Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Sacramento California
302 PM PST Tue Dec 10 2019
rain returns to the region tonight tapering to a few light showers
on Wednesday. Widespread precipitation expected later in the week
and into the weekend. Drier weather Sunday and early next week.
after the morning fog in the valley, much of the low cloud deck was
slow to erode. Now the clouds associated and ahead of the frontal
band along the coast are moving into our area and will spread
over the Sierra early this evening. It has been a sunny,
seasonably pleasant day in the Sierra as the higher elevations
were above the fog and well to the east of the advancing cloud
deck from the west for most of the day. But that will change this
The frontal system will move into the region, spreading light rain
over much of our County Warning Area beginning late this afternoon and continuing
into the evening. Radar echoes along the nrn California coast this morning
are now moving onshore at press time, and the timing of the upper
trof suggests the frontal precip should end by Wed morning as the
trof moves through our region. Satellite trends suggest a brief
period of clearing early Wed morning as the trof passes, followed by
an increase in cloud cover during the day. Behind the trof, heights
rebuild on Wed as a ridge axis slides across the region. Some light
sprinkles or showers are still possible mainly in the nrn part of
our County Warning Area on Wed, where warm air advection (about 7 deg c/12hr in the 850-700 mbs
layer) could result in some light precip.
An increase in precip is forecast over norcal Wed nite and Thu as
a strong 150kt pac jet south of the goa (gulf of ak) low reaches the
coast and focuses a +3 precipitable water anomaly into our County Warning Area. The U.S. West Coast
Arkansas tool shows that a "weak" Arkansas (tpw/moisture plume) along 140w
will make "landfall" during this time (late on 12/11 and into
12/12). Rising 500 mb heights will steer the storm track and main
dynamics a little farther north and into the pac northwest. But the strong
warm air advection Wed nite and Thu (over 5 deg c/12 hr in the 850-700 mbs layer)
will interact with the moisture plume resulting in steady precip
over norcal, mainly over the nrn mtns and areas north of I-80. An inch
or so of precip is forecast over Shasta co during this time, with
amounts tapering off rapidly swd to the I-80 corridor. South of
I-80, only a few hundredths are expected as the storm track shifts
Looks as if it will be a little wetter in our County Warning Area on Fri from
earlier forecasts as 500 mb heights begin to fall slowly as the goa
trof emerges from its source region. This will keep the tpw plume
over our County Warning Area through the day. Comparing our quantitative precipitation forecast with the European model (ecmwf)
ensembles we are generally a little low/light on the quantitative precipitation forecast and we
made a last minute upward adjustment this afternoon...but still
we may be a little too light. The bulk of the precip is still
forecast over the Sierra.
Snow levels will be rising Wed nite and Thu to 8kft or so limiting
winter travel impacts, until they lower below the passes Fri nite.
Extended discussion (saturday through tuesday)...
By Saturday morning expect ongoing precipitation mainly over the
mountains and foothills, although a few lingering showers may be
possible in the valley. Snow levels will range from 4-5kft Saturday,
although snowfall accumulations themselves will only be a few inches.
Some minor mountain travel impacts are anticipated Saturday due to
this lingering precipitation. Models have trended a bit later to
bring dry weather back to the area, as a few light lingering snow
showers will be possible through Sunday morning.
Upper level ridging builds into the area Sunday and Monday. Mostly
clear skies and average temperatures are expected early next week.
Ensemble guidance shows the potential for another system to impact
norcal Tuesday, although confidence with this system remains low
at this time.
Deteriorating conditions expected as a system moves across
northern California tonight. Light rain is expected at rdd/rbl
between 04-06z, spreading south towards Sacramento terminals
shortly after. MVFR/IFR conditions likely overnight at these sites,
with possible LIFR conditions returning to southernmost