Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sacramento California
945 PM PST Fri Dec 6 2013
a cold weather system will move over northern California tonight
bringing widespread precipition to the area. Snow showers will
linger over the Sierra Nevada into Saturday while the rest of the
area clears out. Heavy accumulating snow is expected for the
mountains, with 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 I-5 tonight into early
radar indicates widespread precip ahead of the cold front nearing a
line from Mt. Shasta to Bridgeville areas...just northwest of our County Warning Area.
Liquid rainfall amounts have not impressive with most amounts less
than a tenth of an inch thus far due to dry sub-cloud layer and lack
of interaction with a decent tpw plume. Low snow levels have been
impressive...with spotters reporting up to a couple of inches of
snow around the rdd (3-4" north of rdd) area making roads a
mess...as well as along the I-80/US-50 corridor where snow has
fallen into the low foothills...Auburn/Grass Valley/Nevada City with
an inch or two of accumulation in some locations. High resolution
models indicate the frontal band of precip will be dropping into the
80/50 corridor between 06z and 11z...and by 13z or 14z ending in the
valley but lingering in the siernev. Snow in Shasta co should
diminish by around midnight. The cold Arctic air has/will result in
high slrs and a dry powerdery snow. This type of snow will add
nothing to the water equivalent of the anemic snowpack.
Mainly areas along the 80/50 corridor will likely see most of their
quantitative precipitation forecast as the front drops sewd during the night. There should be a band
of .10 to .25 inch amounts with the front which may only be over one
area for 1-3 hours generally...giving a general indication of how
much more quantitative precipitation forecast we can expect...with the higher amounts over the West
Slope siernev. Given slrs of 20-25:1 it still looks as if 1 to 2
feet of snow could fall/accumulate over the siernev above 5 kft with
several inches below that. Dewpoints have slowly been on the rise
ahead of the front in the srn part of the valley where the wbz are
forecast to stay above freezing through the duration of the precip.
The NAM seems to be trending well with the wbz basically showing the
wbz lowering to the ground from Corning...to Oroville... Loma
Rica...east of Rocklin/Folsom...Ione...to LaGrange. So anywhere
along and then north and east of that line is likely to get snow.
The wbz does arc to the northwest from Durham...to Willows to Yountville by
4 am Sat morning...but little precip is expected to be left over at
this time. If showers linger behind the front into mid to late
morning...then rain/snow showers are possible in the SAC and sck
areas per se. Jhm
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 near
the valley floor. 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.