging
becoming main synoptic feature.

&&

.AVIATION...

Areas MVFR/IFR with lcl LIFR over mtns obscurring trrn til 03z Wed
as Pac storm moves thru interior NorCal. Slight chc of isolated -
tsra btwn 18z-01z. Snow levels 5500-6500 feet this morning, lowering
to 4500-5000 feet this evening. In Central Vly, areas Sly sfc wind
up to 20 kts with lcl gusts to 30 kts possible til 18z Tue. Over
mtns, areas of SWly sfc wind gusts 35-45 kts next 24 hrs.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 7 AM this morning to 7 PM PST this
evening for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&

$$