Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 222329
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
329 PM PST Thu Feb 22 2018

.SYNOPSIS...
A cold system will bring low elevation snow to the mountains
into early Friday, which could impact travelers. A few showers are
possible in the Valley. Below normal temperatures will continue.
Additional systems likely next week.

&&

.DISCUSSION...
Vort max that was along the CA/OR coast dropping SSEwd, mainly
along the Wrn side of the CWA. Clouds and scattered showers will
spread SEwd over the CWA into this evening. Very low WBZs will
result in low snow levels dropping into the foothills. AT press
time, the snow level had dropped to Applegate around 2 kft. SPC
SREF plumes suggest the Srn portion of the CWA will be more
unstable, roughly from SMF Swd into the Nrn SJV where isolated
storms are possible through late afternoon, than the Nrn portion
of the CWA. This is due to the timing of the vort max dropping Swd
this afternoon. Effective Shear indicates the potential for some
mid level rotation/rotating storms. The limiting factor is the
elevated instability forecast in the Bufkit forecast soundings and
not surface based convection. Central Valley rotating storms are
rooted in the boundary layer with low LCLs. Dewpoints in the 30s
makes this more problematic. Can`t rule out a possible funnel
cloud, but less likely there would be an actual touchdown. Looks
as if the HRRR suggests some isolated cells in the extreme Srn
Sac, Solano Co, Nrn SJV areas this afternoon. The 12z NAM 3 km
keeps the precip in frozen form and basically out of the Valley.
Timing of the snow advisory looks good with precip winding down
around 06z.

Precip amounts will be light with this system due to a dry air mass
with PWs ranging from .13 at REV to .34 at MFR and .44 at OAK. These
values are below average. AMSU data also indicate very low PW over
the region. However, good dynamics and cold temps will result in
some brief periods of heavy showers with high snow ratios. For
example for BLU, using the max temp in the profile, a snow ratio of
16:1 to 18:1 is calculated for today. The average SLR is 9:1.
Don`t expect normal snow water equivalents with these next few
systems in the short term.

After the wx system/trof moves through the area tonite, dry Nly
winds will increase over the CWA, weakening Fri afternoon. Winds
may continue to mix the Nrn and Wrn zones overnite, while
decoupling will occur on the E side of the Valley. With clearing
skies overnite, it will be another cold nite, with a brisk Nly
wind on the W side of the Vly/lee-side of the Coastal Range.

A couple more wx systems are forecast to drop out of the GOA in
the highly amplified pattern along the W Coast of NOAM in the
short term. The next system on Sat, followed by another late Sun
into Mon. These systems will be similar, cold with relatively low
QPF. The WR US West Coast AR Landfall tool (Probability of IVT) is
virtually "bone dry" for the next 16 days. What moisture there is
will be spilling over the Pac Ridge and eroding as it hits the
Pac NW and spreads inland into our CWA. Even though Norcal will
have the continuation of a cold, showery pattern, the QPF will be
relatively low, albeit with high snow ratios. Some modest
modification in temps is expected later in the weekend and into
early next week.  JHM

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)

Models show a continuation of the wet pattern as several shortwaves
dive southward from the Gulf of Alaska. Compared to this week`s
systems, the upcoming storms have the potential to pull in some
moisture from the Pacific before spreading precipitation inland.
Given the cold nature of these systems, snow levels will remain
low.

Models are showing better agreement compared to previous
forecast runs, though differences remain in terms of timing and
precipitation amounts. The first system moves in Monday into
Tuesday. Early precipitation estimates suggest 0.10-0.30 inches
across the Valley, and around 0.25-0.65 inches over the mountains,
though these amounts will likely change as we get closer to the
event. A brief break is expected late Tuesday into Wednesday as
weak upper ridging builds in. Then, a stronger and wetter storm
will move across the area late Wednesday into Friday. Precipitation
will initially impact the northern mountains on Wednesday before
spreading across the forecast area on Thursday. Stay tuned to see
how these systems develop as model uncertainty remains.
Temperatures will remain below average as cold air continues to
filter in.

&&

.AVIATION...

VFR conditions expected next 24 hours except local MVFR vicinity
scattered showers and thunderstorms 22Z-02Z and widespread IFR
over the northern Sierra with snow showers thru about 03Z Friday.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 10 PM PST this evening for Burney
Basin / Eastern Shasta County-Motherlode-Northeast
Foothills/Sacramento Valley-West Slope Northern Sierra Nevada-
Western Plumas County/Lassen Park.

&&

$$



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