Area Forecast Discussion
Issued by NWS San Joaquin Valley, CA

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FXUS66 KHNX 182355
AFDHNX

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
455 PM PDT Fri Aug 18 2017

.UPDATE...Updated Air Quality section.

.SYNOPSIS...A weak upper level storm system will remain
anchored off the southern California coast through Tuesday and
bring a chance of mainly afternoon showers and thunderstorms
to the Sierra. The threat of thunderstorms will extend into
the Kern county mountains and desert and the Sierra foothills
by early next week with the arrival of monsoonal moisture.

&&

.PREV DISCUSSION... /issued 330 PM PDT Fri Aug 18 2017/

DISCUSSION...Satellite imagery shows an abundance of cumulus
buildups over the Sierra this afternoon. Atmospheric warming
and increased stability will produce a scattering of showers
and isolated thunderstorms by late afternoon. With precipitable
H2O values near 1.5 inches, a few of the most well developed
thunderstorms will bring briefly heavy rain and gusty winds.
Like yesterday, much of the convection will dissipate by sunset
only to rejuvenate in similar fashion Saturday and again Sunday.
If you`re a novice to radar imagery, the echoes that are being
detected on Edwards AFB radar this afternoon is not precipitation,
even though it looks like it. One has to look at satellite imagery
to know that this is AP, since skies are mostly sunny in the
Mojave desert, so don`t be fooled by the radar.

Otherwise, a deep easterly flow aloft prevails over the central
California interior this afternoon. Any remnants of high Sierra
convection during the next couple of afternoons will be carried
into the foothills and the east side of the San Joaquin Valley
north of Kern county. This easterly flow is associated with a weak
closed low near Point Conception which the models hold nearly
stationary through at least Tuesday. As this feature pivots
northeastward, it is likely to tap into some deeper mid level
moisture later this weekend. Although the NAM 12 is faster and
more aggressive than the other models, all of them forecast
700 mb-300 mb winds to shift to southeasterly by Sunday night.
This is a direction that will surely bring an influx of monsoonal
moisture into our CWA by early next week.  The resulting effect
will be a noticeable increase in clouds and humidity along with
an increasing threat of thunderstorms over the mountains and
desert. This could happen as early as Sunday night, but is more
likely to occur Monday through Tuesday. Southeasterly flow
patterns like this can carry a few showers and thunderstorms out
of the mountains into the San Joaquin Valley. Right now we are
not confident enough to raise valley POPS any higher than 14
percent during the Sunday night through Tuesday period. We will
have to adjust valley POPS upward if the monsoon becomes more
active than we are currently anticipating. Nonetheless, it is
one part of this weather pattern that we will closely monitor
in the days ahead.

By the middle of next week, the models forecast this closed low
to open up and move inland. The remains of this low will either
get absorbed by the approach of an upper level trough near the
Pacific Northwest coast Wednesday or get kicked eastward by this
trough into the Great Basin Thursday. Either way, winds aloft
will gradually shift to a westerly direction during this period
and finally bring an end to the thunderstorm threat. A slightly
drier and more stable air mass will settle into the CWA by next
Friday as a result. Temperature-wise, little diurnal change is
expected through the weekend. Max temps will be a few to several
degrees lower Monday and Tuesday due to increased cloud cover,
however clouds will also keep min temps a bit higher than they`ve
been and it will feel rather muggy, even in the desert. Next
Friday has the potential to be our warmest day of the next 7 days
as an upper level ridge of high pressure builds into California.

On a final note...we have been fielding many questions lately as
to how the state of the weather will be during Monday morning`s
solar eclipse. A view of it may be partially blocked by clouds,
especially over the higher terrain. Whatever the case, you should
never look directly at the eclipse with the naked eye nor with a
camera, sunglasses, telescope or binoculars unless these devices
have a special solar filter.

&&

.AVIATION...
VFR conditions will prevail across the central CA interior during
the next 24 hours.

&&

.AIR QUALITY ISSUES...
On Saturday August 19 2017... Unhealthy for sensitive groups in
Fresno and Kern Counties and Sequoia National Park and Forest.
Further information is available at Valleyair.org

Additionally...an Air Quality Alert has been issued for smoke
from the South Fork wildfire in Mariposa county. The Air Quality
Alert is in effect for Mariposa county until Monday afternoon,
August 21st.  For additional information, go to
http://www.mariposacounty.org/index.aspx?nid=1434 or call
(209)966-2220.

&&

.CERTAINTY...

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

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...
NONE.
&&

$$

public...Durfee
avn/fw...Bean
synopsis...Durfee

weather.gov/hanford



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