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Area forecast discussion
National Weather Service Sacramento California
401 am PDT Wed Sep 18 2019

Synopsis...
showers and a chance of thunderstorms are expected across the
region today as another weather system moves through. Showers will
linger across the mountains on Thursday, then dry and milder
weather is expected heading into the weekend.

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Discussion...
satellite imagery shows front moving slowly inland (southeastward)
from northwest California. Radar indicates some light showers across
the northern Sacramento Valley and mountains to the west and north
of Redding. Skies remain mostly clear to the south across the
southern two thirds of the forecast area. Current temperatures
vary from the 30s in the colder cloud-free mountain valleys ahead
of the front, to the upper 60s across milder portions of the
Central Valley.

Little change to forecast. Front spreads showers south to about
the Sacramento area this morning before slowing across the
northern San Joaquin Valley this afternoon, with scattered
showers and thunderstorms expected later this morning into this
evening (mainly Sacramento area northward) with the upper trough
and approaching vort. Quantitative precipitation forecast expected to vary widely given the
convective nature of the system with amounts varying from a few
hundredths of an inch in the valley to locally a half inch or more
over the northern Sierra.

The convective potential is still on track with model forecast
soundings indicating sufficient cape in the afternoon for potential
for thunder mainly north of the SAC area, per the Storm Prediction Center outlook. Href
indicates a high probability of >40 dbz echoes over the West Slope
Sierra in the afternoon which could correlate to heavy showers
lowering snow levels to pass levels once again. Motorists should be
alert for sudden bursts of snow over the high trans-Sierra passes
in the afternoon.

Model agreement in timing and position of a vort Max moving through
the upper trof in the afternoon and the NAM refl prog suggests
ingredients coming together for possible rotating storms in the
"chico (area) convergence zone." Similar to Mon, the low level
shear is weak (light winds), but the upper level vort Max, if
forecast with any reasonable accuracy, would augment the mid level
cyclonic flow possibly yielding a lower end mesocyclone.

Unsettled weather should exit most of the region on Thursday with
skies clearing out from west to east. The exception would be the
Sierra where cyclonic flow is forecast to linger for most of the day
causing some wrap around moisture and showers. With the mean trough
centered over Nevada, meridional height gradients set up supporting
a period of dry, northerly flow over the valley on Thursday and
continuing into Friday. Wind fields do not appear particularly
strong although gusts could 15-20 mph or so on Thursday afternoon
and evening. A continued moderation in the temperatures will
accompany this shift in the winds with highs in the mid 80s in The
Vly on Friday. This is still 1 to 9 degrees below average except
in the Carquinez Strait where readings will be slightly above
normal.

&&
Extended discussion (sunday through wednesday)...

Progressive Pacific flow will usher in another upper trough which
is set to arrive on the West Coast by Sunday morning. The upstream
height falls ultimately shift the transient ridge into The Rockies
by the afternoon. Some increase in cloud cover is likely over
sections of interior northern California as high-level moisture
streams off the Pacific. However, valley highs should have no
issue reaching the upper 80s which is close to climatology for
mid-September. A few scattered showers will be possible over the
mountains of northern Shasta County extending into western Plumas
County as well the northern Sierra crest.

The biggest question mark ahead is where this trough situates
itself as it will dictate whether the pattern will be unsettled or
possibly dry and breezy. Deterministic models continue to waver
with the 06z GFS and 00z European model (ecmwf) taking an upper low along the
California/Nevada border into the Desert Southwest. Ensemble
spaghetti plots show plenty of diversity in its solutions to
support tracks west and east of this scenario. Will continue to
maintain a period of northerly flow early next week which ensures
valley highs stay in the upper 80s to low 90s. The parent upper
low potentially stalls in the southwestern U.S. For part of next
week which would dictate the prevailing flow aloft. While not in
the forecast, the 00z European model (ecmwf) shows downsloping northeasterly flow
which suggests highs in the upper 90s early next week. So there is
some room for temperatures to rise higher than currently in the
forecast. ~Bro

&&

Aviation...

A frontal system is currently moving through northern California
spreading light to moderate rain to portions of the area. Most
ceilings and visibilities will be in the MVFR range; however,
heavier showers could briefly drop some sites to IFR conditions.
Isolated thunderstorms are possible through 06z across taf sites
krdd and krbl. ~Bro

&&

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