Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 241649
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
849 AM PST Wed Dec 24 2014

.Synopsis...
Valley fog this morning...otherwise increasing clouds as a cold
Pacific storm moves towards the coast. Rain and mountain snow this
afternoon and tonight as the system moves through. Dry conditions
with breezy north winds on Christmas day Thursday. A slight chance
of light showers across the northern mountains Sunday...otherwise
dry Friday through the middle of next week.

&&

.Discussion...
Forecast still in good shape this morning as system begins to move
onshore. Patchy dense fog remains from KSAC to Modesto this
morning but expect conditions to improve over the next several
hours. Have extended dense fog advisory until 11am to account for
this. Leading edge of rain now extends from central Siskiyou county
back to Humboldt Co. This band of precip will shift ESE during the
day and into the Valley after noon. Latest hi-res guidance shows
heaviest precip across mountains and eastern side of the valley
with only light amounts elsewhere. Snow will begin after then
noon-1 pm timeframe and shift south over the northern Sierra
throughout the afternoon. Heaviest looks to fall between the 2-5pm
timeframe and will cause travel headaches across the Sierra as
snow levels drop rapidly and southwest winds pick up. Going winter
wx advisory will continue as is. Precip should shift south of the
area by 7-8 pm timeframe leaving dry conditions and clearing skies
overnight.

CEO

.Previous Discussion...
A Pacific frontal system now approaching
the north coast is spreading cloudiness over the north half of the
CWA this morning. Farther south...clearer skies are allowing for
fog to form in the southern Sacramento valley...the Delta...and
northern San Joaquin valleys. Several locations in this area are
reporting 1/4 mile or less so the dense fog advisory will remain
in place. This fog should clear quickly this morning as the cloud
cover spreads southward. By mid day today...the main frontal band
is forecast to have advanced into much of NORCAL with
precipitation likely from about interstate 80 northward. By 00z
this afternoon...most locations across NORCAL will have seen some
precipitation with several inches of snow possible over the higher
elevations of the Sierra. Snow levels will start out high this
morning at generally above 8000 feet as the cold front approaches
then drop quickly later today and this evening as cold air behind
the front sweeps inland. By this evening snow levels will have
dropped to between 3000 and 4000 feet. This rapidly moving system
is not expected to drop a lot of precipitation but between the
several inches of snowfall and breezy southwest winds expected
over the Sierra...the winter weather advisory now in place from
noon through 8 pm tonight will be left as is. Models show the
front exiting to the southeast of the CWA by late this evening. By
12z Christmas morning...winds aloft and at the surface transition
to northerly which should bring an end to any precipitation
threat. Skies should clear quickly on Thursday with northerly
pressure gradients increasing as high pressure aloft begins
pushing inland and surface high pressure noses into the Pacific
northwest. MFR to SAC surface gradients of about 6 mb with good
upper level support will keep breezy north winds going Thursday
night and Friday morning. The breezy north winds should keep even
patchy fog from forming in the valley and skies should remain
mainly clear. Fair skies and decreasing winds on Friday should
allow daytime highs to climb to near normal. Lighter winds Friday
night and Saturday will allow at least patchy fog to form in the
valleys Saturday morning with dry mild weather continuing into
Saturday.

&&

.Extended Discussion (Sunday through Wednesday)

Extended models are trending towards a north to south oriented
jetstream over NorCal through early next week with surface high
pressure settling into Oregon and the western Great Basin. This
pattern setup, or course, will likely generate breezy to strong
northerly winds. Several pulses move through NorCal which will
periodically enhance these winds. By Wednesday, the ridge axis
moves closer to the coast which will weaken winds.  JClapp

&&

.Aviation...

Areas of dense fog in the central valley this morning with LIFR
conditions to the interior and western portion of valley through
19-21z. Areas of MVFR in BR and -RA expected after morning fog. A
storm system will move through the area later today with mountains
becoming obscured and rapidly lowering snow levels in the
afternoon and evening. Breezy to strong northerly winds to develop
late this afternoon into evening. JClapp/JBB


&&

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

dense fog advisory until 11 am pst this morning carquinez strait
and delta...northern san joaquin valley...southern sacramento
valley.

&&

$$







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