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FXUS66 KSTO 111137

Area Forecast Discussion
National Weather Service Sacramento CA
337 AM PST Wed Dec 11 2019

Light showers possible on Wednesday and Thursday. Widespread 
precipitation expected later in the week and into the weekend. 
Drier weather Sunday and early next week. 


A weak weather system moved through NorCal last night spreading 
precipitation across the area. The upper level trough has moved east 
into Nevada with only lingering showers remaining early this 
morning. Areas of light fog have developed behind the frontal 
passage. Fog could become denser in areas this morning, though it
will be dependent on high cloud cover.

Upper level flow expected to become more zonal over NorCal today. A 
shortwave along a deep trough centered around the Gulf of Alaska 
pushes into the West Coast today, bringing an increased chance of 
light precipitation to areas generally north of I-80 this afternoon. 
Increased precipitation is expected across most of the area 
overnight Wednesday into Thursday, tapering off significantly south 
of the Sacramento area. Heaviest precip amounts forecast in northern 
Shasta County and mountains north of I-80 with amounts up to 0.75 to 
1 inch possible. Lighter amounts expected in the rest of the Valley 
with only a few hundredths south of Sacramento up to around a 
quarter of an inch in the northern Sacramento Valley. Snow levels 
will mainly be above 7000 to 8000 feet with minimal accumulation 

Ensembles continue to suggest precipitation chances continue Friday 
through Saturday as impulses move along the zonal flow. The majority 
of precipitation will remain in the foothills and mountains with 
precipitation amounts of half an inch to an inch and a quarter 
forecast. The Valley could see anywhere from a few hundredths of 
an inch to a quarter of an inch. Snow levels will begin to drop 
Friday to around 6000 to 7000 feet before dropping below pass 
level overnight Friday with light accumulation down to 4000 to 
4500 feet possible. Snow accumulation around 5 to 10 inches is 
possible over the Sierra which could cause hazardous travel
conditions over the Sierra passes. HEC


To conclude the weekend, broad troughing will have shifted into 
the Four Corners which allows for heights to build over the region
from Sunday into early next week. This supports a period of dry 
weather accompanied by mostly sunny skies and near average 
temperatures. From Tuesday onward, there is a greater amount of 
uncertainty in the offshore (upstream) pattern. Progressive flow 
will carry a longwave trough across the central Pacific this 
weekend with its fate dictating impacts over northern California. 
While 06Z GEFS/00Z ECMWF ensemble means show a trough entering the
picture next Tuesday/Wednesday (December 17/18), deterministic 
guidance show the potential for a closed low to separate itself 
from the prevailing westerlies. Such solutions are wildly variable
so confidence is low in the eventual outcome. However, did raise 
the chances for precipitation in the forecast due to the presence 
of the trough. ~BRO 


Widespread MVFR/IFR conditions at TAF sites through 20z. -SHRA 
possible across Central Valley through 16z. Precipitation returns 
this afternoon with -RA possible after 23z. Light winds generally 
less than 10 kts expected. 



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