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fxus65 kpsr 210510 
afdpsr

Area forecast discussion
National Weather Service Phoenix Arizona
1010 PM MST Tue Aug 20 2019

Update...updated aviation discussion.

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Synopsis...
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.

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Discussion...
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 Storm Prediction Center
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.

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Aviation...updated at 0510 UTC.

South-central Arizona including kphx, kiwa, ksdl, and kdvt:
benign weather conditions will persist through Wednesday as an
area of high pressure in the eastern Pacific predominates. This
will result in light, diurnal winds and mostly clear skies.
Isolated convection is expected Wednesday, but will remain
confined to southeastern Arizona, with no impacts in the Phoenix
area.

Southeast California/southwest Arizona including kipl and kblh:
no aviation weather concerns expected. Clear skies and diurnal
winds will prevail at both kipl and kblh.

Aviation discussion not updated for amended tafs.

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Fire weather...
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.

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Climate...

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

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Spotter information statement...
spotters should follow Standard reporting procedures.

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Psr watches/warnings/advisories...
Arizona...excessive heat warning until 8 PM MST Wednesday for azz530>546-
548>556-559>562.

California...excessive heat warning until 8 PM PDT Wednesday for caz561>570.

Excessive heat warning from 11 am to 8 PM PDT Wednesday for
caz560.

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