Scientific Forecaster Discussion
fxus66 ksto 051152
Area forecast discussion
National Weather Service Sacramento California
352 am PST Thu Dec 5 2019
scattered light showers today before another winter storm will bring
periods of rain and mountain snow and hazardous travel Friday and
into the weekend.
Just a brief break in the wet wx for today as short wave ridging
builds over the County Warning Area in the wake of the departing low pressure system
to our east and the next mid latitude cyclone near 140w. Lingering
showers noted in the Sierra and over the 80/50 corridor with the
Colfax snow profiler showing the snow level around 7 kft. Although
many areas will not see precip today, models suggest some light warm air advection
precip may develop or spread nwd in our County Warning Area over areas mainly to the
north of the I-80 corridor during the day. Topographic/orographic
influences from the upslope sly flow from the rising 500 mb heights may
be enough to squeeze out a few hundredths. Any partial clearing
early this morning will be short-lived, as the GFS simulated
satellite indicates clouds just offshore will overspread norcal
during the day.
The next winter storm (mentioned above near 140w) is shaping up to
be another impactful winter storm that will bring hazardous mountain
travel Friday into the weekend. U.S. West Coast Arkansas landfall tool
shows a moderate tpw plume (ar) at 37-41 deg north latitude putting the
80/50 corridor right in its path. Heavy mountain snow with
significant travel impacts is expected to begin late Friday for
elevations above 6000-6500 ft. Models have been slowing down the
onset of the precip in our cwa, E.G. BUFKIT for blu begins precip
around 05z Sat (or 9 PM PST fri). Will continue the Winter Storm
Watch as another model run or two could assist in locking in better
timing of the precip.
Another soaking rain is expected with 1 to 3 inches of rain forecast
for the valley and 4 to over 6 inches of liquid precip for the nrn
mtns/West Slope Sierra, with as much as 7 inches in Shasta co. (The
quantitative precipitation forecast follows fairly closely with the European model (ecmwf) ensemble spread for the
valley sites, but exceeds the mean of the ensemble for blu by
about an inch and a half.) Once again this storm has the potential
for several feet of snow to accumulate over the High Sierra. Snow
levels will be above 6 kft at the onset, and then should lower
below 6 kft on sun as the upper low drops swd over our norcal.
Satellite indicates a well developed comma cloud (baroclinic leaf)
with lots of cold, unstable air associated with it. As this
colder air approaches our area Sat, and moves over County Warning Area on sun, we
have introduced a chance of thunderstorms in the valley and West
Slope Sierra foothills. Small hail/graupel is likely with these
storms as indicated by the "thin cape" on the forecast soundings.
Precip forecast to wind down sun nite, followed by dry wx to start
the work week.
Additionally, sly pressure gradients should tighten dramatically
with the approaching surface front leading to wad criteria for the
valley as a 50+ kt low level jet (nam 925 mbs winds) develops Fri evening.
Breezy to windy conditions should last into Sat. Similar to the last
event, the isothermal lapse rates caused by the warm air advection will preclude
downward momentum Transfer of all of the low level jet wind. As a rule-of-
thumb, the peak wind gusts end up to be about 2/3 of the 925 mbs
winds in these isothermal profile cases. Jhm
Extended discussion (monday through thursday)...
Ensemble and model guidance hold firm on ridge building over the
Pacific northwest early in the week. This will lead to at least a
short stretch of dry weather, with the potential for some fog
Tuesday morning. Looking towards to the mid to late portion of
the work week, model and ensemble guidance continue to deviate
from each other. Much like yesterday afternoon's runs, there is
uncertainty as to when and where the next trough sets up. Some
ensemble members suggest we could see light precip over northern
California, with the nbm being one of them. Ended up going a blend
of the nbm to get some pop north of I-80 for Wednesday and then
over the Cascades and coastal range for Thursday. Confidence is
low on this occurring for the time being, thus we'll be keeping a
close eye on how the models and ensembles handle the situation.
Temperatures are forecast to be within +/-5 degrees or so of
their seasonal average for both daytime highs and morning lows.
Stratus and patchy areas of dense fog have been observed at the
taf sites this morning. With moisture lingering expect to see
periods of local IFR/LIFR with low cigs and br through the
morning. Showers currently linger in the Sierra this morning but
will become more focused in the northern portion of the valley
and coastal and Cascade Mountain later today.
Winter Storm Watch from Friday evening through Sunday afternoon
for West Slope northern Sierra Nevada-western Plumas