Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 222323
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
323 PM PST Fri Feb 22 2019

.SYNOPSIS...
Next chance of precipitation will return over the weekend and
continue into next week.

&&

.DISCUSSION...
Pleasant/dry wx today will begin to sour this weekend before a
wet pattern re-emerges next week. Clouds expected to increase
late tonight and Sat morning as an upper low from B.C. drops Swd
into WA this weekend. Increasing cloud cover over the Nrn zones
should inhibit frost from forming, while recovering dewpoints
could lead to some frost over the Srn zones before enough cloud
cover arrives Sat morning.

The upper low over the Pac NW doesn`t make much progress other than
to retrograde slightly as it phases with or engulfs the Rex portion
of the Omega-Rex Block along 150W. The Rex portion undercuts the
ridge early next week resulting in a muti-day precip event for
Norcal. Until then and for the weekend, most of the precip will
be north of the 80/50 corridor. Snow levels will initially range
from 2500 ft in the Shasta County mountains to near 4000 ft over
the "Coastal Range." At these levels, travel along Hwy 299 could
be impacted with several inches of snow Sat nite and Sun as this
system taps into a weak AR that should begin to move inland on the
24th, and before snow levels rise during the week ahead. Have
issued a Watch product for this area, thinking that it will be
more of an advisory situation for zone 63, while the potential
exists for an upgrade to a warning in zone 13 and the other zones,
eventually.

From Sun nite into the middle of next week, precip will become
become steadier and heavier over Norcal as the "phasing" mentioned
above occurs. The West Coast AR Landfall Tool implies that this
moisture plume (AR) will impact Norcal through the 27th (Wed) before
a brief break. For the 80/50 corridor, precip is forecast to
move into that area early Mon, and should become heavy over the
Hwys 70/36 corridors by Mon afternoon. Winds will also be
increasing Mon and Tue as the bulk of the AR moves inland. The WAA
from the AR will raise snow levels to at least 5-6 kft, if not
higher over the Sierra.

The timing of the weather systems, duration of the AR, and the
relatively low snow levels initially, followed by rising snow levels
during the event, warrants segmenting the WSW product. Not the
easiest way to handle this, but the different areas will be impacted
differently, both in time and quantity of precip. After snow levels
rise above 5 kft, there is little impact for zones 13 and 63, while
impacts continue for the Sierra. The runoff from the precip through
the middle of next week will cause rivers to rise, so expect some
hydro impacts as well.  JHM

&&


.EXTENDED DISCUSSION (Tuesday THROUGH Friday)

The forecast remains on track, with an active weather pattern
continuing to look likely for the extend portion of the forecast.
QPF amounts on Wednesday suggest less than 1" across the Valley,
and 0.50-1" over the mountains. Hazardous mountain travel is
anticipated, with snow levels around 5,500 to 6,000 feet.
Forecast models disagree with how quickly to eject this system on
Thursday, with the ECMWF being more progressive. Thus, lingering
mountain showers could be possible on Thursday. Otherwise, dry
conditions are expected for most areas as weak ridging develops
between systems. A return of precipitation is likely late Friday
evening into next weekend as the next system approaches from the
west, bringing widespread precipitation across interior NorCal.

&&

.AVIATION...



Northerly winds will prevail today, before becoming light and
variable as we head into the evening and overnight hours. VFR
conditions are expected.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday evening through Tuesday morning
for Burney Basin / Eastern Shasta County-Mountains Southwestern
Shasta County to Northern Lake County-Western Plumas
County/Lassen Park.

Winter Storm Watch from Monday morning through Wednesday morning
for West Slope Northern Sierra Nevada.

Winter Storm Watch from Saturday evening through Monday morning
for Shasta Lake Area / Northern Shasta County.

&&

$$


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