Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 151754
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
950 AM PST Mon Dec 15 2014

.Synopsis...
Periods of valley rain and mountain snow through mid-week as
several weather systems move through the area. Brief drying
expected Thursday, then another system is forecast to bring a
return of showers on Friday.

&&

.Discussion...
Upper level trof and front along the coast forecast to swing inland
later today and tonite. Both features forecast to weaken as the jet
sags Swd into Socal quite a bit S of our area...and upper level
divergence weakens in the afternoon. The various QPF progs have been
lifting the precip Nwd...mainly affecting the NW portion of our CWA
today and then the N and NE portions of our CWA this evening...as
the short wave energy lifts NEwd across interior Norcal.

Water Vapor imagery seems to indicate the vort max is a tad farther
S than forecast...more in line with the GFS but a little faster.
Have a "gut feeling" the vort max will track farther S than forecast
with more QPF than forecast over the Srn portion of the CWA over the
W Slope Siernev...mainly later this afternoon and this evening. With
this progression of events and the synoptic pattern...and a more
neutrally-tilted trof...the flow over the Siernev will be more from
a SE to S trajectory (or parallel to the Siernev) than SWly so less
favorable orographic effects are expected overall. With snow
levels near 4500 ft...the snow will be more evenly distributed over
the Siernev than a normal winter event.

We did update to a WSW for the Siernev zones with the expectation
the precip would wind down by around midnight. With the trof axis
still forecast along the coast at 06Z this may be a little premature
but can adjust in the timing if needed.

The next wave comes in late Tue/Wed and will be deeper/colder with
some to many areas likely getting more precip than today.   JHM


.Previous Discussion...

Cool and unsettled weather will continue across the area through
much of the week. Post-frontal unstable airmass, and developing
warm advection ahead of the next wave, will support a continuing
chance of showers across the region tonight into Tuesday. The next
wave, presently approaching 40N/140W, will spread more
precipitation into the area by later Tuesday followed by a third,
weaker system later Wednesday.

All told, some healthy additional precipitation totals will be
possible by mid-week. Looks like much of the region will see
another inch or so of rain by later Wednesday while the higher
elevations will likely see a foot or more of snow. All this QPF
will be spread out over 3+ days, so impacts are expected to be
relatively minor. Snow levels will be fairly moderate, ranging
mainly from 3000 to 4500 feet and chain requirements will be
likely at times the next few days over the northern Sierra passes
as periods of precipitation move through.

Looks like ridging will bring a brief return of dry weather to
NorCal on Thursday.

&&

.Extended Discussion (Friday through Monday)...
Precipitation with the next system will move into the coastal
range and Shasta County late Thursday and spread over all of
interior Northern California on Friday and Saturday. The 0z ECMWF
remains wetter than the GFS so low confidence in amounts. The
trough shifts to the east as an upper level ridge builds over the
eastern Pacific and into Northern California late Saturday into
Sunday. This will bring some drier weather and a return of fog and
near to a little above normal high temperatures.

&&

.Aviation...
Storm system moving widespread precipitation into the central
valley is causing a mix of VFR/MVFR at TAF sites. Local IFR
conditions possible during heavier showers. Snow levels around
3500 ft in Shasta County and around 4500 ft over the Sierra with a
mix of MVFR/IFR and LIFR expected across the mountains. Winds at
TAF sites should be light 6-12 kts with occasionally breezy
periods. Precipitation will dissipate tonight.

&&

.STO Watches/Warnings/Advisories...None.

&&

$$





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