Area Forecast Discussion
Issued by NWS Sacramento, CA

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402
FXUS66 KSTO 181128
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
328 AM PST Sun Feb 18 2018

.Synopsis...
A cold weather system brings breezy winds and snow over the
mountains and foothills Sunday into Monday, impacting travel.
This will be followed by near record cold temperatures with a
possible widespread freeze.

&&

.Discussion...
The eastern Pacific ridge has begun to retrograde westward as a
shortwave trough slides southward along the Pacific Northwest.
Skies remain mostly clear across interior Northern CA, though a
few light radar returns are being detected just north of our CWA.
South winds / onshore flow has begun to pick up this morning
across much of the region ahead of the cold front. These winds are
keeping the boundary layer well-mixed: temperatures are running
several degrees warmer than 24 hours ago.

Precipitation associated with the aforementioned trough will begin
to spread across the northern mountains this morning, and down
through the Sierra through the course of the day. The system taps
into very little moisture, so it will difficult to manage more
than a sprinkle or flurries across lower elevations. A few inches
of light, fluffy snow are expected across the higher elevations of
the Sierra later today, with periodic showers continuing into
Monday. Snow levels are projected to lower rapidly with this
system, and a dusting of snow will be possible across the
foothills. Snow accumulations are not expected to be impressive,
but this holiday weekend traditionally sees a high volume of
traffic over the mountains and snow could cause exacerbated
impacts.

What will likely be the coldest air of the winter season will
begin to impact portions of the region tonight. For lower
elevations, the coldest temperatures tonight will likely be across
the northern Valley, where skies clear and winds slacken. Skies
should clear further Monday night into Tuesday morning, and
currently appears to be the coldest night of this event.
Widespread freeze conditions may continue Tuesday night. NAEFS
ensembles are characterizing this event as roughly 2-3 standard
deviations colder than normal, and with a 5-10 year recurrence
interval. With forecast lows on the order of 10-20 degrees colder
than normal, record minimum temperatures will be possible across
Valley climate sites.

    Feb 18       Feb 19      Feb 20      Feb 21      Feb 22
MOD 29(1956)     29(1955)    28(1955)    29(1955)    32(1975)
RBL 29(1956)     28(2006)    28(2006)    27(2006)    30(1969)
RDD 29(2013)     25(2006)    25(2006)    29(2006)    29(2011)
DTS 31(1990)     31(1882)    31(1953)    31(1955)    32(2013)
SAC 29(1956)     33(1961)    30(2013)    29(2006)    30(1975)
SCK 29(1975)     28(1955)    28(1955)    27(1953)    28(1951)

Widespread frost is expected. Agricultural impacts could be
significant, due to an early bloom from unusually mild weather in
recent weeks. People should begin planning now on ways to protect
sensitive plants, outdoor pets, and outdoor pipes.

Dang

&&
.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Series of cold shortwave lows continue to drop through the Pacific
Northwest and Norcal through the extended period. Extended models
in good agreement at this time in dropping one of these troughs
through Norcal on Thursday bringing light rain to much or all of
the CWA. This cold system is not expected to contain a lot of
moisture so precipitation amounts are expected to be minimal but
snow levels will be quite low at between 1000 and 2000 feet. If
current timing holds then Friday looks dry under a brief period of
northerly flow between systems. There should be enough sun on
Friday to bring daytime highs from a little below normal to a
little above normal. Next system on track to drop into the Pacific
Northwest and Norcal late Saturday. This system looks to be a
little more shallow so best threat of precipitation will be the
northern mountains and Sierra Cascade crest. Overnight lows will
remain on the cool side through the extended period but valley
lows should stay above freezing while daytime highs over next
weekend should come in around normal.

&&

.AVIATION...

Generally VFR conditions next 24 hours TAF sites.  Cold low
pressure system drops through Norcal today and tonight bringing
periods of light rain or snow for occasional MVFR cigs TAF sites
through about 00z. Periods of IFR over mountains through 12z
Monday with continued occasional showers. Snow levels 3000-5000
feet this morning dropping to below 1000 feet tonight. Mainly
northwest winds 15 to 30 mph decreasing after about 06z tonight.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 10 AM this morning to 4 PM PST
Monday for West Slope Northern Sierra Nevada.

Freeze Warning from 2 AM Monday to 8 AM PST Wednesday for
Carquinez Strait and Delta-Central Sacramento Valley-Clear
Lake/Southern Lake County-Motherlode-Mountains Southwestern
Shasta County to Northern Lake County-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Shasta Lake Area / Northern Shasta County-
Southern Sacramento Valley-West Slope Northern Sierra Nevada.

&&

$$



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