Debris Flows and Heavy Rain in California, December 20 to 22, 2015

Area Forecast Discussion
National Weather Service Sacramento CA
354 AM PST SUN DEC 20 2015

.SYNOPSIS...
Series of storms to move through early this week with
rain...mountain snow and gusty winds. Brief break possible mid
week with another significant storm expected around Christmas
eve, Thursday. Some clearing forecast for Christmas day. Travel
impacts likely at times through the week due to mountain snow.

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.DISCUSSION...
Brief break in precipitation early this morning as flat high
pressure ridge slides over the state. Clearing skies earlier this
evening allowed some fog to form over the northern San Joaquin and
southern Sacramento valleys with visibility locally dropping to
one quarter mile. A cold frontal band now just off the north coast
has begun spreading mid level cloudiness across the north state
this morning. This has brought slowly improving visibility as the
cloud cover has spread southward. Eureka radar currently showing
precipitation just off the coast. Models bring this precipitation
into the northwest corner of the state this morning with
precipitation moving into the coast range within the next few
hours. Current surface observations indicate snow levels could
start out fairly low at around 2500 to 3000 feet or low enough to
impact major coast range passes. Snow amount estimates of a few
inches warrant the continuation of snow advisory there.
Precipitation continues into the southern Cascades later this
morning then slips southward into the northern Sierra late this
morning or early afternoon. At this time...snow advisories
currently in place look good and will not update any hazards this
update cycle. Moisture and orographics likely to keep showers
going through the night over the Sierra range with decreasing
shower threat elsewhere.

Next impulse forecast to track through Norcal tomorrow afternoon.
This system of warmer origin will come with higher snow levels so
snow impacts will mainly be limited to the Sierra Cascade range.
Gusty winds will also accompany this system with GFS and NAM
models both showing a better than 10 mb surface gradient from MFR
to SAC Monday afternoon. 24 hour snow totals for the higher
elevations of the Sierra still generally remain below 10 inches so
snow advisory in place through Tuesday mid day still looks okay
for now. By Tuesday night...slight upper ridging should reduce
shower activity significantly. With basically zonal flow across
the north state on Wednesday...can not rule out some light shower
activity but significant precipitation is not likely.
Therefore...many lower elevations could see about a 24 hour break
in the wet pattern Wednesday while the mountains see continued
light showers.

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.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Active pattern continues through Thursday as one last system in
this series slides down in northwesterly flow into NorCal. While
likely not as strong as systems earlier in the week, significant
travel impacts are likely to persist over the mountains through
Christmas eve with lower snow levels. Pattern change develops
Christmas and continues into the weekend as flow across the
eastern Pacific amplifies and a ridge shifts onto the west coast
resulting in a few days of drier weather for interior NorCal.
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.AVIATION...

Generally VFR conditions except for areas of IFR/LIFR in FG across
the Central Valley from KMYV southward thru about 17Z. The next
system moves through NorCal today with widespread MVFR over the
valley and IFR/LIFR developing by 20Z. South winds 15-25 kts
develop across the Central Valley with stronger winds across
higher terrain.
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.STO Watches/Warnings/Advisories...
Winter Weather Advisory from 10 AM this morning to 10 AM PST
Tuesday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Winter Weather Advisory until 4 PM PST this afternoon for
Mountains Southwestern Shasta County to Northern Lake County-
Shasta Lake Area / Northern Shasta County.

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