Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 162326
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
326 PM PST Tue Dec 16 2014

.Synopsis...
The next weather system is along the California coast and will be
moving inland tonight and Wednesday. Another round of valley rain
and mountain snow is expected. Brief drying on Thursday, then
another weather system is forecast to return showers on Friday.

&&

.Discussion...
Break in weather systems has allowed some clearing/warming in the
valley leading to cumulus congestus over the Motherlode this
afternoon. Showers should be developing early this evening over
the Siernev ahead of the offshore system.

The comma cloud (next system) along the coast is generally moving
Ewd at about 20 kts and will overspread interior Norcal tonight into
Wed morning. The leading edge of the rain has crossed the coastal
range and is nearing western Solano Co at press time. Jet energy
remains off the coast and is digging the offshore trof...so the
Ewd movement of the system will be slower than with a more Wly
jet. Higher resolution QPFs show some variability in the Ewd
movement of the QPF...but generally 02z-04z looks to be the timing
for the SAC area with about 04z or so for the Siernev...and that
is when we begin the WSW for the Siernev. Snowfall amounts from
6-10 inches are forecast from tonight into Wed evening over the
Siernev with isolated amounts near a foot. Rainfall from 1/4 to
1/2 inches forecast for the valley with 1.0 to 1.50 inches over
the Siernev/higher terrain for the same time frame.

The GFS cross section along the I-80 corridor forecasts the best
large scale ascent over the W Slope Siernev around 06z...weakening
by 12z Wed. Thus...the heaviest snow is forecast to fall from the
late evening into early morning hours. The heavier snow and greater
fall rates will lower snow levels down to around 4 kft to 4.5 kft
over the Siernev as the WAA ahead of the frontal band this evening
will likely raise it to possibly near 5 kft.

A few C-G strikes are noted offshore...and given the jet location/
orientation...the SW corner of our CWA (around Solano Co) has a
chance of having a storm move/develop into that portion of our CWA.

Large scale ascent weakens on Thu as the short wave energy zips
well to our SE...but weak cyclonic flow prevails in its wake over
Norcal. Showers are likely to continue over the Siernev with
diminishing activity elsewhere in our CWA.

Ridging rebuilds on Thu with a break before Fri`s system. Stratus/
fog should prevail in the valley on Thu while showers end over
the higher terrain. Fri`s system quickly follows the ridge and will
move quickly across Norcal with lighter QPFs. The progressive
pattern is due to consolidated Wly flow over the Ern Pac and a
strong Pac jet that builds Nwd into the Pac NW for a dry weekend
for Norcal.   JHM

.Extended Discussion (Saturday through Tuesday)

Medium range models similar in transitioning to a drier weather
pattern over the weekend into early next week. Lingering shower
threat will continue into Saturday, mainly in the mountains.
Models strengthen upper high in EPAC with associated upper ridging
building over Interior NorCal. Some light overrunning WAA precip
continues to be depicted over the Shasta mountains Sunday, otherwise
dry weather with the main weather concern being impact of valley
fog nights and mornings. Heights continue to increase Monday into
Tuesday with warming AMS. Above normal temperatures expected in
areas not affected by fog with 50s to lower 60s for the Central
Valley and mainly 40s to 50s for the mountains and foothills.

&&

.Aviation...

Dtrtg conds this eve as Pac fntl sys movs inld. Wdsprd MVFR with
areas IFR and Lcl LIFR ovngt into Wed mrng with impvg conds by Wed
aftn. Sn lvls in Shasta mtns arnd 035 ft AMSL and arnd 045 ft AMSL
nr I-80/HWY 50. SWly flow alf tngt vrg to NWLy aft 08z. Lcl SEly
sfc wnd gsts to 25 kts in Cntrl Vly this eve.

&&

.STO Watches/Warnings/Advisories...
winter weather advisory from 8 pm this evening to noon pst
wednesday above 4000 feet in the west slope northern sierra
nevada...western plumas county/lassen park.

&&

$$






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