000
FXUS66 KSTO 211316
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
516 AM PST Wed Nov 21 2018


.SYNOPSIS...
Cooler and wetter weather spreads into the area by today 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...

Satellite imagery this morning shows the first of a series of
storm systems pushing towards the West Coast with the radar
showing some light rain over the coastal range. This will continue
to advance eastwards and eventually bring widespread rain to the
area later this afternoon and evening. Snow levels will be falling
during this time frame as well, with the Sierra seeing
accumulating snow around 6000 feet. These conditions will make
driving conditions hazardous so motorists should take extra
precautions if they must go out.

As we head into Thanksgiving, the first storm system will be
exiting the region with a second and more potent system arriving
late in afternoon and into the evening hours. Periods of heavy
rain and mountain snow are expected with this system in addition
to gusty southerly winds. These conditions will continue as we
head into Friday, so travelers will once again have to take extra
precautions as these conditions will make driving difficult. By
late Friday night and into Saturday morning, the low will be
exiting the region. Showers will likely linger over the Sierra and
its associated foothills, but elsewhere conditions should be dry.
Finally, we`re still concerned about the newly burned areas as
they will likely see debris/ash flow from moderate rain. The
major question is whether rain intensity could reach sufficient
levels to cause more hazardous debris/ash flows. 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.
.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)

Drier weather is expected into early next week as high pressure
builds into California. A weak system could bring a few showers
over the northern mountains on Sunday, but guidance has trended
drier with this feature, so only light precipitation amounts are
expected. A return of precipitation is likely during the second
half of next week across interior NorCal. However, model uncertainty
is evident in terms of precipitation amounts, and timing, with the
latest ECMWF run being quicker than the GFS. Regardless,
widespread precipitation, locally gusty winds, and cooler
temperatures could be possible. Seasonal temperatures expected
across the area during the extended period, with Valley highs in
the mid 50s to low 60s, and 40s to 50s over the mountains.

&&

.AVIATION...

Areas of MVFR/IFR with local IFR conditions possible next 24
hours as Pacific storm moves inland. Southerly wind gusts 15-25 kts
possible between 18z Wed - 03z Thurs mainly from KSAC southward.
Wind gusts over the Sierra up to 45 kts are possible.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from noon PST today 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 today to 10 AM PST Friday for
West Slope Northern Sierra Nevada-Western Plumas County/Lassen
Park.

Wind Advisory from 4 PM Thursday to 7 AM PST Friday for Central
Sacramento Valley-Northern Sacramento Valley-Southern Sacramento
Valley.

&&

$$