Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 191756

Area Forecast Discussion
National Weather Service Phoenix AZ
1056 AM MST Mon Aug 19 2019

.UPDATE...Updated aviation discussion.


Temperatures will remain several degrees above normal as moisture
and thunderstorm activity will be very limited through Wednesday.
Another Excessive Heat episode with daily record high temperatures
is expected on Tuesday and Wednesday across southern Arizona and
southeast California. Moisture will return by Thursday to begin a
gradual cooling trend and isolated thunderstorms back into the
forecast for south central Arizona for the second half of this
week, with the best chances on Friday.


Mostly clear skies and dry conditions with dew points in the 30s
and 40s have once again allowed for efficient radiational cooling
as temperatures have cooled into the upper 70s to mid 80s. Unlike
yesterday when a thick middle and high cloud layer helped keep
temperatures down in the mid 100s, clear skies under an expanding
mid-to-upper level ridge of high pressure centered over west
Texas will allow temperatures to warm up into the upper 100s and
lower 110s today across the lower deserts. High temperatures will
approach excessive heat thresholds for many locations, but should
generally remain below them as the increased heights aloft will
not quite translate to very much boundary layer warming as H850
temperatures only increase into the 28-29 deg C range. Convection
is not expected anywhere in Arizona today except possibly in far
southeast Cochise County, but even there subsidence and limited
moisture will most likely prevent any convection from occurring.

Temperatures will increase above excessive heat thresholds for
most lower desert locations tomorrow and Wednesday as the mid-
to-upper-level anticyclone broadens and become more zonally-
oriented. Forecast H500 heights will increase to around 594 dm on
Tuesday before dropping a bit on Wednesday as the high pressure
center moves west, whereas forecast H850 temperatures will
increase to 30-32 deg C on Tuesday and 31-33 deg C on Wednesday,
about a degree warmer than last week`s excessive heat episode.
Therefore, our Excessive Heat Warning across most of the lower
deserts of Arizona and southeast California continues, with daily
records likely being set on Wednesday and possibly on Tuesday. A
Gulf moisture surge associated with an increase in convection on
both sides of the Gulf of California on Wednesday should occur
over the lower deserts Wednesday night into Thursday to bring an
end to this excessive heat episode for most locations. Although
there is a chance that this warning will need to be extended into
Thursday for some portions of southeast California, higher
confidence in the moisture surge across the lower deserts should
only allow for isolated locations to exceed excessive heat
thresholds, with temperatures on Thursday being similar to today.

An easterly wave currently located east of the Yucatan Peninsula
that will trigger the aforementioned convection on Wednesday will
move northwest on Thursday and Friday to help cause a northward
expansion of strong convective complexes over the Sierra Madre.
In addition, nearly all models show the development of a tropical
cyclone just off the coast of Mexico by the middle of the week
that should track northwest about 100-200 miles west of Cabo San
Lucas by Friday. This should help advect significant upper level
moisture into southern Arizona and southeast California for Friday
and Saturday with regular Gulf surges maintaining dewpoints in
the mid 50s to mid 60s. At the very least, decreased heights aloft
and increasing cloud cover will allow temperatures to decrease
back to near normal levels in the lower to mid 100s over the lower
deserts with isolated to scattered thunderstorms each afternoon
and early evening over the higher terrain of Gila County.

Unfortunately, models are slightly less bullish on convection for
the lower deserts for the second half of this week than they had
been before. The biggest culprit appears to be the lack of a well
defined high pressure center which was previously forecast to move
west or northwest into the Great Basin, but is now ill-defined.
This prevents favorable north-northeast steering flow from
developing on Thursday or Friday and prevents wave energy and mid-
to-upper level moisture associated with a frontal system moving
into the Central Plains to move into the Desert Southwest to help
trigger higher terrain thunderstorms. Although significant
outflows from the north and/or east do not appear to occur on
Thursday or Friday, scattered to widespread convection should
develop over southern Arizona that may push outflows and
additional moisture into the south central Arizona, so we are
still maintaining 10-20 POPs on Friday. We cannot rule out
isolated convection on Thursday either, but midlevel stability
will most likely suppress convection. The best chance for north-
northeast steering flow now appears to occur on Saturday as the
ridge builds in from the northwest, but we may be on the subsident
side of the inverted trough associated with the tropical cyclone
tracking northwest parallel, but just offshore Baja California.
Thus, we have 10 POPs for Saturday before keeping PoPs limited to
the higher terrain of Gila County for Sunday as the ridge moves
over northern Arizona. Although we should have a break in
convection for early next week, the monsoonal moisture will stick
around to help set up a potentially busier convective period for
the second half of next week with deeper moisture that the broader
model ensemble continues to latch onto.


.AVIATION...Updated at 1755 UTC.

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

A benign weather pattern will continue over the next 24 hours
with light winds less than 8 kts and diurnal tendencies, but with
some variability through early afternoon and again late this
evening. VFR conditions should persist with mostly clear skies,
aside from occasional clouds based around 14 kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry southwest flow aloft will keep skies generally clear next 24
hours. Winds aob 10 kts are expected for the most part, favoring
the south to southwest at KBLH and southeast for much of the day
at KIPL then turning towards the southwest after about 02z this
evening. Some variability is possible Tuesday morning. No
aviation concerns for at least the next 24 hours.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday:
Excessive heat is expected on Wednesday as temperatures across the
lower deserts 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 isolated thunderstorms into
the forecast for the higher terrain of Gila County and perhaps the
lower deserts including the Phoenix metro whose best chances will
be on Friday. Thunderstorm chances will begin to decrease on
Sunday, but elevated moisture levels should keep temperatures
within a few degrees of normal. Minimum relative humidity values
will be below 10 percent on Wednesday before an upward trend
commences on Thursday through the weekend to keep most 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 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
CLIMATE...Rogers/Kuhlman/MO is the U.S. government's official web portal to all federal, state and local government web resources and services.