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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 241005
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
305 AM MST Sat Aug 24 2019

.SYNOPSIS...
Although temperatures will be cooler the next couple days, 
humidity levels will be significantly elevated. After the slight
possibility of an early morning shower or even a thunderstorm 
near the Phoenix metro, chances will be relegated to areas south 
of Phoenix this afternoon and evening. Drier air will move into 
the region through the first half of next week, basically ending 
storm chances. Temperatures will start climbing again early next 
week with Tuesday and Wednesday possibly topping 110 degrees 
across the lower deserts.

&&

.DISCUSSION...
Boundary layer moisture continues to be quite high across 
southern Arizona due to multiple instances of storm outflows and 
gulf moisture surges over the past couple days, but storm activity
on Friday was overall minimal. A northerly dry flow aloft set up 
yesterday as the high pressure ridge strengthened to our northwest
leading to warming aloft and increasing subsidence. 00Z soundings
for Phoenix and Tucson showed modest elevated CAPE, but a couple 
different warm layers aloft mostly suppressed storm potential. 
Some hi-res CAMs have been showing the potential for some isolated
to scattered elevated showers and storms early this morning 
across Pinal and Maricopa Counties, but the latest runs have for 
the most part backed off on this potential. This is likely due to 
the general lack of convection over northern Mexico and the 
absence of any significant gravity waves that could have acted as 
a trigger.

The rest of the today will still pose a small threat for isolated
afternoon showers and thunderstorms south of Phoenix, but the 
drying aloft will also be mixing out some of the lower level
moisture, gradually decreasing instability by this evening. Sunday
looks even more quiet with virtually no chance of storms other
than south of Tucson. Temperatures this weekend will be somewhat
more comfortable and right around seasonal normals, but increased
surface dew points will make higher humidity levels more
noticeable. 

The high pressure ridge centered to our northwest on Sunday will
gradually shift southeastward over our region early this week 
with heights staying near steady-state, but low level 
conditions will be warming as the atmosphere dries out more each 
day. The warm air aloft and fairly dry conditions will keep storm 
chances near zero on Monday and Tuesday. Models continue to show 
high temperatures quickly trending upward early next week with 
highs 3-5 degrees above normal on Monday and 6-8 degrees above 
normal by Tuesday. The possibility for a marginal excessive heat 
episode is becoming a bit more likely for Tuesday and Wednesday 
for the warmest deserts with median guidance showing highs topping
out 109-112 on Tuesday, but will hold off on any headlines for 
now.

Eventually, models mostly show the high center shifting just to 
our northeast, likely setting up a more favorable southeasterly 
moist flow. This should bring back storm chances as early as 
Wednesday across the eastern Arizona high terrain and possibly 
over the Arizona lower deserts at some point late next week. 
Heights aloft later next week still look to be toward the upper 
end of climo normals, but the likely increase in low level 
moisture should lower temperatures to just a bit above normal.

&&

.AVIATION...Updated at 0500 UTC. 

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

Westerly winds will mostly prevail overnight with some gusts. The
winds at KIWA are a little more suspect and may be variable at 
times. 

Late tonight toward sunrise, there are indications that a batch 
of more modest intensity showers will develop over Pinal County 
and into southern/eastern portions of metro Phoenix. If that were 
to happen, most likely timing would be between 11Z-16Z. Confidence
remains too low at this point to insert -SHRA into the TAFs. 

Southeast California/Southwest Arizona including KIPL and KBLH:
South and southeasterly winds will prevail today and tonight as
one Gulf surge damps out and possibly another comes in late 
tonight. Not enough instability for storms though. In fact, 
anticipate clear skies except perhaps for some hazy conditions in 
the morning over the Imperial Valley - depending upon development 
of another surge. 

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday: 
Unusually dry conditions for late August will prevail through the 
first half of next week with only slight chances of high terrain 
thunderstorms later in the week. With this dry airmass, temperatures 
will once again warm near excessive levels as afternoon highs peak 
some 5F-10F above average. Afternoon humidity levels will fall into 
the teens during the early part of the week, but increase closer to 
a 20-30% range late in the week. Occasionally gusty upslope terrain 
winds are likely through the week, though nothing too unusual for 
the season. 

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Kuhlman
AVIATION...Deems/Rogers
FIRE WEATHER...MO