Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 202328
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
328 PM PST Tue Nov 20 2018

.SYNOPSIS...
Cooler and wetter weather spreads into the area by Wednesday and
continues into early Saturday. Newly burned areas could
experience ash/debris flows. Snow over the higher elevations of
the Sierra will make travel difficult for the Thanksgiving travel
period.

&&

.DISCUSSION...
The persistent ridge which was been over the West for an extended
period is shifting eastward into the Rocky Mountains. A low
pressure trough is just off the coast and will begin to move
inland late tonight, with precipitation spreading across the area
on Wednesday. Commuters and holiday travelers are urged to allow
extra time for driving since this is the first widespread rain of
the season and roads will be particularly slippery due to an
accumulation of oil through the summer and early fall.

Snow will begin to develop over the southern Cascades by late
morning, over the northern Sierra by early afternoon. A Winter
Storm Warning is in effect from noon Wednesday through 10 am
Friday morning. Snow levels will initially be around 6000 feet,
rising to around 7000 feet in the early evening, then drop back
again to to around 6000 feet by early Thursday. Travelers over
the passes should start early in the day Wednesday if they must
travel. Drier conditions are expected for the weekend.

In addition to our wet roads in the valley, those that need to
travel over the Sierra tomorrow should be mindful of the snow
levels since they are forecast to fall to around 6000 feet in the
evening hours. Accumulating snow is forecast at these levels,
thus the Winter Storm Watch remains in effect from tomorrow
afternoon through Friday morning.

Thanksgiving Day we will see our first system exiting, with
another and more potent storm arriving in the afternoon and
evening. There may be a brief lull in the intensity of rain
during the day, but it will remain possible through the day with
some light snow continuing through the mountains. Thursday
afternoon and evening southerly winds will increase and become
quite breezy into the night, gusting to 45 mph over the northern
Sacramento Valley, to 30 mph over the central and southern
Sacramento Valley. These winds will last through the overnight
hours into Friday. Driving high profile vehicles may be difficult.
Tree branches may come down.

Periods of moderate to heavy rain are expected, with the heaviest
expected Thursday night into early Friday for areas north of I80.
Some locations in the higher mountains could see storm totals
through Saturday morning of up to 8 inches of liquid equivalent,
though much of this will be in the form of snow. The Valley should
get generally 1-2 inches, up to 3 inches around Redding. Newly
burned areas will likely see ash flow from moderate rain, with
about 4-6" of rain. The major question is whether rain intensity
could reach levels sufficient to cause more hazardous debris flows
as a cold front moves inland. There is still quite a bit
uncertainty, but there is enough potential to keep the Flash Flood
Watch in effect for debris flows at the Mendocino Complex, the
Carr, Delta, Hirz fires and the Camp Fire. Flooding outside of the
burn areas is not expected, except for local ponding on roads
with clogged drains.

Conditions on mountain roads will gradually improve on Friday as
the low exits the region and snow levels start to rise. Wet
conditions will continue across the area through the day. EK/KR

&&

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)
Drier weather is expected late Saturday into Sunday as the trough
shifts eastward into the Great Basin with just some lingering
showers, mainly in the mountain. A ridge of high pressure is
expected to build into CA, bringing dry conditions into early next
week. A weak system moves north of the forecast area late
Saturday into early Sunday, and might bring some light
precipitation mainly over the northern mountains. Model guidance
indicates the possibility of additional precipitation moving
across interior NorCal by the middle of next week, but model
differences exist in terms of timing and precipitation amounts.
Forecast highs will be in the mid 50s to low 60s across the
Central Valley, and 40s to 50s over the mountains.

&&

.AVIATION...
Widespread MVFR with local IFR likely to continue in the Central
Valley as smoke from the Camp Fire persists; improving conditions
mainly after 03z-06z Wednesday. Otherwise, VFR conditions
elsewhere. Winds less than 10 knots at TAF sites. Precipitation
will be reaching the Coastal Range mainly after 09z-12z Wednesday.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Wednesday afternoon through Friday
morning for Central Sacramento Valley-Clear Lake/Southern Lake
County-Mountains Southwestern Shasta County to Northern Lake
County-Northeast Foothills/Sacramento Valley-Northern Sacramento
Valley-Shasta Lake Area / Northern Shasta County-Western Plumas
County/Lassen Park.

Winter Storm Warning from noon Wednesday to 10 AM PST Friday for
West Slope Northern Sierra Nevada-Western Plumas County/Lassen
Park.

&&

$$


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