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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 201958

Area Forecast Discussion
National Weather Service Phoenix AZ
1258 PM MST Tue Aug 20 2019

Another excessive heat episode with daily record 
high temperatures is expected on Wednesday across southwestern and
south-central Arizona and southeast California. Thunderstorm 
activity will be very limited on Wednesday due to the persistence 
of a dry air mass. Moisture will return by Thursday beginning a 
gradual cooling trend and bringing isolated thunderstorms back 
into the forecast for south central Arizona during the second half
of this week, with the best chances on Friday and Saturday. 


A very warm and dry air mass is in place across the state, as
evidenced by the limited cumulus development across Arizona this
afternoon on visible satellite imagery. The observed 850-mb
temperature at Phoenix was close to record values per the SPC
Sounding Climatology, at 27.8 C. Afternoon surface observations
reflect the very warm air mass, as temperatures are already
approaching 110 degrees over the desert areas as of 1 PM PDT.
Still expecting a high temperature of 113 degrees at Phoenix and 
114 degrees at Yuma today. Another day of excessive heat is likely
on Wednesday across the region, with high temperatures very
similar to today. Officially, the high temperature is forecast to
reach 114 degrees at Phoenix, which is likely to shatter the
record of 110 degrees set in 2007. 

A cooling trend will commence on Thursday into the weekend, as
high temperatures return closer to normal. Overnight low
temperatures will also be fairly pleasant for late August, falling
into the low 80s in Phoenix and 70s across the remainder of the
desert locations. This will occur coincident with a gradual
increase in low-level moisture, as a complicated upper-level
pattern takes hold. Strong westerlies will be present across the
Pacific Northwest into the Inter-Mountain West, while a series of
weak upper-disturbances move across northwest Mexico. With an
elongated ridge axis in place across central Arizona into central
New Mexico, this should keep the influence of upper-level
disturbances primarily confined to southern Arizona. However, a
slight chance of showers and thunderstorms may still exist across
south-central Arizona on late Friday into Saturday given the
elevated moisture values.

The persistence of strong westerlies aloft will keep the
southwestern U.S. mid-upper level anticyclone relatively
suppressed and elongated, meaning rich Monsoonal moisture will
continue to remain positioned southward across northwest Mexico.
A gradual rise in heights will also bring high temperatures back
to a few degrees above normal by early next week with minimal
precipitation chances. All in all, expect very quiet weather most
of next week for late August. There are some indications a more
significant moisture increase could occur later next week, but
this far out confidence remains quite low. 


.AVIATION...Updated at 1740 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: 
There are no aviation weather concerns through the rest of the TAF
period. The winds are slowly becoming westerly and should complete
the switch within the next two hours across all TAF sites. Wind
speeds will remain mostly at or below 10 kts, except for a few 
afternoon breezes. The winds are expected to become easterly 
tonight but will be very light and variable at times.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no major aviation weather concerns through the TAF
period. The winds will favor a more westerly direction across the
region through the afternoon and evening with some stronger gusts
possible west of the Colorado River, including KIPL. The winds 
will become light and variable again tonight. 

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday: 
A gradual increase in moisture will occur into Friday, with
isolated thunderstorms returning into the forecast for the higher
terrain of Gila County and possibly the lower deserts including
the Phoenix metro on Friday and Saturday. Temperatures will also
return closer to seasonal normals through the weekend.
Thunderstorm chances will begin to decrease on Sunday and Monday.
Minimum relative humidity values will increase from the 10 to 20
percent range on Thursday to the 20 to 30 percent range by the
weekend. Winds will follow typical diurnal patterns through Friday
before westerlies generally prevail across the south central
Arizona zones for the weekend with afternoon and early evening
gustiness each day. Another gradual warming and drying trend will
occur into Tuesday as high pressure rebuilds over the Southwest. 



Record High Temperatures

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

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 until 8 PM MST Wednesday for AZZ530>546-

CA...Excessive Heat Warning until 8 PM PDT Wednesday for CAZ561>570.

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



FIRE WEATHER...Rogers/Hopper