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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 211740

Area Forecast Discussion
National Weather Service Phoenix AZ
1040 AM MST Wed Aug 21 2019

.UPDATE...Updated Aviation Discussion.


Excessive heat with record high temperatures is expected again 
today across south central Arizona into southeast California with
only very isolated thunderstorms over far eastern Gila County. 
Moisture will return by Thursday morning to begin a gradual 
cooling trend and bring isolated thunderstorms back into the 
forecast for south central Arizona Thursday through Saturday, with
the best chances over Pinal and Gila Counties. A gradual warming
trend will begin on Sunday and continue into the first half of
next week with minimal rain chances. 


An elongated, zonally-oriented subtropical ridge at mid-to-upper
levels stretches from just off the coast of California into 
Oklahoma, with its center along the New Mexico-Arizona border. 
This is continuing to promote dry and clear conditions that are 
allowing for efficient radiational cooling once again this evening
into the upper 70s and 80s across the lower deserts despite our 
excessive heat in the afternoons. Although H500 heights are 
expected to decrease slightly from yesterday's 593-594 dm values
as the ridge center shifts west and consolidates off the coast of
Baja California, H850 temperatures that were around 31 deg C over
south central Arizona and 28-30 deg C over southeast California 
will increase slightly today. This will allow temperatures to
increase 1-2 deg F relative to yesterday over south central 
Arizona with more significant warming of 3-5 deg F over southwest
Arizona and southeast California. Therefore, the Excessive Heat 
Warning remains in effect for much of south central and southwest
Arizona along with southeast California as temperatures warm up
into the 112 to 116 deg F range for most desert locations with
isolated hotter temperatures in southeast California. This will
easily shatter the 110 degree record for Phoenix set in 2007, and
will challenge the 115 degree record in Yuma set in 1969. 

A cooling trend will begin on Thursday as significant increase in
southwest-moving convection over the Sierra Madres causes a
moisture surge that moves up the Gulf of California into the 
lower deserts of both southeast California and Arizona. This
should end our excessive heat episode as temperatures generally
stay below 110 degrees in the Phoenix metro and 112 degrees over
southwest Arizona and southeast California. Additional cooling
will occur on Friday and Saturday to get temperatures closer to
normals in the lower to mid 100s as repeated Gulf surges maintain
moisture over the region as an inverted trough moves northwest
through the Gulf of California to trigger additional convection
over the Sierra Madres into Sonora and even southern Arizona. At
the same time, a weak shortwave disturbance from the west will
move into the Great Basin and Colorado River Valley for Thursday
evening into Friday. Although an outflow boundary and/or the 
westerly shortwave might be able to trigger a rogue convective 
cell late Thursday into Friday, midlevel stability aloft left 
behind by the ridge of high pressure should suppress thunderstorm
development and only allow for 10 PoPs or less with better 
chances closer to Tucson. 

Rain chances for Friday afternoon through Saturday evening also do
not look very promising as the inverted trough moving northwest
over the Gulf of California meet resistance from the subtropical
ridge as it gradually builds back in from the northwest. Although
we should see at least isolated showers and thunderstorms over the
higher terrain of eastern Arizona on Friday and Saturday,
northerly steering flow on the eastern edge of the subtropical
ridge will most likely keep outflows and storms away from the
lower deserts and the Phoenix metro. In addition, there is more
confidence among the model ensemble that a tropical cyclone that is
expected to develop in the East Pacific over the next 24-48 
hours will remain parallel to, but several hundred miles west of 
Baja California into the weekend. Thus, any chance we had of the
inverted trough associated with this system from impinging upon
this ridge appears to be pretty bleak. Although the operational 
GFS remains a notable outlier, less than 20 percent of the GEFS 
ensembles and even fewer ECM ensembles have any measurable 
rainfall for the Phoenix metro. Therefore, we have kept PoPs at or
below 10 percent for Friday and Saturday for most areas with 
10-20 PoPs over portions of Pinal and Gila Counties. These 
minimal rainfall chances should end by Sunday as the ridge 
reasserts itself over the region. 

Despite the presence of an elongated ridge over the Desert 
Southwest through the first half of next week, rich Monsoonal 
moisture will continue to remain positioned southward across 
northwest Mexico. Although the gradual rise in heights aloft will 
increase high temperatures back into the upper 100s and suppress
convection through subsidence associated with the ridge, this 
moisture should keep temperatures just below excessive heat 
thresholds. Models continue to indicate that a more significant 
moisture increase could occur during the second half of next week,
with some potential for relatively strong dynamics associated with
an inverted trough that splits off a relatively strong trough and
frontal system moving through the eastern two-thirds of the U.S.
on Wednesday into Thursday. Although confidence this far out is
still low, both the GEFS and ECM ensembles show better rain 
chances for the second half of next week that may try to help keep
us from having the driest monsoon season on record for Phoenix.


.AVIATION...Updated at 1740 UTC.

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

No significant aviation concerns expected through the TAF period
as VFR conditions and diurnal wind trends continue to persist. A
few gusts into the teens will be possible this afternoon before
tapering off this evening. Not expecting any convection or
outflows to impact the terminals this afternoon, but some 
guidance is now indicating an outflow reaching the terminals 
tomorrow afternoon. Will not mention in the TAFs at this time as 
confidence remains low.

Southeast California/Southwest Arizona including KIPL and KBLH:

Few aviation concerns are expected through the TAF period under
VFR conditions. Winds this morning will continue to remain light
and at times variable before increasing this afternoon. A few
gusts into the teens will be possible this afternoon before
tapering off this evening.

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday: 
Elevated moisture levels will allow for isolated thunderstorms
over primarily Pinal and Gila Counties with minimal chances for
most of Maricopa County including the Phoenix metro. This increase
in moisture will also allow temperatures to return closer to
seasonal normals Friday through the weekend. Thunderstorm chances
will decrease on Sunday through the first half of next week as
temperatures warm back up several degrees above normal as high
pressure rebuilds over the Southwest. Aside from values in the
teens on Friday over southeast California, minimum relative
humidity values will generally be in the 20 to 30 percent range
through Sunday before dropping back into the teens for most areas
on Monday and Tuesday. Winds will follow typical diurnal patterns
on Friday before westerlies generally prevail across the south
central Arizona zones for the weekend into early next week with 
afternoon and early evening gustiness each day. 



Record High Temperatures

Date        Phoenix         Yuma
----        -------         ---- 

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 until 8 PM MST this evening for 

CA...Excessive Heat Warning until 8 PM PDT this evening for 

     Excessive Heat Warning until 8 PM PDT this evening for CAZ560.



FIRE WEATHER...Hopper/Rogers