NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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000
FXUS66 KSTO 162255
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
255 PM PST Sat Feb 16 2019


.SYNOPSIS...
Cold upper low will bring showers through Sunday. Heaviest
precipitation over the Sierra with significant travel issues
through Sunday morning. Dry Monday and Tuesday. A weak, cold
system is possible Wednesday and Thursday.

&&

.DISCUSSION...
Broad upper trough is positioned over much of the western states
with disturbances contining to rotate through. Not much moisture
left for the system to work with, however the cold cyclonic flow
and unstable air associated with the system is very efficient with
the low water content. Core of coldest air will drop south over
NorCal tonight with snow levels falling from the current
3000ft level down to 2000ft or even localized 1500ft level. Best
snow will be over the Sierra from I80 south where 1 to 2 feet will
be possible above 4000 feet...2 to 6 inches is expected for
foothill locations between 1500-3000 feet, locally higher in more
intense convective showers. Isolated thunderstorms with small hail
will be possible through about 7 pm for Valley locations mainly
between Red Bluff and Oroville.

Lower snow level down to Redding area may put a few flakes in the
air, but low moisture in the northwest west flow will keep
accumulation generally below an inch, but can`t rule out isolated
2 or 3 inches over some of the higher foothill locations.

Convective showers will continue Sunday for the Sierra with 2-4
inches of additional snow at pass level, higher amounts Calaveras
and Tuolumne counties, where up to 6 inches will be possible.

Upper low will move east Sunday night into Tuesday with high
pressure providing a return to dry weather. Breezy north winds are
expected to develop on the back side of the low during this
period. Morning lows will be on the cold side in the low to mid
30`s. The breezy winds will act to diminish the freeze threat.

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)

Amplified flow extending from the eastern Pacific into the
continental U.S. will allow another cold upper low to impact the
region mid-week. This particular system is evident in water vapor
imagery as it approaches the far western side of the Aleutians and
eventually into the Bering Sea. Lowering heights drop down from
the Pacific Northwest into Northern California by late Wednesday
night. The general southward trajectory of this upper low across
the western U.S. is not ideal for substantial moisture
availability. Cold temperatures aloft will steepen lapse rates
enough to produce bands of showers, particularly in regions where
orographics come into play. This will prove to be another cold
Gulf of Alaska type system with 1000-500 mb thicknesses dropping
into the 525-530 dm range. Consequently, snow levels will again be
low accompanied by high snow-to-liquid ratios. While subject to
change, snow levels may drop below 2000 feet carrying a threat for
accumulations into the lower foothills. The primary opportunity
for precipitation will be Wednesday afternoon until Thursday
morning. Some timing issues exist with the 12Z ECMWF slower in
clearing the region on Thursday.

In the wake of this system, general meridional flow transports
cold continental air southward into Northern California. Although
the air mass will moderate considerably, it should allow for below
normal temperatures on Thursday and Friday. On Thursday,
persistent cold advection will see areas of Burney Basin struggle
to get out of the upper teens to low 20s which is roughly 10 to 15
degrees below climatology. By Friday morning, chilly temperatures
will be evident over the entire region as the Valley stays around
the freezing mark while footill locations sit in the low 20s.
Into the mountains, minimum temperatures may dip to near 10
degrees.

Regarding the next precipitation chances, the GFS/ECMWF solutions
are at odds with one another on the evolution of another Gulf of
Alaska system for next weekend. The 18Z GFS favors an elongated
longwave trough pattern across the Pacific Northwest which would
force the storm track to the north. On the contrary, the 12Z ECMWF
stalls an upper low well offshore which builds heights along the
West Coast. Cannot put too much stock into either solution given
model inconsistencies and high ensemble spread. Kept some
precipitation chances in on Saturday for northern zones but with
limited confidence. BRO


&&

.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.


&&

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

&&

$$
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