Flash Flood Watch 2, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
322 AM PST Sun Jan 20 2019

.SYNOPSIS...
Valley rain and heavy mountain snow today as Pacific frontal
system passes through. Breezy south winds will accompany the
front. Mountain travel impacts likely. Scattered afternoon and
evening thunderstorms possible. Sierra showers ending Monday. Dry
conditions Monday night through the end of the week.

&&

.DISCUSSION...
Radar and satellite imagery showing Pacific frontal system now
just moving on to the north coast. Current model timing moves the
front into the northern Sacramento valley by around 15z, into the
southern Cascades and far northern Sierra by 18z, into the
southern Sacramento valley by 21z then northern Sierra after 00z
Monday. Maximum surface gradient and therefore strongest winds
will be just ahead of this frontal timing with strongest winds
dropping southward as the front moves to the southeast. May see
some windspeeds briefly topping wind advisory criteria but likely
only of one hour or so and not planning on issuing wind advisory
for this short duration event. As mentioned by previous shift,
stability progs are indicating a large area of instability along
the frontal band so scattered afternoon and evening thunderstorms
a good bet.

Snow levels start out around pass level then drop slightly during
the day. With significant snowfall expected over the Sierra
Cascade passes, winter storm warning now in place still looks
good. Heaviest snowfall accumulations over the northern Sierra
will be between 00z and 06z Monday as frontal band moves through
and snow levels continue to drop. Heavy snow and wind will combine
to make mountain travel very hazardous especially during this
time. Frontal band slips mainly south of the forecast area by
Monday morning bringing decreasing showers. Main upper trough
moves into the Great Basin Monday afternoon. Shortwaves dropping
down back side of this trough could bring a few showers Monday
afternoon but significant snowfall accumulation is not expected
although cold air filtering in behind the front will drop snow
levels below 4000 feet.

Upper ridge builds in starting Monday night bringing shift to
northerly flow and drying. At this time, this dry period is
expected to last through the coming week. Breezy north wind may be
enough to minimize nighttime valley fog north of the Sacramento
area but will leave patchy fog in forecast for Tuesday/Wednesday
mornings south of the Sacramento where nighttime and morning
winds should be a little lighter.

 &&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

An amplified ridge across the eastern Pacific will keep conditions
benign through late next week. Early morning fog will be possible
most mornings. Clear skies will contribute to warmer than normal
high temperatures each day, with lows generally around climatological
values. Highs in the valley each day will be in the 60s, with the
upper 40s and low 50s seen at higher elevations. The ridge looks
to stay across the western CONUS through the weekend.

&&

.AVIATION...

Areas of IFR/MVFR conditions will remain possible across northern
California this morning. MVFR ceilings are forecast to return to
the valley later as another system moves across the area.
Southerly winds may gust as high as 30 knots at TAF sites.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 9 AM this morning to 4 AM PST Monday
for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&