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000 
FXUS66 KSTO 101850
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1050 AM PST Tue Dec 10 2019

.SYNOPSIS...
Quiet weather today with areas of morning valley fog and near 
average temperatures. Rain chances return to the region tonight 
with more widespread precipitation expected late in the week.

&&

.DISCUSSION...
The Morning Update: Visibilities have improved as dense fog near the 
I-5 and Hwy 99 corridor from the Sac vicinity to SCK and MOD has now 
lifted into the proverbial low gray cloud deck. Much of the low 
cloud deck may persist throughout the day, lifting this evening when 
the frontal system moves into the region, spreading light rain over 
much of our CWA late this afternoon and evening. Radar echoes 
showing up along the Nrn CA coast at press time. Timing of the upper 
trof suggests the precip should end by Wed morning as the trof moves 
through our region. Behind the trof, heights rebuild on Wed as a 
ridge axis slides into the region. Some light sprinkles/showers are 
still possible mainly in the Nrn part of our CWA on Wed, where 
warm-advection may allow for some lingering light precipitation.
JHM

.Previous Discussion...
The upper ridge that brought the dry weather to the region on Monday 
is moving east as the next upstream short-wave trough approaches. 
Satellite imagery shows high clouds out ahead of the system already 
moving over NorCal.

Fog has been developing in the southern half of the Sacramento 
Valley southward into the northern San Joaquin Valley since late 
Monday evening. So far, the most extensive dense fog has been in 
the Sacramento area with patches of dense fog in around Modesto 
and across portions of the Motherlode. The extensive high clouds 
may limit further spread of dense fog, and forecast soundings 
indicate it may lift to a low stratus deck this morning.

Dry weather will persist today, but a brief period of light 
precipitation is expected across NorCal tonight as the short-wave
and a shot of warm-advection move through interacting with TPW of
around 3/4 to 1 inch over the area. QPF of less than a tenth of 
an inch is likely in the valley, with less than a third of an inch
over the mountains. An inch or two of snowfall accumulation will 
be possible above 5500-6000 feet.

Ridging in the wake of tonight's wave gradually flattens the
remainder of the week as strong mid/upper level jet across the
eastern Pacific reaches the West Coast. Most of the forecast area
is expected to see dry weather Wednesday, except across the
mountains north of Redding where warm-advection may allow for some
lingering light precipitation.

Light precipitation is then forecast to increase again along a 
stalled out front by Thursday as warm-advection ramps up and 
deeper moisture spreads into the region. The front may sag far 
enough south later Thursday into Friday to allow light 
precipitation to spread across the Sacramento region and into the 
I-80 corridor over the northern Sierra. Snow levels will be above 
8k ft limiting winter travel impacts.

&&

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...
Fairly zonal upper level flow is expected to remain over NorCal 
Saturday. Weak shortwave along the flow will bring a chance of 
light showers to the area Friday night through Saturday. Another 3
to 6 inches of snow accumulation is possible Saturday mainly for 
elevations above 5000 feet, though dropping snow levels early 
Saturday could bring light accumulation down to 4000 to 4500 feet.
This could cause some travel issues over mountain passes 
Saturday.

Zonal flow breaks down behind this weather system as upper level
ridging builds over the area Sunday into Monday, bring dry
weather back to interior NorCal. Ensembles hinting at wetter
weather system impacting the West Coast beginning Tuesday. However
confidence is low at this time, so keep an eye out for forecast
updates. HEC 

&&

.AVIATION...
IFR/LIFR visibilities across the Central Valley due to fog 
through 18-21Z, generally areas Marysville and south. Otherwise, 
VFR visibilities and winds generally less than 10 kts.

&&

.STO WATCHES/WARNINGS/ADVISORIES...None.  

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