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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 192123

Area Forecast Discussion
National Weather Service Phoenix AZ
223 PM MST Mon Aug 19 2019

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 tomorrow 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.


Our region still lies between weak troughing to our west and
strong ridging to our east, resulting in another day of dry
southwesterly flow a loft. The aforementioned ridge has shifted
towards western Texas today and has caused our 500 mb heights to
increase a bit too (591 dm across our far western forecast area to
594 dm across the eastern forecast area). This increase in heights
combined with dry flow aloft will bring the forecast area mostly
clear skies today with temperatures a few degrees warmer than 
what was observed yesterday. Lack of substantial moisture is 
evident in the current SPC mesoscale analysis which only depicts 
0.5-0.8 inches of PWAT over the region. Across the state, the 
greatest moisture exists across far southern Arizona, close to 
the border. With a stable airmass aloft and limited moisture, 
storm chances will remain pretty much non-existent today, even 
across southeast Arizona. The HREF depicts that the best chances are
around and south of the AZ/Mexico border. Thus, another quiet day
is in store for us with high temperatures approaching 110 degrees
this afternoon.

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.


Thursday through Monday: 
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 and Monday,
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 Monday 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...Hernandez/Hopper