Area Forecast Discussion
Issued by NWS San Joaquin Valley, CA

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FXUS66 KHNX 201200

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
500 AM PDT Tue Jun 20 2017


.DISCUSSION...Although today is officially the last full day
of Spring, you would hardly realize it, weatherwise. The kind
of heat we`ve been experiencing isn`t even typical for Summer.
Records for high temperature and for high minimim temperature
were shattered in many locations Monday and they will be
challenged each and every day for the remainder of this week.
When Summer officially begins at 9:24 pm PDT today, most of
us may blame Mother Nature for being too anxious to bring us
an early preview of blazing hot weather.

The biggest blame should go to a very strong upper level ridge
of high pressure currently centered over Arizona. The models
agree in keeping this ridge anchored over the Desert Southwest
through the end of this week. That means our current heat wave
will be persistent and unrelenting, at least through Friday
now. Those models are forecasting little change in 500 MB heights
and 850 MB temperatures during this time. So if you guessed
that we`ll be extending the Excessive Heat Warning for the San
Joaquin Valley, foothills and the Kern County mountains and
desert, then you`re exactly right.  In fact, the models are now
suggesting that this high pressure ridge will hang tough and
keep a firm grip on much of the West coast through the weekend.
That`s at least two more days of triple digit heat in the lower
elevations Saturday and Sunday. Numerical guidance is doing
what it normally does and is trending temperatures toward
climatological normals in the 5 to 7 day period, but we know
this is not very realistic while this high pressure ridge remains
over California.

Both the GFS and ECM solutions try to bring a weak short wave
trough into the Pacific Northwest early next week. Chances are
that this trough will just be deflected farther northward as
the ridge remains anchored over the Desert Southwest. Nonetheless,
the passage of an upper level trough to our north could allow
an onshore flow to develop along the central California coast
by next Tuesday which in turn could bring a shallow intrusion
of marine air into the San Joaquin Valley and finally drop high
temperatures below the century mark. Much of the CWA outside of
the San Joaquin Valley will probably see little if any appreciable
cooling early next week.

Bottom line is, there will not be any change in the air mass over
the CWA for at least the next 5 days. Since winds will be rather
light, the air mass will continue to stagnate underneath this
ridge with increasingly poor air quality in the San Joaquin
Valley. Dewpoint temperatures have been extraordinarily high
throughout much of the San Joaquin Valley lately, especially over
the eastern third of the valley. So the added humidity will just
exacerbate an already unpleasantly high heat risk in this region.
Many have asked why dewpoints have been so high in the San Joaquin
Valley. The ridge is keeping the door to the tropics shut.
However, after a very wet Winter, there`s an abundance of water
around. Besides heavy irrigation, there`s still water in the
Tulare Lake basin. Additionally, rivers are running deep because
of large water releases from the dams. Minor flooding is possible
along any of the rivers downstream as a result. Those rivers are
also more dangerous than meets the eye and are running swift and
cold. Too many people have ignored the dangers of the rivers and
either drowned in them or became victims of a water rescue. It`s
simply not worth risking your life to cool off in these rivers!

Another potential hazard from all of this hot weather is the fire
danger. This will be an increasing threat through the Summer
months as fuels continue to dry out.  Over the high Sierra,
melting snow could provide enough moisture for the development of
towering cumulus each afternoon during this heat wave. Although
this strong high pressure ridge will suppress most convection, an
isolated thunderstorm or two cannot be ruled out along the Sierra
crest during the next few afternoons, so folks in the back
country should be mindful of this if they are doing any hiking.


Isolated thunderstorms can be expected over the higher elevations
of the Sierra between 21Z today and 06Z Wednesday. Otherwise, VFR
conditions will prevail across the central California interior
during the next 24 hours.


On Tuesday June 20 2017... Unhealthy for sensitive groups in
Fresno... Kern... Kings and Tulare Counties and Sequoia National
Park and Forest.
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...
Excessive Heat Warning until 11 PM PDT Thursday



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