Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sacramento California
354 PM PST Fri Dec 6 2013
a cold weather system is beginning to bring widespread
precipitation to northern California this afternoon and will
continue into Saturday. Heavy accumulating snow for the mountains,
significant snow in the foothills, and potentially some
accumulating snow down to the valley floor in the northern
Sacramento Valley. A wet snow/rain mix is possible for rest of the
Sacramento and San Joaquin valleys east of i5 tonight into early
another very cold night last night under mostly clear skies and
light winds. Widespread subfreezing temperatures were observed
across the entire region, with low to upper 20s in the valley, and
single digit temperatures in the High Sierra communities. Record
temperatures have been broken in some locations as a result of
this Arctic air mass. High temperatures today have stalled under
increasing clouds as a result of an approaching winter system.
WV imagery shows a rapidly strengthening low bowing out from the
northerlies offshore of Oregon state this afternoon. Models
indicate this system will continue to intensify as it drops south
into California. This system is expected to be very cold
especially when considering the cold antecedent conditions in
place. This system will be a very efficient snow producer with
snow to liquid ratios up to 25 to 1. This is primarily due to dry
air beneath cloud bottom, cold air in and above the clouds, and
favorable storm scale dynamics. Snow levels are forecast to remain
near the valley floor along the east side of the Sacramento and
San Joaquin valleys and only lift above the valley floor where a
relatively warm, moist air mass seeping in from the Bay area will
make it difficult. The boundary between this warmer, moist air
mass and the approach cold air mass will be along the wet bulb
zero line and Divide the valley between rain and snow with a rain
snow mix near this dividing line.
Radar imagery indicated that precipitation began between noon and
1pm in extreme northern Shasta County and over the coastal range.
Spotters in Redding, Sims, Burney, Cottonwood, and Cobb Mountain
have already reported snow either in the air or on the ground as
of 2pm. The bulk of the precipitation is expected to move into the
area by 4pm in the coastal range and northern Shasta County before
spreading southward through Saturday morning. 1-3 inches of snow
is possible for the communities of Chico, Redding, and Red Bluff.
The foothills could see 3 to 12 inches, and the High Sierra
between one and two feet of fluffy snow. Elsewhere in the valley,
snow will struggle to accumulate but still may fall as a slushy
rain/snow mix. Areas west of i5 south of Willows should only only
see rain. South to southwest winds will accompany the
precipitation and could drastically drop visibilities when
combined with snow. Expect dangerous driving conditions and extended
traffic delays. Downed trees and power lines are a possibility in
the foothills due to the weight of accumulating snow. Lingering
snow showers are possible in the Sierra through midday Saturday.
The winter system exits into the Great Basin area late Saturday.
Dry and cold northerly flow develops again behind this system and
result in conditions similar to earlier this week. Extremely
cold overnight temperatures are likely between Saturday night and
Extended discussion (tuesday through friday)
Light, dry northerly flow will continue during the first half of
next week. This will bring continued colder than normal
temperatures along with subfreezing to near freezing overnight conditions.
Upper level ridge will build over California on Wednesday and
quickly shift eastward so we expect overnight lows to finally be
above freezing by Thursday morning.
Longer range models continue to indicate a breaking down of the
upstream ridge as a Gulf of Alaska low pressure moves through the
region late next week. The extended models (ecmwf, GFS, gem) are
now delaying the arrival of this next storm so that it looks to
move inland closer to Thursday morning. However, the models
continue to show notable differences in the track and which areas
of California will be most impacted by precipitation. Have continued to
try and take an average of all the models to show unsettled
weather for the second half of next week.
A low pressure system moves through the area tonight bringing
lowering ceilings and precipitation with snow levels down to
around 500 ft. IFR conditions with areas of LIFR are possible from kcic
and north until around 18z. Snow is expected at krdd and krbl with
snow accumulations around 3 inches possible at krdd and about an
inch around krbl. Rain and snow mix are possible around ksmf and
kmhr aft 06z. Southerly winds 10 to 20 knots will turn northerly
around 18z Saturday. Improving conditions are expected Saturday
afternoon and evening as the system moves off to the east.
Winter Storm Warning until 7 am PST Saturday Burney Basin /
eastern Shasta County...mountains southwestern Shasta County to
Northern Lake County...northeast foothills/Sacramento Valley...
northern Sacramento Valley...Shasta Lake area / northern Shasta
County...western Plumas County/Lassen park.
Winter Storm Warning from 10 PM this evening to 4 PM PST
Saturday motherlode...West Slope northern Sierra Nevada.
Winter Weather Advisory until 4 am PST Saturday Clear
Lake/Southern Lake County.
Winter Weather Advisory from 7 PM this evening to 7 am PST
Saturday central Sacramento Valley.