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NWS Area Weather Forecast Discussion
County Warning Area [CWA]: STO
Regional NWS Weather Office: Sacramento, CA

690
FXUS66 KSTO 161051
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
251 AM PST Sat Feb 16 2019

.SYNOPSIS...
Cold upper low will bring continued showers through the weekend.
Heaviest precipitation over the Sierra Cascades for significant
travel issues through Sunday morning. Otherwise minor mountain
travel impacts. Isolated thunderstorms in the valley today. Dry
Monday and Tuesday. Lighter precipitation Wednesday and Thursday.

&&

.DISCUSSION...
Broad upper trough over the western U.S with multiple shortwaves
forecast to pivot through norcal over the next few days. One of
these shortwave troughs moving through norcal this morning
bringing scattered showers across the CWA. With mainly northwest
flow aloft, Coast range and northern mountains of interior Norcal
are mainly rain shadowed so precipitation rates are fairly light
there. Therefore will allow winter storm warning there to expire
at 12z this morning. Better orographics, although not idea, over
the Sierra Cascade range bringing higher precipitation rates there
and will leave winter storm warning up for these zones. Stability
progs showing some minor instability this afternoon so may see a
few thunderstorms in the valley where there may be enough partial
clearing to allow some surface heating.

Subsequent shortwaves rotating through will bring continued
showers tonight and Sunday although with main trough axis well
east of the state Sunday afternoon final shortwave will bring only
light precipitation. This is a good thing since very cold airmass
will bring low snow levels even lower with light snow possible in
the Redding area during the early part of the day on Sunday. Last
in series of shortwaves drops into the southern CWA Sunday night
and south of the forecast area by Monday morning. Therefore
should see last of the shower activity ending during the night.

Upper flow turns more northerly on Monday between upper trough
over the Great Basin and high pressure ridging over the eastern
Pacific. This will bring warming and drying on Monday although
daytime highs are still forecast to come in a few degrees below
normal. High pressure pushes in over the west coast on Tuesday
bringing continued dry conditions and fair skies.

Short term forecast ends Tuesday night with a chance of light
precipitation over the northern most part of the state as a small
trough drops down the coast out of the Gulf of Alaska.

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)

Highly amplified upper air pattern over Wrn NOAM will result in a
negative 2-3 height and temp anomaly over the Great Basin during the
EFP. Short wave energy from the Bering Sea yields the development of
an inside slider and deep closed low as it dives Swd over CA
during the middle of the week. As the low dives Swd into Socal on
Fri, short wave ridging builds into the Pac NW bringing dry wx
and a moderation in temps to our CWA.

The ECMWF is faster in returning precip to our CWA on Wed than
the GFS, otherwise the overall pattern of the models is reasonably
similar. The continental trajectory of the "slider" will mean
limited precip, but the cold air and high snow ratios will result
in some accumulating snow in the Sierra and some light snow in the
low foothill locations and Nrn Sac Vly Wed and Thu. The 1000-500
mb thicknesses are very low for Norcal, dropping to the 520s dm
range across our CWA Thu morning, and over the NErn zones
including the Burney Basin Fri morning, with some moderation in
temps elsewhere. The coldest temps look to be Thu/Fri morning.
Highs Thu are forecast to be some 10-20 degrees below normal, and
lows Fri morning are forecast to be some 6 to 15 degrees below
normal. These temps could be considered a one in very 1 to 2 year
event. Valley lows will likely be near the freezing mark with low
20s in the foothills and readings near 10 degrees in the
mountains. Clouds and a cold Nly wind Thu and Fri will also result
in wind chills not too common in our area...it will feel cold by
Norcal standards!   JHM


&&

.AVIATION...

VFR conditions except for MVFR/IFR conditions in passing showers,
and possible brief low-topped thunderstorms with graupel/small
hail. Prevailing IFR to LIFR conditions over mountain locations
given continued moderate/heavy snow showers. Scattered rain
showers to continue over the Valley today with south winds 10-20
kts.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 4 AM PST Sunday for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Winter Storm Warning until 4 AM PST early this morning for
Burney Basin / Eastern Shasta County-Shasta Lake Area / Northern
Shasta County.

&&

$$

Forecast Discussion from: NOAA-NWS Script developed by: El Dorado Weather




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