Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 250535

Area Forecast Discussion
National Weather Service Phoenix AZ
1035 PM MST Sat Jun 24 2017

.UPDATE...Updated Aviation discussion.



High pressure will keep temperatures hot this weekend. A slight
chance of afternoon thunderstorms will return to the mountains of
eastern Arizona Sunday and Monday. Toward the end of the week
temperatures will edge down and chances for afternoon
thunderstorms will increase.



Visible satellite imagery shows mostly sunny conditions over the
desert southwest. Some scattered mid/upper level clouds are moving
northward over Maricopa and Yuma counties. Dewpoints are in the
40s with 50s near the Colorado River area---indicative that
monsoon conditions are not far away in time.

High pressure over Arizona is keeping us under the high heat
again with temps expected as high as 115 today in the lower
deserts, about 9-10 degrees above normal. Sunday should be the
same although some moisture is likely to back into portions of
eastern Arizona, increasing the chances for some afternoon
thunderstorms, especially over the higher terrain. Monday could
see more thunderstorms to the east again although coverage appears
to be less than on Sunday. Temps will moderate later in the week
down to 107 in Phoenix; not a huge drop but closer to normal.



Today and Tonight...

00Z Sat weather balloon soundings highlighted the plume of boundary
layer moisture, mainly below 4000 feet, that seeped north through
the Colorado River Valley the past few days then curved toward
Phoenix. Yet despite this moisture increase, the flow aloft at and
above 300 mb was highly anti-cyclonic and very stable. Ditto for
Saturday afternoon and evening over our forecast area, southeast CA
to south central AZ. And for the most part, the lower Colorado River
Valley moisture seepage is over.

However, significant boundary layer moisture from west Texas is
expected to push to the eastern AZ border today. New Mexico easterly
winds are remnants of a back door cold front that pushed into
northeast New Mexico/Texas panhandle from Nebraska/Kansas Friday
afternoon. Models forecast a few late day thunderstorms over the
higher terrain of the White Mountains which should stay east of our
southern Gila County zone.

The Excessive Heat Warning continues today as desert temperatures
rebound into the 115-117 degree range.

Sunday and Monday...

Significant boundary layer moisture from New Mexico will continue to
push deeper into southeast AZ Saturday night and Sunday. At the same
time, models continue to forecast a weak Pacific disturbance at and
above the 300 mb level to move from offshore San Diego into
southeast AZ by Sunday evening. A better chance of late afternoon
and evening thunderstorms are forecast over parts of southeast AZ
and the White Mountains, and close enough to mention a threat of
thunderstorms over the higher terrain of our southern Gila County
zone. Could convective outflows from southeast AZ move toward the
Phoenix area for gusty winds, blowing dust, and a spike in humidity
Sunday evening. This question will be better answered perhaps Sunday
afternoon as we see the character, coverage, and behavior of these
predicted storms in southeast AZ.

Following the Sunday evening upper level disturbance in southern AZ,
a more anti-cyclonic flow regime at and above 300 mb will likely
stabilize the atmosphere over our forecast area Monday despite a
moisture increase. Upper level deformation is still seen in the
models  over far southeast AZ/northwest Mexico for a continuation of
afternoon/evening storms. Otherwise partly cloudy skies and dry
weather will cover our forecast area Monday, with the exception of a
slight chance of thunderstorms over the higher terrain of southern
Gila County bordering the White Mountains.

Tuesday through Friday...

Somewhat stable northwesterly flow aloft at and above 300 mb level
will develop over most of AZ.  At the same time drier 850/700 mb
winds veer from the west to thin the boundary layer moisture in our
forecast area. This should preclude any thunderstorm developments in
our area of responsibility

High temperatures are forecast edge lower this period as the center
of the high pressure system and thicker atmosphere move south into
northwest Mexico.


South-Central Arizona including KPHX, KIWA, and KSDL:

Persistence forecast with typical diurnal wind shifts and
afternoon breeziness Sunday across the Phoenix area. Any
thunderstorm activity that develops during the late afternoon is
expected to remain confined to the higher terrain of Gila County.
However, there is a slight chance that an outflow boundary
propagating from the east or southeast could produce brief wind
gusts along with blowing dust and reduced visibilities in the late
afternoon or evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather impacts expected. Winds will retain a
southerly component at KIPL/KBLH with winds generally less than 10
to 15 kt.

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:

A high pressure system will remain over the southwest allowing temperatures to
continue to soar well above normal. Temperatures will begin to decrease through
the week as the high pressure weakens and some moisture moves into the region.
The moisture is primarily moving into southeastern California/
southwestern Arizona and eastern Arizona. The best chances for any thunderstorms
will be confined to the mountainous regions including the Rim and southern Gila County.
However, dry lightning will be a concern as the amount of precipitation with any
storm may be limited. Otherwise, minimum relative humidity will be in the 8-12
percent range for desert locations, a little higher over the higher terrain, and
both with meager overnight recoveries. Finally, winds look to be the strongest
Tuesday evening and into Wednesday with gusts up to 30 mph possible though much
of the area.



Record highs this week:

Date           Phoenix            Yuma
----           -------            ----
6/24         118 in 1929        120 in 1957
6/25         120 in 1990        119 in 1994
6/26         122 in 1990        122 in 1990


Spotters may be needed in southern Gila County on Sunday afternoon/evening.


AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ530-533-534-

     Excessive Heat Warning until 9 PM MST /9 PM PDT/ Sunday for

     Heat Advisory until 9 PM MST Sunday for AZZ557-558-563.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ560-561-563-

     Excessive Heat Warning until 9 PM PDT Sunday for CAZ562-564-565.



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