Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 042153
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
153 PM PST Sun Dec 4 2016

.SYNOPSIS...
A small upper level disturbance will bring chances for small
amounts of rain and snow to the valley and mountains Sunday into
Tuesday. Colder air in the Valley could result in near to below
freezing overnight temperatures Tuesday and Wednesday mornings. A
stronger system will move into the region Wednesday night into
Thursday and will bring wind, rain, and snow through Friday.

&&

.DISCUSSION...
High clouds will continue to drift across the region through the
night as a weak system moves through the northern part of the
state late this afternoon and tonight. A second system will move
through the region Monday night and Tuesday. This system looks
like it may bring light amounts of precipitation to a greater part
of the region. Precipitation amounts with both systems look to be
very light and no significant problems are expected.

Temperatures at night will be getting colder with overnight low
temperatures Monday night and Tuesday morning looking like
they will be in the mid 30s for the Valley with some of the
coldest spots dipping into the lower 30s. The coldest morning
still looks to be on Wednesday. At this time widespread morning
lows around freezing with some of the colder spots dipping into
the upper 20s still looks possible. Mountain valley lows will dip
into the single digits and teens. There may be high clouds
drifting over the region along with some north wind in spots that
may help to mitigate the low temperatures slightly from current
forecast so we will have to monitor that for possible changes.

The cold morning lows will be short lived as a stronger more
significant system moves into the region Wednesday night into
Thursday.


.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Precipitation will increase over our area during the day on
Thursday as strong warm air advection occurs aloft. Under a nearly
zonal flow, models indicate offshore PW values may range from
1.3-1.5 inches. Given the amount of moisture this system has to
work with we could be looking at some very heavy rainfall,
especially for mountain locations. Additionally, as indicated by
the high PW values, we expect this system to be very warm with
relatively high snow levels (7000+ ft) for some locations.

However, models have been indicating that cold air may be trapped
near the surface as this system moves onshore. Thursday morning,
wet bulb cooling may bring snow levels down to ~1500 ft near the
Shasta Lake area creating for a difficult forecast. If this does
happen, the cold air will be slow to mix out during the day but
snow levels would be expected to gradually rise Thursday into
Thursday night. For other locations, especially those south of
Shasta County, the cold air may mix out rather quickly and push
snow levels up the the aforementioned 7000+ ft. Given the complex
nature of the snow levels on Thursday, confidence is rather low
and details will need to be worked out as we get closer to this
event.

By Friday afternoon, the heaviest precipitation should be beyond
us with post frontal showers continuing. Zonal flow should
continue for Saturday although most areas should see a break from
the precipitation with the exception of some showers in the
northern mountains and valley. Finally, on Sunday model solutions
begin to diverge as the GFS brings in yet another system into our
area. This is in contrast to the ECMWF which keep precipitation
confined to the northern valley/mountains and areas northward.
Wilson


&&

.AVIATION...

Light and variable flow expected to continue through the evening
and most of the night with areas of MVFR and local IFR in BR/FG.
Dry northerly surface flow expected to begin tomorrow morning
although will likely be too late to prevent fog for places like
KSCK. Besides this, VFR conditions expected for the next 24 hours.

&&

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

$$


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