Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 160557
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1055 PM PDT Sun Apr 15 2018

.SYNOPSIS...
Pacific storm moves through the area on Monday with rain, mountain
snow, thunderstorms, and gusty wind. Dry Tuesday, then wet weather
Wednesday into Thursday, with dry weather returning Friday.
&&

.DISCUSSION (Today through Wednesday)...
Update: Frontal band will be moving through the SErn half of the CWA
for the rest of the night including the 80/50 corridor. Chain
controls now up for the Sierra with the Colfax snow profiler showing
the snow level around 4 kft. Higher resolution REF progs indicate
the areal coverage of snow showers will be decreasing after midnight
and chain controls may be lifted in a few hrs.

Cold air mass will be moving over Norcal on Mon with area of open
cellular cloudiness offshore moving inland with the upper trof on
Mon. Any partial clearing overnite and early in the morning will
cloud-up as the colder air moves inland. A shower/storm could pop up
anytime/anywhere on Mon but should not last too long. Should see
some higher cloud tops on Mon compared to today, but shear will be
weaker as the system moves overhead.

Broad cyclonic flow and very cold 5H temps (-32 C) are forecast to
be over Norcal on Mon. A broad area of negative LI`s is forecast
over the region Mon afternoon but shear values weaken with the broad
trof overhead and weak wind fields. The forecast shear values do not
support rotating storms in our area and the very thin CAPE forecast
in the Valley soundings reduces the possibility of large hail. The
deep layer of thin CAPE with the very cold air aloft (-3.6 5H temp
anomaly) sets up the possibility of cold air funnels, and brief,
short-lived storms. JHM

.Previous Discussion...

Pacific frontal system is spreading precip into western and
northern portions of the CWA attm. Breezy to windy conditions
have developed across Interior NorCal ahead of the front. Wind
advisory has been expanded to include all of the Sacramento Valley
and Delta as winds are stronger farther south than guidance
suggested. Peak wind speeds expected now through early evening in
the Central Valley. Higher resolution NAM-3KM and HRRR showing
precip continuing to spread inland through the afternoon into
evening. Models showing some Neg LI`s and increased CAPE along the
front in the northern third of the CWA so an isolated afternoon
thunderstorm with the front is possible. Helicity values are
favorable for rotating storms, however CAPE is somewhat limited
today.

Precip turns showery over much of the Central Valley this evening,
while heavier precip spreads into the Western Plumas mountains
and Sierra Nevada. Snow levels this evening will initially be
around 4500 to 6500 feet, but lower overnight behind the front.
Heavier precip decreases in the Western Plumas mountains after
midnight and in the Sierra Monday morning. Snow levels Monday
morning expected to be around 3000 feet.

Large cold upper trough moves into NorCal Monday with cyclonic
flow keeping showers...heavy at times..in the forecast Monday.
Large area of neg LI`s depicted across Interior NorCal with
highest values north of a KUKI-KOVE-KSVE line. 5H temps lower
to -34 Deg C over this area in the afternoon with steepening
lapse rates. 0-1 KM and 0-6 KM Bufkit vertical wind shear profiles
suggest potential for isolated rotating storms. Abundant small
hail likely with stronger cells and possible isolated funnel
clouds or weak tornadoes.

Storm total QPF looks to be up to around 0.60 inches or less in
the Central Valley, locally higher with stronger convective
storms. Liquid totals in the foothills and mountains through
Monday range generally from around 0.5 to 1.5. Snow totals of 6 to
12 inches, locally higher, expected in the mountains of Western
Plumas and Northern Sierra Nevada above 5000 feet. Winter weather
advisory in effect as mountain travel likely to be difficult with
slick roads, reduced visibility, travel delays, and chain
controls.

Showers decrease Monday night with drier and warmer weather
returning Tuesday as upper ridging moves into NorCal. Ridging
shifts east into the Great Basin late Tuesday, as next Pacific
storm spreads precip into the CWA by Wed afternoon/night.

PCH

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Forecast models indicate an upper low moving quickly overhead
with a few lingering Sierra showers on Thursday. The upper low
will shift to the east as a ridge of high pressure builds in. As a
result, dry and warmer conditions are anticipated Friday into
next weekend. Daytime highs could be 5-10 degrees above seasonal
normals, with Valley highs potentially reaching the upper 70s to
low 80s.

&&

.AVIATION...

Precipitation will continue across the area with occasional MVFR
conditions over the Central Valley and widespread IFR over the
northern Sierra Nevada through Monday. South wind gusts 20-25 kts
Central Valley through 03z Monday. Thunderstorms possible over
the Central Valley through 03z Monday, and again Monday afternoon
after 18z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 7 PM PDT Monday for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Wind Advisory until 8 PM PDT this evening for Carquinez Strait
and Delta-Central Sacramento Valley-Northern Sacramento Valley-
Southern Sacramento Valley.

&&

$$



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