000
FXUS66 KSTO 212321
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
321 PM PST Thu Nov 21 2019

.SYNOPSIS...
Dry weather with mild daytime temperatures and light winds through
the weekend. Breezy conditions over the Sacramento Valley on
Monday with the potential for a winter storm by Tuesday into the
Thanksgiving holiday. Much cooler temperatures in store for next
week.

&&

.DISCUSSION...
Mid-afternoon water vapor imagery shows the upper low spinning
over northwestern Arizona with general northeasterly flow aloft on
the backside. Diurnal heating combined with some instability
aloft has produced some enhanced cloud cover over the west slopes
of the Sierra, generally from Plumas county southward. Otherwise,
conditions are fairly quiet with lighter winds as height and
pressure gradients have weakened.

Split flow over the eastern Pacific will allow another upper low
to emerge but this stays well offshore with little to no impact
over northern California. Temperatures will gradually rise each
day into the weekend as a building west-east oriented ridge begins
its brief dominance. Light winds and mostly sunny skies should
ensure quiet weather for the weekend as Valley highs soar into the
upper 60s to low 70s. While not expecting any daily records to be
broken, this will easily be 6 to 12 degrees above climatology,
locally higher departures into the Sierra. Many higher mountain
locales could see high temperatures into the low 50s. However,
overnight lows across the region remain closer to average as high
pressure supports favorable radiational cooling effects. ~BRO

&&
.EXTENDED DISCUSSION (Monday THROUGH Thursday)...
Next week will bring about a number of changes to the weather
with dry, breezy conditions on Monday before turning cool and wet
from Tuesday into the Thanksgiving holiday.

Global ensembles continue to show the pair of systems with the
lead one tracking through the Pacific Northwest toward the central
Rockies. Models have been trending a bit weaker so overall
impacts may be less than earlier perceived. The forecast KSAC-KMFR
gradient is only on the order of 8 mb which is on the lower end
of the spectrum of northerly wind events. While humidities are not
expected to be particularly low, the extremely dry fuels in place
still warrant the potential for an elevated fire weather risk on
Monday.

On the heels of this initial shortwave will be a much stronger
system with widespread precipitation likely to enter the picture
Tuesday into Thanksgiving. The upper ridge parked over the east
Pacific (i.e., 140W) will retrograde enough to allow this system
to be a rain/snow maker versus another dry wind event. Overall
details are subject to change given there is quite a bit of spread
among ensemble members. Many of the GEFS members are on the
lighter side while the ECMWF ensembles are more adamant about the
wet pattern. While forecast amounts will waffle a bit, there is
greater confidence in the cooler temperatures and lower snow
levels. Mountain snow is likely at or below pass levels during the
Tuesday through Thanksgiving Day timeframe. Given this is one of
the busiest periods for travel, this system bears watching. ~BRO

&&

.AVIATION...
VFR conditions, winds generally less than 10 kts expected. Local
MVFR conditions possible over Northern San Joaquin Valley Friday
AM.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$