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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 260003
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
503 PM MST Sun Aug 25 2019

.UPDATE...Updated aviation discussion.


&&

.SYNOPSIS...
A warming trend can be expected during the first half of the
workweek. This will lead to high temperatures several degrees
above normal on Tuesday and Wednesday. Portions of south-central
Arizona are expected to reach excessive heat thresholds. Humidity
will trend down as well...most noticeably on Tuesday. Humidities
trend up Wednesday and especially Thursday with an increase in
storm chances...mainly for the higher terrain of south-central
Arizona. Temperatures trend down on Thursday as well. Humidities 
and storm chances fluctuate over the weekend with the best chances
for storms on Sunday. 

&&

.DISCUSSION...
Monday through Wednesday...
Lower troposphere remains quite humid but satellite imagery for
early this afternoon shows very little activity thanks to having too
much CIN. There may still be something that PoPs over Joshua Tree
N.P. and maybe eastern Gila County. There will still be fairly
humid conditions tomorrow as another seep of Gulf of California
moisture comes in tonight. But, high pressure (currently centered
near San Francisco) continues to expand southeastward (increasing
CIN). By Tuesday, the high becomes centered east-southeast of 
Arizona. In the process, there will be a warming and drying trend.
In fact, temps over portions of south-central AZ reach high Heat 
Risk thresholds and thus an Excessive Heat Watch is in effect. The
Watch goes through Wednesday as the high center shifts westward a
bit. Models dip temperatures slightly on Wednesday (with an 
increase in humidity) but not enough to eliminate the high Heat 
Risk. PoPs go up over our easternmost areas due to the moisture
increase. 

Thursday through Sunday...
There is good model agreement that temperatures trend down for
Thursday. This appears to be due to a more significant influx of 
moisture that comes in Wednesday night and Thursday morning 
(mainly near and east of the Lower Colorado River Valley) in the
wake of an inverted trough passing to the south. By Thursday 
afternoon, the 500mb high becomes centered near San Diego leading 
to a northerly component to the steering flow. While this can be 
favorable for getting storms to advect off the Rim and Yavapai 
County, there will be a trade off with drier air coming in.
Temperatures are anticipated to nudge upward again on Friday with
additional drying (though the southeast quarter of AZ looks to 
maintain some decent moisture). Anticipate our western areas will
approach excessive heat thresholds. Over the weekend, the high
center tracks northeastward to near the AZ/UT border. This leads
to more of a favorable steering flow pattern an in turn better
moisture availability (possibly aided by another inverted trough)
with Sunday having higher PoPs than Saturday. Temps nudge back
down as well.  

&&

.AVIATION...Updated at 0002 UTC. 

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Westerly surface winds are likely to prevail through most of the
period, especially at KPHX and KDVT, with only some variability in
outlying terminal sites Monday morning. An hour or so of gusty
conditions around midnight (gusts to 20 kts) cannot be ruled out,
but is less likely than previous nights and has been left out of 
the TAF. VFR conditions should persist through Monday with 
increasing SCT- BKN cirrus. 

Southeast California/Southwest Arizona including KIPL and KBLH:
Wind shifts and occasional gustiness will be the main forecast
challenge through Monday afternoon. Some form of a southerly wind
component should prevail, though large amounts of variability are
possible - particularly at KIPL. Confidence in timing any wind
direction shifts and potential gusts is only low to moderate. SCT-
BKN cirrus should persist through much of the forecast period. 

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday: 
Humidities trend up on Wednesday and Thursday. Storm chances
increase as well - for south-central Arizona. Humidities and storm
chances fluctuate at the end of the week and over the weekend with
the best chances for storms on Sunday. Minimum humidities are
expected to be lowest on Friday but remain above 10%. Similarly,
overnight recovery is anticipated to be lowest on Friday night but
remain at least fair. Apart from thunderstorms, no strong wind
events are anticipated but breezy to locally windy conditions from
a Gulf surge couldn't be ruled out.

&&

.SPOTTER INFORMATION STATEMENT... 
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from Tuesday morning through Wednesday 
     evening for AZZ534-537-538-540>544-546-548>551-553>556-558-
     560-562.

CA...None.
&&

$$

DISCUSSION...AJ
AVIATION...Benedict/MO 
FIRE WEATHER...AJ