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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 170542
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1040 PM MST Fri Aug 16 2019

.UPDATE...
Updated Aviation

&&

.SYNOPSIS...
Temperatures will cool ever so slightly over the weekend, however 
remaining several degrees above normal. Moisture will remain very 
limited and little, if any storm activity should be expected until 
the middle of next week. Another excessive heat episode looks 
likely early next week as temperatures trend upward beginning Monday 
with record high temperatures possible by during the middle of the 
week. 

&&

.DISCUSSION...
Quite a few interesting and subtle processes going on this evening 
across the region. First, 00Z KPSR and KTWC sounding data sampled an 
environment of deep westerly flow and boundary layer moisture 
profiles fairly hostile towards sustained deep convection. Despite 
only 7-8 g/kg mixing ratios in the sfc-H7 layer, outflow boundaries 
from terrain driven convection were marginally sufficient to spur 
additional storms into lower elevations. In fact, a southerly 
outflow boundary interacted with South Mountain to pop a storm over 
Tempe earlier this evening when investigation of the corresponding 
sounding would never suggest storm initiation. It appears within the 
westerly flow, a weak shortwave was aiding in large scale ascent 
through much of south-central Arizona this afternoon. 

In addition, objective analysis indicated a narrow finger of 10 g/kg 
moisture protruding into south-central Arizona while outflows 
initiated robust theta-e advection along the steep moisture gradient 
aligned adjacent to the Phoenix metro. Given the poor sounding 
profiles, loss of daytime heating, and lack of additional outflows, 
any remaining meager instability should focus storms towards SE 
Arizona. Meanwhile, the flow pattern and mesoscale features through 
the region were also promoting quite a shallow Gulf surge through SW 
AZ/SE CA. While this will likely result in little more than a humid, 
uncomfortable night, its always quite interesting seeing KNYL 
dewpoints jump from 40F to 67F in one hour. Made notable adjustments 
to dewpoints and sky cover based on observational trends while also 
expiring the current excessive heat warning. 

&&

.PREVIOUS DISCUSSION...
/410 PM MST Fri Aug 16 2019/
With an upper trough passing across the Intermountain West, heights 
will be suppressed ever so slightly this weekend and bring slightly 
cooler temperatures, even with a dry air mass remaining in place. 
Thus, high temperatures are expected to generally remain below 110 
degrees and barely provide a brief respite from the excessive heat. 
Unfortunately, mid-level heights will re-build Sunday and Monday, 
first across the southern High Plains, and then expanding 
northwestward early next week. The mid-level anticyclone is expected 
to become positioned near the Four Corners on Wednesday, and over 
Arizona on Thursday with 500-mb heights potentially approaching 597 
dm. With the center of the anticyclone essentially directly overhead 
central Arizona during the middle of the week, another excessive 
heat episode looks very likely. Current forecast high temperatures 
will exceed 110 degrees across most lower desert areas Tuesday and 
Wednesday. Record high temperatures are expected to be challenged 
during the middle of next week. Thus, another Excessive Heat Warning 
has been issued on Tuesday for southwest and south-central Arizona 
and on Wednesday for the majority of the forecast area. 

With the general westward shift in the mid-level anticyclone, 
eventually Monsoonal moisture will be drawn back into Arizona, 
closer to seasonal normals by Thursday and Friday. This will bring a 
return to slight chance of thunderstorms across the higher terrain 
of south-central Arizona, and potentially the lower deserts by 
Friday. There is some hint that a glancing influence of a shortwave 
trough embedded in westerlies could enhance thunderstorm chances on 
Friday, but given it's still 7 days out uncertainty is very high. 
However, at a minimum it looks like there's still at least some hope 
for a return to a more active regime for thunderstorm activity 
across south-central Arizona before August concludes. 

&&

.AVIATION...Updated at 0540 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: 
Very little aviation concern through Saturday night under mostly 
clear skies. Winds have finally returned to a SW direction after 
early evening outflows, and a switch to easterly should occur 
somewhat later than usual towards sunrise Saturday. Similarly, 
confidence is moderate that the switch back to a westerly component 
will occur by late Saturday morning with the possibility of some 
occasional gusts late in the afternoon. 

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation concerns through Saturday night under clear skies. 
A few evening wind gusts are still possible, but overall trends 
support shifts in wind direction and speeds very similar to the past 
several days with more variability during the morning hours and 
prevailing southerly type winds in the afternoon. 

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday: 
After somewhat cooler temperatures Sunday, a warming trend will
take place through the middle of next week. High temperatures will
climb well above normal by next Tuesday and Wednesday with many
desert temperatures rising in excess of 110 degrees. Moisture
starts to move into the region during the latter half of next week
and cools temperatures slightly. Conditions will remain relatively
dry through Tuesday, with rain chances relegated to far eastern
Gila County. Precipitation chances begin an upward trend Wednesday
through Friday, with storms favoring the high terrain in central
Arizona. Minimum RH values through Wednesday will generally range
from 8 to 12 percent before an upward trend commences Thursday 
and Friday. Winds will follow slope/valley patterns and favor 
southwest/west directions in the afternoons and evenings. 

&&

.CLIMATE...

Record High Temperatures

Date        Phoenix         Yuma
----        -------         ----
Aug 17    114 in 2013    114 in 1992
Aug 18    112 in 2011    116 in 1960
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

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 11 AM Tuesday to 8 PM MST Wednesday 
     for AZZ531>546-548>551-553>556-559-560.

     Excessive Heat Warning from 11 AM to 8 PM MST Wednesday for 
     AZZ530.

CA...Excessive Heat Warning from 11 AM to 8 PM PDT Wednesday for 
     CAZ561>570.

&&

$$

DISCUSSION...MO/Hernandez/Rogers
AVIATION...MO
FIRE WEATHER...Hernandez 
CLIMATE...Rogers/Kuhlman/MO