Area Forecast Discussion
Issued by NWS San Joaquin Valley, CA

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FXUS66 KHNX 120926

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
126 AM PST Tue Dec 12 2017

.SYNOPSIS...High pressure will remain over the region for
the next several days with continued dry weather.


.DISCUSSION...Very dry easterly flow aloft continues this morning
as the rex block pattern of high pressure to the north and low
pressure to the south of the Central California Interior
continues. Many many consecutive hours of single digit relative
humidity values in the Sierra and foothills where the dry air can
move in from the east. Other locations facing the San Joaquin
Valley are being influenced by air that is not as dry.

With the calender saying mid December, the long nights. dry air
and clear skies are allowing for a continuation of cold overnight
temperatures at most lower elevation locations and this trend will
continue for the next several days. High pressure aloft has really
locked in an inversion at elevations ranging from 4500 to around
7500 feet over the entire area and below this inversion there is
very little mixing of air. Over the San Joaquin Valley, terrain is
trapping the airmass and daily increases in particulate matter
will cause increasingly dirtier air which may allow for additional
fog formation. Over the past few nights, fog has been very
isolated with most locations seeing visibility reduced to about
one mile at the worst. This situation is not causing much concern
from a traffic perspective and even though airport sensors may
show a downward trend in visibility, values have not reached
critical levels for impacts on transportation. Will continue to
monitor overnight in case things change.

Forecast models keep the ridge in place over the region for the
next several days, and shunt any incoming Pacific weather systems
well to the north. One is progged to move across Oregon and into
the Great Basin on Friday night however the only affect over the
forecast area may be some high clouds.

Through Tuesday of next week, forecast models continue to hold the
ridge in place for dry weather. After Tuesday, the latest ECMWF
model develops and drives into the area a deep, closed low. It
must be noted that this solution was NOT present on the previous
run of this model. Additionally the GFS is hinting at a deepening
trough and a fundamental shift in the longwave pattern way out
Wednesday of next week. Model accuracy gets RATHER LIMITED this
far into the future, therefore as of today this forecaster is not
putting too much confidence in this change in the pattern. The
pattern will undoubtedly change sometime and maybe for only for a
short time. As of this morning a wait and see approach seems to be
the most reasonable way to proceed.


Widespread MVFR visibility in haze and mist will persist in the San
Joaquin Valley, with areas of IFR and local LIFR/VLIFR in fog
developing until 18Z Tuesday. Otherwise, VFR conditions will prevail
over the central CA interior during the next 24 hours.


On Tuesday December 12 2017... Unhealthy in Kern County. Unhealthy
for sensitive groups in Fresno... Kings and Tulare Counties.
Fireplace/Wood Stove Burning Status is: No Burning Unless
Registered in Fresno... Kings... Madera... Merced and Tulare
Counties. No Burning For All in Kern County.
Further information is available at



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 for additional information
an/or to provide feedback.


.HNX Watches/Warnings/Advisories...


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