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Area forecast discussion
National Weather Service Sacramento California
430 am PST Thu Dec 12 2019



Synopsis...
light rain at times today, heaviest in the mountains. Heavier
snowfall returns to the Sierra Friday through Saturday, creating
mountain travel issues. Drier weather Sunday and early next week.
&&

Discussion...
upper level flow is mainly zonal over California this morning.
Radar shows lingering showers remain over portions of the valley
and mountains as a weak Pacific moisture plume continues to
stream into norcal. Scattered showers will continue throughout the
day, heaviest in the mountains and foothills. Snow levels are
high, generally above 7500 to 8000 feet, with little to no
accumulation expected at pass level.

Precipitation continues Friday through Saturday as another
slightly stronger weather system moves into norcal. Another influx
of Pacific moisture will produce liquid precipitation amounts of
around half an inch to one and a half inches in the foothills and
mountains. Precipitation will be lighter in the valley with around
a quarter to a half of an inch in the northern Sacramento Valley,
tapering significantly south of I-80. Heaviest precipitation
expected Friday afternoon through early Saturday. This will be a
colder system with snow levels dropping to around 6500 feet
Thursday night through Friday. Snow amounts up to 5 to 10 inches,
locally up to 15 inches, are forecast in the Sierra through early
Saturday for elevations above 6500 feet. A winter storm advisory
has been issued. Travel is discouraged during this time as gusty
winds and periods of heavy snow will create hazardous travel
conditions over Sierra passes.

Precipitation turns showery Saturday with additional snow amounts
of a few inches forecast Saturday through Sunday morning. Snow
levels are expected to fall to around 4000 to 45000 feet behind
the frontal passage. Light snow accumulations under an inch are
possible down to the foothills. Current ensembles indicate another
brief increase in precipitation is possible Saturday evening
producing a quick few inches of additional snowfall over mountain
passes. Precipitation will taper off overnight Saturday, clearing
out by Sunday morning.

Upper level ridging builds over the area Sunday bringing dry
weather for the beginning of the work week. Hec

&&

Extended discussion (monday through thursday)...
the start of next week will be dry as models build heights across
the region. Forecast 1000-500 mb thicknesses should remain on the
low side in the wake of the earlier frontal passage. In spite of
the ridging aloft, this will keep monday's temperatures close to
climatology, roughly in the mid 50s over the valley with 30s into
the mountains. While skies will be mostly sunny to start the week,
a projected pattern shift is likely by sometime mid-week. A
somewhat complicated pattern offshore remains unresolved by the
available guidance. Multi-day ensemble trends have shown a slowing
of this system, generally arriving on Wednesday. It appears some
form of closed low will pinch off from the prevailing westerlies
which frequently offers low predictability. Recent runs of the GFS
and European model (ecmwf) differ in timing and placement, with the former
bringing the better threat for precipitation. At a minimum, expect
an increase in cloud cover Tuesday into the subsequent days.
Looking farther out, the gefs atmospheric river ensemble
probabilities show a signal for wet conditions likely during the
Dec 21/22 timeframe. Such values are higher than usual this number
of days out, so this bears watching as well. ~Bro

&&

Aviation...
widespread MVFR/IFR conditions expected through 20z Thursday.
MVFR/IFR conditions may continue through Tuesday evening in the
northern Sacramento Valley. Light winds generally less than 12
kts.

&&

Sto watches/warnings/advisories...
Winter Weather Advisory from noon Friday to 10 am PST Saturday
for West Slope northern Sierra Nevada-western Plumas
County/Lassen park.

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