Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 182339
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
339 PM PST Sun Nov 18 2018

.SYNOPSIS...
Dry with near to above normal daytime temperatures and cool
nights through Tuesday with smoke in the valley due to the Camp
Wildfire. Cooler temperatures this week with a series of wet
systems the middle to end of this week. Snow over the higher
elevations of the Sierra for possible travel difficulties during
Thanksgiving travel weekend.

&&

.DISCUSSION...
High pressure will remain over the region into next week. Little
change in conditions is expected through at least Monday.  Smokey
skies will continue due to the light winds. Temperatures will
continue to be cooler than normal at night to near normal during
the day in the valley from the central Sacramento valley southward
to above normal for the north end of the valley, foothills and
mountains.

Tuesday the ridge will begin to shift to the east and a trough
begins to approach the coast. This will reverse the wind
direction and bring southerly winds to the region that will help
to bring better dispersion of the smoke. Rain looks to start
late Tuesday night along the coast spreading inland during the
morning and early afternoon hours for the valley and mainly
afternoon and evening hours for the western slopes. Some rain or
high elevation snow may start a few hours earlier up around
Lassen Park. We do have some concern that the system may slow
slightly and could impact when snow will start to fall below major
pass levels. Current thinking is that it will be sometime during
the late afternoon to early evening hours on Wednesday when the
snow will start to become a problem.

The good news with this event is that it will have enough rain to
end new wildfire concerns. The bad news is it will be the start of
watching for ash, mud and debris flows. Current thinking is that
it may be mostly ash that might get moved around with this first
event with perhaps the second system in the week having a better
potential for mud and debris flows.

&&
.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Thanksgiving Day will see our first wet system exiting, with a
stronger, wetter storm arriving in the afternoon and evening.
There may be a brief lull in intensity, but rain remains possible
through the day. Periods of moderate to heavy rain are expected,
with the heaviest at this point expected late night Thursday into
early Friday. This will likely cause local ponding on roads with
clogged storm drains. Commuters or Thanksgiving holiday travelers
should allow extra time for driving.

Newly burned areas will likely see ash flow. The major question
is whether rain intensity could reach levels sufficient to cause
more hazardous debris flows. While rain amounts should be
significant, this system may lack enough convective aspect to
bring heavy enough rain. Uncertainty is high now, but the
potential should become as we get closer to the event.

Storm total rainfall amounts through early Saturday could range
from 1.5" inches in the Valley to around 2 to 5" in the foothills
and mountains. Accumulating snowfall could be significant through
the period, causing potential travel difficulties across the
passes during the busiest travel period of the year. Snowfall
amounts could well exceed a foot, so plan ahead for wintry driving
conditions in the mountains. Accumulating snow in the mountains
during the Thanksgiving travel period could cause significant
delays on mountain roads above 5500 feet. Those planning to
travel should closely monitor the forecast to determine the best
times to drive. EK

&&

.AVIATION...

Widespread MVFR with local IFR likely to continue in the Central
Valley the next 24 hours as smoke from the Camp Fire persists. VFR
conditions elsewhere. Generally light winds at TAF sites. Local
east wind gusts northern Sierra Nevada thru 18Z.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$


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