Area Forecast Discussion
Issued by NWS San Joaquin Valley, CA

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FXUS66 KHNX 211131
AFDHNX

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
422 AM PDT Thu Sep 21 2017

.SYNOPSIS...Cooler than normal weather will prevail over the
district through the weekend as an upper level storm system
remains anchored over the Great Basin. Scattered showers can
be expected over the mountains into this evening with snow
showers over the higher elevations of the Sierra. Winds will
also be strong and gusty through and below the mountain
passes into this evening.

&&

.DISCUSSION...A cold front will herald the arrival of Fall-like
weather across the central California interior today. The timing
couldn`t be more appropriate since Autumn officially begins at
1:02 pm PDT tomorrow. This cold front shows up quite well on
current satellite imagery and is defined by a band of clouds and
precipitation from Merced county and Mariposa county southward
into Fresno county. A 120-knot jet aloft that is aligned with the
surface cold front will support gusty winds with this frontal
passage southward through the CWA this morning. Up to five
hundredths of an inch of rain has fallen with this system so far
in the San Joaquin Valley north of Fresno county where wet roads
will be slick for early morning commuters. We don`t expect rain
amounts to be much more than that on the valley floor as this
cold front sweeps southward this morning. The foothills and
higher elevations of the Sierra will fare a little better with
precip amounts from this system through at least the midday hours
today thanks to added orographic lift associated with the 300 mb
jet.

In the southern Sierra, up to a few tenths of an inch of precip
is likely today and, for the first time in months, the highest
elevations of the Sierra will see a small accumulation of snow.
Local snow accumulations of 1 to 3 inches is possible in the
Sierra above 7,000 feet. Long after the surface cold front has
exited south and east of the CWA, the upper level trough that
follows it will continue to produce instability showers and
possibly even isolated thunderstorms with small hail over the
Sierra into the early evening hours today. SPC has this area
outlined for a general risk of thunderstorms today which we`ve
introduced into the gridded forecast.

By this afternoon, the core of the upper level jet will be
positioned over Kern county and will support strong winds through
and below those mountain passes. Although today will be a
blustery day over much of the CWA, it appears the strongest winds
will accompany and immediately follow the surface cold front.
This will be especially true in the San Joaquin Valley where a
wind advisory in effect. While a general diminishing trend in
wind speeds should begin over most of the San Joaquin Valley after
midday today, we will keep the wind advisory in effect through at
least early this evening. Afterward, the models forecast surface
pressure gradients to decouple rather quickly, so in most valley
locations winds should die off shortly after sunset. We`ll let
the decision rest with forecasters on the day shift as to whether
the expiration time of that wind advisory can be shortened for
the San Joaquin Valley. Meanwhile, strong and gusty winds should
continue into this evening in the Kern county mountains and
desert. A high wind warning remains in effect for these areas
where winds could gust up to 75 mph today.

Aside from the wind, the big weather story today will be the
cooler temperatures. There`s no doubt that highs this afternoon
will be a good ten to fifteen degrees below normal. In most
places, today will be the coolest day since the 12th of June.
Hikers and campers in the high Sierra may be in for a rude
awakening today as temperatures dip into the 30s and are
accompanied by snow showers. Although most of the moisture
associated with the upper level trough will be wrung out over the
Sierra during the next 18 hours, clouds will likely linger along
the western slopes and along the valley facing slopes of the
Tehachapi mountains through Friday morning. Elsewhere, as winds
die and the atmosphere stabilizes, temperatures will plummet to
chilly levels tonight where skies are clear. Minimum temperatures
will be in the lower 40s in the chilliest locations of the San
Joaquin Valley and the Kern county desert by daybreak and warm no
higher than the 70s Friday afternoon.

The models are in pretty good agreement that the upper level
trough will remain centered over the Great Basin through at
least Monday. This will keep a north to northeast trajectory of
air coming into California directly out of western Canada during
this period with a continuation of cooler than normal
temperatures. Each day will dawn chilly but mellow into
comfortable, pleasant afternoons with plenty of sunshine.

A noticeable warming trend will get underway by Tuesday and
continue undaunted through the end of next week as an upper level
ridge of high pressure builds into California from the Pacific.
Maximum temperatures will rebound rather nicely during this
period. (Imagine trying to keep a ping pong ball under water.)
By Wednesday, Autumn weather will be all but a memory and
Summerlike warmth will be back. High temperatures in the San
Joaquin Valley, lower foothills and the Kern county desert
will be in the 90s during the second half of next week while
the nights will be relatively cool (mid 50s to mid 60s).

&&

.AVIATION...
Scattered showers will produce areas of MVFR and local terrain
obscurations over the southern Sierra Nevada and Kern County
mountains through 03Z Friday, while lingering upslope clouds will
persist along north and west facing slopes through 12Z Friday. Winds
will gust near 35 knots along the west side of the San Joaquin
Valley and near 50 knots over the Kern County mountains and deserts
through 03Z Friday. Areas of MVFR and local IFR visibilities will be
possible in blowing dust from the gusty winds. VFR conditions are
otherwise expected over the Central CA interior during the next 24
&&

.AIR QUALITY ISSUES...
None.

&&

.CERTAINTY...

The level of certainty for days 1 and 2 is high.
The level of certainty for days 3 through 7 is high.

Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.php for additional information
an/or to provide feedback.

&&

.HNX Watches/Warnings/Advisories...
High Wind Warning until 11 PM PDT this evening ABOVE 1500 FEET
FOR THE CAZ095-098-099.

Wind Advisory until 11 PM PDT this evening CAZ089-091.

&&

$$

public...Durfee
avn/fw...JEB
synopsis...Durfee

weather.gov/hanford



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