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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 172315

Area Forecast Discussion
National Weather Service Phoenix AZ
415 PM MST Sat Aug 17 2019

Updated Aviation


Temperatures will cool ever so slightly this weekend but remain
several degrees above normal. Moisture and thunderstorm activity
will be very limited through the middle of next week as record high
temperatures and another Excessive Heat episode is expected for
Tuesday and Wednesday across most of southern Arizona and southeast
California. Moisture should return by Thursday to begin a gradual
cooling trend and bring back at least a slight chance of
thunderstorms back into the forecast for south central Arizona late
next week.


West-southwesterly flow is engulfing the Desert Southwest and
pushing moisture east and of the forecast area. This mornings
Phoenix sounding showed a PW of 0.87 inches, which is just above the
10 percent moving average. Current Mesoscale Analysis also indicates
a dry atmosphere overhead with PWs between 0.4 and 0.8 inches in our
forecast area (the greatest PWs are over La Paz, Yuma, and Maricopa
counties). Mid-level temperatures have also continued to warm since
yesterday and as a consequence it`s going to be extremely difficult
to get any storms going today, especially with very limited
instability. CAPE is pretty much non-existent today, with the
greatest instability in far southeastern Arizona. The HREF is in
agreement, showing the best chances for isolated convection in far
southeast Cochise county. Otherwise, temperatures today will rise
into the 105-110 degree range with mostly clear skies in place.

Similar conditions and temperatures are expected on Sunday as H500
heights remain around 589-590 dm and H850 temperatures remain around
28-29 deg C. Mid-level heights will once again rebuild and intensify
Sunday into Monday, first across the south central Texas before
shifting northwest into Big Bend. Temperatures will begin to
increase on Monday, but we should remain below excessive heat
thresholds since the height increases aloft will not translate to
warmer boundary layer temperatures. However, the mid-to-upper-level
anticyclone is expected to become positioned near the Four Corners
for Tuesday into Wednesday, with H500 heights increasing to around
594 dm under mostly clear skies. Forecast H850 temperatures exceed
31 deg C on Tuesday and 32 deg C on Wednesday, about a degree warmer
than than the past week`s excessive heat episode, so another
excessive heat episode appears likely with daily records likely
being set on Wednesday and possibly on Tuesday. Therefore, the
Excessive Heat Warning for Tuesday and Wednesday over the lower
deserts of Arizona has been expanded for some higher elevation
locations in Pinal and Gila County including Superior, Globe, and
Miami that were left out of this previous event. In addition, the
latest model guidance shows temperatures increasing more rapidly
than previously thought across southeast California on Tuesday, so
we have included them in the Warning for Tuesday in coordination
with neighboring offices. We have even added in western portions of
Joshua Tree National Park for Wednesday that are usually left out of
these warnings as some of the more accessible portions of the park
will experience excessive heat.

Fortunately, this excessive heat episode should be shorter than
what we saw this last week as moisture returns from the south
Wednesday night into Thursday, though this may not return fast
enough for southeast California where an extension may be needed.
A fast-moving easterly tropical wave currently east of the Lesser
Antilles should cross the Yucatan early this week and progress
into central America and Mexico by Wednesday, triggering a
significant amount of convection in the tropics. At the same time,
a weak frontal system over the Central Plains and thunderstorms
on the lee side of the Rockies should help bring in moisture from
the northeast as the center of the anticyclone slides westward.
Both of these features should at least end the excessive heat,
though thunderstorms may be limited on Thursday to the higher
terrain north and east of Phoenix and over southeast Arizona as
midlevel stability left behind by the ridge aloft will still be in
place. However, better shower and thunderstorm chances for the
lower deserts including Phoenix are expected on Friday as we may
have better northeasterly steering flow along with convection to
our southeast to help multiple outflow boundary collisions
overcome any leftover midlevel stability. Regardless, we will
finally have a return of a monsoon pattern which appears to hang
around for a bit with perhaps some influence from a tropical
cyclone as both the ECMWF and GFS show development once the
easterly wave crosses into the East Pacific later next week.


.AVIATION...Updated at 2315 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major aviation concerns through Sunday evening with just
increasing high cirrus decks. Winds could remain somewhat variable
into the evening before settling on a WSW component; and confidence
is only moderate regarding the timing of the switch back to easterly
overnight. Similar to today, the wind transition Sunday afternoon
will also likely incorporate several hours of light and variable

Southeast California/Southwest Arizona including KIPL and KBLH:
Little to no aviation concerns through Sunday evening under
increasing thicker cirrus decks. Winds will favor a general
persistent southerly component with the typical diurnal variation
between a ESE and WSW. Some occasional gusts to 20 kt are possible.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday:
A warming trend will take place through the middle of next week.
High temperatures will climb well above normal by next Tuesday and
Wednesday with many desert temperatures increasing into the 110 to
115 degree range and some isolated spots exceeding 115 degrees in
southeast California and southwest Arizona. Moisture should return
by Thursday to begin a gradual cooling trend and bring back at
least a slight chance of thunderstorms into the forecast for the
higher terrain of Gila County and perhaps the lower deserts
including the Phoenix metro Friday and Saturday. Minimum relative
humidity values through Wednesday will decrease into the single
digit range before an upward trend commences Thursday through
Saturday to keep all areas in the 15 to 25 percent range. Winds will
follow typical diurnal patterns with afternoon and early evening



Record High Temperatures

Date        Phoenix         Yuma
----        -------         ----
Aug 17    114 in 2013    114 in 1992
Aug 18    112 in 2011    116 in 1960
Aug 19    113 in 1986    116 in 1915
Aug 20    112 in 1986    114 in 1982
Aug 21    110 in 2007    115 in 1969
Aug 22    113 in 2011    115 in 1969
Aug 23    114 in 2011    115 in 2011


Spotters should follow standard reporting procedures.


AZ...Excessive Heat Warning from 11 AM Tuesday to 8 PM MST Wednesday
     for AZZ530>546-548>556-559>562.

CA...Excessive Heat Warning from 11 AM Tuesday to 8 PM PDT Wednesday
     for CAZ561>570.

     Excessive Heat Warning from 11 AM to 8 PM PDT Wednesday for



FIRE WEATHER...Hernandez/Hopper

Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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