Area Forecast Discussion
Issued by NWS Sacramento, CA

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845
FXUS66 KSTO 191651
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
950 AM PDT Fri Apr 19 2019

.SYNOPSIS...
Chance of showers and thunderstorms across Norcal late today
through Saturday. Snow levels generally above major pass levels.
Cooler Saturday then fair skies and normal temperatures Sunday
through mid week. Slight chance of showers northern mountains by
the end of the week.

&&

.DISCUSSION...
Morning Update: High cloudiness from the upper low vcnty 31N/123W
will remain over the CWA as the low moves towards the Socal coast.
The incoming neutral to slightly positively tilted upper trof will
"kick" the upper low Ewd while dragging a frontal boundary across
our CWA late tonight and Sat when precip chances should increase in
our CWA. NAM 19/00z REF prog suggested some higher REFs and possible
thunderstorms developing E of I-5 mainly over Shasta Co around
sunrise on Sat, with a few cells developing during the late
afternoon and early evening over the Srn portion of the CWA and
Sierra. Forecast BUFKIT soundings indicate this will not be a case
of surface-based convection, but elevated instability above 4-5 kft,
and correlated with the dynamics from the upper trof. CAPE is rather
"thin" so hail is not the main concern, but light wind fields (slow-
moving cells) and PW over an inch in the morning could yield some
locally heavy rain rates with the cluster or short segments of cells
east of I-5 in Shasta Co. There is a small chance of seeing some
half inch per hour rates, but no indication of anything heavier than
that.

The trof is forecast to be narrower than yesterday, so the precip
chances should wind down in our CWA Sat nite, then dry and milder wx
on Sun. Still expecting 10 to 15 degrees cooler temps on Sat over
today, and then milder temps up to 9 degrees above normal N to near
or slightly below normal temps S on Sun.   JHM

.Previous Discussion...

Fair skies over the forecast area this morning under upper level
ridging over the western U.S. Temperatures are running a few to
several degrees higher than 24 hours ago after a few to several
degree climb in highs on Thursday. Upper ridge axis shifts eastward
today as a weak trough drops out of the Gulf of Alaska. Increased
onshore flow and slight airmass cooling will bring a little cooling
today but daytime highs will remain well above normal. Models still
paint some light precipitation over the coast range by 00z this
afternoon spreading westward overnight. MU cape progs showing enough
instability for a thunderstorm threat through the evening hours
until daytime surface heating drops off. Main upper trough drops
southeast through Norcal on Saturday. This will bring shower chances
southward to about Sacramento with enough instability for return
chances of thunderstorms. Cooler air associated with the trough will
bring a significant drop in daytime temperatures with highs forecast
at or a little below normal. Upper ridge begins building in late
Saturday night with an end to any shower threat expected by Sunday
morning.

Fair skies and warming put daytime highs back to several degrees
above normal by Sunday afternoon. More warming expected on Monday
as the upper ridge builds along the west coast with daytime highs
back to 10 to 15 degrees above normal.

&&

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)


Not much change to the extended forecast this morning as model
guidance continues to support high pressure building over the
area. We`ll see another warm week with high temperatures
featuring the 80s for the Valley while the ridges top in the 60s
and 70s. The warmest days look to be Tuesday and Wednesday with
temperatures then decreasing each day due to a broad upper level
trough moving into the region. This will bring at at least an
increase in cloud cover to northern California and perhaps even
some isolated showers and thunderstorms.
&&

.AVIATION...


For most of the day, VFR conditions will prevail with high clouds
eventually filtering over the region. Showers and isolated
thunderstorms will arrive to the northern most TAF sites late
this afternoon and evening, before spreading southward on
Saturday. For now, we`re expecting VFR conditions to prevail but
heavier showers and thunderstorms may produce MVFR or IFR
conditions at times.
&&

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

$$



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