Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 220447
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
847 PM PST Tue Feb 21 2017

.Synopsis...

Significant flooding concerns continue as runoff from the recent
rain continues to work its way downstream adding stress to
waterways. Showery weather with a few thunderstorms expected into
mid-week, then another storm potentially late in the weekend.

&&

.Discussion...
Showery weather over the interior will continue overnight before
tapering late. One last wave will be moving through overnight to
keep enough instability and showers around overnight. Will most
likely extend the Winter Storm Warning until 6 am due to the
additional snow and icy roads. Several inches of additional
snowfall is likely overnight. Small creeks seemed to have peaked
earlier in the day around the region and continue to recede
tonight. Most of the smaller rivers in the foothills/mountains
have peaked as well. The main steam rivers in the valley will
continue to remain high for the foreseeable future.

The trough will remain over the region on Wednesday and a weak
wave will move through in the afternoon and evening to produce
some showers with a slight chance of a few thunderstorms. The
best chances for precipitation look to be over the north end of
the valley and over the mountains. Snow levels will remain low and
near 3000 feet during the day.

A few showers may linger over the mountains on Thursday but for
the most part expect dry weather to return.

.Previous Discussion (Today through Friday)...

Water vapor imagery and model analysis this afternoon shows the
main moisture plume that brought heavy rain to northern California
is now passed.  Lighter scattered plumes of moisture associated
with scattered showers and thunderstorms were present over the
forecast area this afternoon, with more moisture lined up ahead of
the next shortwave trough of low pressure off the coast.

Latest short term high resolution model guidance continues to
suggest scattered showers and thunderstorms as instability
remains over northern California this afternoon into tonight and
through Wednesday. Heavy rain rates at times, as well as heavy snow
for the Sierra, can be expected tonight as westerly to
southwesterly flow creates good orographic uplift in combination
with the available atmospheric instability. Flooding on roadways
and quick rising streams are possible under locations with
heaviest rainfall. Abundant small hail is also possible under
strongest thunderstorms this afternoon and evening.

Stormy weather will begin to diminish Thursday with some slight
chances for precipitation for the Sierra as a shortwave ridge of
high pressure influences northern California weather before the
next weather making system approaches. Thursday afternoon through
Friday is anticipated to remain mostly dry.

&&
.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)

Models showing upper low dropping south off the California coast
Saturday but diverge with track and intensity. GFS showing a
deeper low and is slower to progress it inland. Secondary low
drops into the area Sunday into Monday with models again showing
differences in strength and track. Solutions overall point to the
potential for an unsettled weekend with the best chances of
precipitation looking to be during the Sunday/Monday time period.
Given North to South track, systems will contain colder air
resulting in lower snow levels with potential for snow into upper
portions of the foothills.


&&

.AVIATION...

Wly flow alf with sct MVFR/IFR conds poss in shwrs with isold
aftn/eve tstms ovr Intr NorCal nxt 24 hrs. Stg SWly sfc wnd gsts
to 40 kts or gtr ovr hyr trrn. Sn lvls 040-045 lwrg to 025-035
Wed.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 10 PM PST this evening for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Flash Flood Watch through Saturday morning for Carquinez Strait
and Delta.

&&

$$


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