Scientific Forecaster Discussion
fxus66 ksto 112257
Area forecast discussion
National Weather Service Sacramento California
257 PM PST Wed Dec 11 2019
light rain at times through Thursday, heaviest in the mountains.
Widespread precipitation also expected later in the week and into
the weekend. Drier weather Sunday and early next week.
precip should increase over our forecast area tonight into 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 130w will
make "landfall" during this time tonight and into Thu. 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 tonite 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. One to two inches of precip is
forecast over Shasta co/wrn Plumas co during this time, with amounts
tapering off rapidly swd to the I-80 corridor. South of I-80/US-50,
only a few hundredths are expected as the storm track shifts nwd.
Snow levels are relatively high due to the transport of subtropical
moisture from Hawaii. The snow profilers indicate a snow level
around 5500 ft at Shasta dam, around 7500 ft at Oroville, and
Colfax, and could rise some 500 to 1500 ft overnite due to the warm air advection.
The wet wx should continue on Fri 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. Model freezing/snow level
forecasts came in a little colder than previous runs and we began
to lower them a little from earlier forecasts. Looks as if the snow
level will drop below the passes along the I-80/US-50 corridor by
Fri morning. There is likely to be enough quantitative precipitation forecast Fri and Fri nite (and
maybe into sat) to warrant a Snow Advisory as the snow levels drop
below the Sierra passes. With the bulk of the precip falling over
the Sierra, our total quantitative precipitation forecast thru 12z sun is now falling in line within
the body of the European model (ecmwf) ensembles box/whisker plots for blu, as an
The precip should turn more showery on Sat as the colder/unstable
air drops into norcal as the upper low and trof move across the
nern pac to the pac northwest/nrn California coast before winding down on sun.
Snow levels should drop to 3500 ft nrn mtns to 4000 ft in the Sierra
late Sat, but by then the precip will be diminishing. Jhm
Extended discussion (sunday through wednesday)...
The weather system bringing multiple days of rain and snow to
northern California will be moving out of the area by early Sunday
morning. As troughing moves towards the southwest US, upper level
ridging begins to build over the West Coast. Dry weather and
mostly clear skies are anticipated by mid-afternoon Sunday, with
these conditions continuing through at least Monday. Tuesday
Onward, models have varying solutions for when another trough
will impact the area bringing additional chances for
precipitation. Went with ensemble guidance for this forecast
bringing back wet weather to the area Tuesday, although confidence
is still low at this time with the onset of any rain.
Temperatures remain fairly steady Sunday into next week with
valley highs topping out in the mid-50s. Mountain locations will
mainly see high temperatures in the 30s.
Widespread MVFR/IFR conditions seen through around 20z. Brief
improvement may be seen before another storm brings -ra after 02z.
MVFR/IFR ceilings return by early morning to all sites. Vcsh
possible around 12z at SAC terminals. Winds generally less than