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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 181147 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
445 AM MST Sun Aug 18 2019

.UPDATE...Updated aviation discussion.


Temperatures will remain several degrees above normal over the
next several days as moisture and thunderstorm activity will be 
very limited through the middle of next week. Daily 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 for
the second half of this week, with the best chances on Friday. 


Aside from a few high clouds, most of the region is experiencing
clear skies and drier conditions as dew points have generally
fallen into the 40s except near Yuma where they are in the 50s.
The combination of clear skies and relatively dry air is allowing
for efficient radiational cooling as temperatures fall into the 
80s and even a few upper 70s. Westerly flow aloft and eventually
at the surface will continue to reinforce our dry conditions as
the subtropical ridge currently centered over south Texas rebuilds
and shifts westward towards Big Bend. Temperatures this afternoon
should be very similar to yesterday as H500 heights will remain in
the 589-590 dm range with H850 temperatures remaining around 28-29
deg C. Hi-res models depict convection developing over northern 
Sonora this afternoon along a tight moisture gradient where PWAT 
values rapidly increase from below 1 inch over southern Arizona to
near 2 inches just a couple hundred miles to the south. A few of
these storms may slip into Pima, Santa Cruz, and Cochise Counties
late this afternoon, but that is the closest convection will get
to us today as our PWAT values generally stay well below 1 inch. 

Temperatures will begin to increase tomorrow, but we still expect
to remain just below excessive heat thresholds at or below 110
degrees as the height increases aloft will not quite translate to
warmer boundary layer temperatures. However, the mid-to-upper- 
level anticyclone will broaden and become more zonally-oriented 
with a center near the Four Corners for Tuesday into Wednesday as
H500 heights increase to around 594 dm under mostly clear skies. 
Forecast H850 temperatures still exceed 31 deg C on Tuesday and 32
deg C on Wednesday, about a degree warmer than last week's 
excessive heat episode. Thus, we still expect another excessive 
heat episode with daily records likely being set on Wednesday and 
possibly on Tuesday, so we have maintained the Excessive Heat 
Warning for Tuesday and Wednesday with no changes with this 
forecast package. There is still a chance that this warning will 
need to be extended into Thursday for some portions of southeast 
California, particularly northeastern Riverside County and the 
Imperial Valley. However, confidence is not quite high enough yet 
to extend this warning as a Gulf surge expected overnight 
Wednesday into Thursday will cause temperatures to decrease near 
or just below excessive heat thresholds. 

The aforementioned moisture surge should end our prolonged break 
in monsoonal activity with repeated bouts of excessive heat. A 
fast-moving easterly tropical wave currently moving into the
Caribbean Sea should cross the Yucatan Peninsula on Monday night
into Tuesday and progress into central America and Mexico by 
Wednesday. This will cause a significant amount of convection on 
both sides of the Gulf of California over the Sierra Madre and 
Baja California that should trigger a Gulf surge Wednesday into 
Thursday. Simultaneously, a front moving into the Central Plains 
along with 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, but thunderstorms will be limited 
to eastern and southeast Arizona including Gila and possibly Pinal
County for Wednesday and Thursday as midlevel stability left 
behind by the ridge aloft will still be in place. Thunderstorm 
chances for the lower deserts including Phoenix will be better on 
Friday as PWAT values increase into the 1.1-1.5 inch range with 
northeast steering flow on the eastern side of the ridge bringing 
outflow boundaries off the higher terrain into the lower deserts.
If Friday is fairly active, Saturday should be a less active day 
with cooler than normal temperatures. Otherwise, the setup for 
Saturday would be similar to Friday with thunderstorms propagating
off the higher terrain, albeit with weaker shear. 

Much of what happens beyond Saturday will depend on whether or 
not a tropical cyclone develops late next week once the 
aforementioned easterly wave moves into the East Pacific. If it
does develop, there is significant uncertainty on where it will
ultimately track even though both the operational ECWMF and GFS 
track it northwestward parallel to Baja California. If this 
occurs, significant southerly moisture surges at the surface and 
moisture aloft at the fringes of the tropical system would result
in very high PWAT values and a prolonged wet period next week for
the lower deserts of southeast California and possibly Arizona. If
not, low grade monsoonal activity primarily confined to higher
terrain locations would be most likely. 


.AVIATION...Updated at 1145 UTC.

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

Although generally dry west flow aloft will persist across the 
central deserts today into Monday, there will be periods of mainly 
high clouds today with SCT-BKN decks mostly above 20k feet. Skies 
become generally clear overnight tonight. Winds will continue to 
exhibit typical diurnal tendencies with speeds mostly below 12kt. 
There may be a few gusts into the teens during afternoon and early 
evening hours but nothing of real significance. Overall no aviation 
concerns for at least the next 24 hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry west flow aloft to keep skies generally clear next 24 hours. 
There will be some high clouds today but skies become clear 
overnight. Winds to favor the south to southwest at KBLH next 24 
hours with speeds mostly 15kt or lower. Maybe a few gusts to near 
20kt during the afternoon. Winds to favor the southeast today at 
KIPL, and then turn more to the southwest during the evening hours, 
with speeds generally less than 12kt. Overall no aviation concerns 
for at least the next 24 hours. 

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday: 
Excessive heat is expected on Tuesday and Wednesday as high
temperatures across the lower deserts will increase into the 112
to 116 degree range with isolated spots exceeding 116 degrees in
southeast California and southwest Arizona. Moisture will return
by Thursday to begin a gradual cooling trend and bring back a
slight chance of thunderstorms into the forecast for the higher
terrain of Gila County and perhaps the lower deserts including the
Phoenix metro on 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 gustiness.



Record High Temperatures

Date        Phoenix         Yuma
----        -------         ----
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...Hopper/Hernandez