Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 191720

920 AM PST Fri Dec 19 2014

Rain and mountain snow today as a Pacific cold front moves through
the region. Drying south on Saturday with a few showers north
especially over the mountains. Precipitation threat pushing
northward on Sunday as high pressure begins pushing northward over
the west coast. Dry all areas Monday/Tuesday under high pressure
ridge. Another system may impact the region Wedneseday evening
into Christmas Day.


Have made a slight update to the forecast this morning. Radar is
showing a swath of precipitation from the Bay Area into the Delta
region into the Sac Metro area and up towards Blue Canyon in the
Sierra. This system has areas of moderate rainfall (snowfall above
5000 ft) and is moving slowly across this region. Have increased
the chance of precipitation for our CWA from the Delta region into
the valley and up into the Sierra where that band is and southward
for late morning into afternoon. Another wave of precipitation
should move in tonight, but a look at hi-res models suggest that
it may arrive later than expected. Will take a look at the timing
and fine-tune details for this afternoon`s package. May need to
consider whether the snow advisory needs to extend into Saturday
morning. JBB

.Previous Discussion...
A Pacific frontal system now moving into northwest California will
be moving across the north state today. Radar is currently showing
light precipitation out ahead of the from from about Red Bluff
westward. Current timing has the main frontal band hitting the
Sacramento valley around 18z then moving into the Sierra by 00z
this afternoon. The frontal system should be mainly south and east
of the forecast area by 12z Saturday morning. Main impact of this
system will be snowfall over the Sierra passes with snow levels
between about 5000 and 5500 feet. Models still put snow amounts at
about 3 to 6 inches with up to about 10 inches at the highest
elevations or enough to cause travel difficulties getting over the
Sierra on a weekend. Flat ridging begins to push up over the west
coast on Saturday but a weak disturbance riding over this ridge
with bring a continued threat of showers over the mountains and in
the valley from about Sacramento northward. Precipitation from
this disturbance should be quite light. Upper ridge continues to
amplify on Sunday and may just see enough clearing over the
southern portions of the CWA to allow for some patchy fog in the
valleys. Some cloud cover is likely though so fog does not look
like it will become too extensive. Sunday looks dry over much of
the north state but all models showing yet another shortwave
disturbance tracking over the ridge Sunday afternoon.
Therefore...have kept shower threat over the northern mountains
and Shasta county although again...precip amounts will remain
quite light. Warmer airmass under the ridge and a little less
cloud cover will allow daytime highs on Sunday to climb to above
normal for this time of year. Upper ridge finally amplifies enough
on Monday to push any precip threat north of the CWA. Morning
valley fog is likely again in the morning Monday burning away in
the afternoon with daytime highs warming to several degrees above

.Extended Discussion (Tuesday through Friday)

Dry weather will continue Monday and Tuesday as high pressure
ridge remains in control across the region. Only impact in this
period will be areas of valley fog each morning with lingering
surface moisture.

The next system of interest is progged to move through late
Christmas Eve into Christmas Day as it drops ESE from the Pac NW into
the Great Basin. The eastern Pac high center placement has
trended a bit farther south. This shift in the high center has
allowed the incoming trough to progress through NorCal rather than
deflect it to the northeast into the Great Basin. These previous
runs hadn`t looked that wet; instead, they looked colder and
drier. Now with this more progressive trough from a moister/less
cold source region, snow levels should stay around 4500-5000 ft
with potentially more snowfall Christmas Eve into early Christmas.
The timing of this system could pose some travel headaches.

High pressure builds in late Christmas Day into Friday with a NNW
jetstream overhead, resulting in increased northerly to
northeasterly low level winds. The warming wind speeds should keep
overnight temps from dropping too low Friday. But once the winds
subside, be prepared for some patches of frost for the Sac
Valley...maybe over the weekend.



Frontal system moving through Northern California this morning.
MVFR/IFR conditions across the Valley in SHRA today, with IFR/LIFR
conditions in the mountains. Freezing levels will be between
5000-6000 ft. SE Valley winds will be 10-20 kt today, decreasing
to below 10 kt tonight.



.STO Watches/Warnings/Advisories...
winter weather advisory until 10 pm pst this evening above 6000
feet in the west slope northern sierra nevada...western plumas
county/lassen park.


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