Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 281130

Area Forecast Discussion
National Weather Service Phoenix AZ
430 AM MST Wed Jul 28 2021

.UPDATE...Updated aviation discussion.


One more day of warmer and minimal storm chances today before rain
chances increase late in the week. Another weather disturbance
expected to affect the region bringing a return to more widespread
shower and thunderstorm activity starting Thursday afternoon,
likely persisting into the weekend. Drier conditions should then
prevail by early next week.


Monsoon activity across the region has returned to a more typical
pattern with storms mainly developing over the high terrain during
the afternoon before sending outflows into the lower deserts.
An overall drier atmosphere across much of southern Arizona
yesterday limited convection to a degree, but an organized
outflow with gusty winds and dust still managed to work its way
through much of the south-central Arizona lower deserts.

Arizona and southern California remain under weak south to
southeasterly flow with the subtropical high now positioned across
the Central Plains. Despite yesterday`s outflow moistening up the
boundary layer further, additional drying aloft today and daytime
mixing is expected to bring less favorable conditions for storms
this afternoon and evening. The 00Z HREF agrees with the lower
potential compared to yesterday, showing isolated to scattered
storms this afternoon along the Mogollon Rim, over extreme
southeast Arizona and maybe a few showers and weak storms over La
Paz and eastern Riverside Counties. The potential for organized
storm outflows into the lower deserts is considerably lower today
than yesterday.

The semi quiet monsoon period will quickly come to an end on
Thursday as a well defined, but weakening upper level trough
currently moving westward through southern Texas moves into the
region. During the day Thursday, broad upper level ascent will
spread westward across much of Arizona as the trough enters the
region. Moisture levels and instability will be sufficient for
scattered to possibly widespread storms developing across the
eastern Arizona high terrain Thursday afternoon with strong winds,
heavy rainfall, and localized flooding all possible. There will
also likely be some isolated storms developing as far west as the
Lower CO River Valley Thursday afternoon, but instability will be
much more limited farther west. As the inverted trough progresses
across Sonora Mexico and southern Arizona Thursday evening into
the early overnight hours, additional storm development will be
possible into the lower deserts. A lower end scenario which seems
to be the current preferred solution (supported by the latest 06Z
HRRR) would mean isolated storms across the lower deserts along a
westward advancing outflow. A higher end, but less likely
scenario would result in a larger cluster of storms and associated
cold pool advancing off the mountains into the lower deserts,
potentially including the Phoenix area. Strong gusty winds and
blowing dust will be the most likely threats over the lower
deserts Thursday evening with locally heavy rainfall also a

Isolated to scattered showers and storms will remain possible
Thursday night, reaching as far west as southeast California. The
inverted trough is forecast to exit the region to the northwest
Friday morning, but not before spreading modest moisture
throughout our area. Storm chances for Friday now look to be a bit
more uncertain with the quicker progression of the inverted
trough. Sufficient moisture will still be in place in most areas
Friday, but some drying is now forecast across eastern Arizona.
Shower and storm chances for Friday afternoon and evening are now
more favored from Phoenix westward with the help of broad ascent
associated with a south to north oriented upper level jet over
California. Should also have some isolated to scattered storms
across the eastern Arizona high terrain, but the drying coming out
of New Mexico should limit coverage. Storm chances Saturday look
to shift back to the east with another jet streak coming up from
the south providing ascent more focused across central and eastern
Arizona while better moisture is again seen across eastern
Arizona. Near to slightly above normal monsoon moisture levels and
forecast soundings for Friday and Saturday point toward a mix of
storm threats from strong winds to locally heavy rainfall and
possible flooding.

Drier air will begin working in from the southwest starting
Sunday, likely ending storm chances across most of the western
deserts and considerably lowering chances elsewhere. The
southwesterly drying flow will result from the subtropical high
weakening while shifting south and an upper level trough becoming
the dominant feature along the West Coast. This unfavorable
monsoon flow pattern should persist through early next week with
another high center forming to our southwest on Monday and even
drier northwesterly flow spreading through the region. Minimal
storm chances over the eastern Arizona high terrain are expected
for Monday and Tuesday, but for now it seems the lower deserts
will be dry for a few days. Eventually, model ensembles show the
new high center shifting northeastward into the Four Corners area
later next week which will likely bring a return to more favorable
monsoon moisture flow out of the southeast and increasing storm

Temperatures today and Thursday should be the warmest of the week
with highs across the western deserts between 109-112 degrees to
102-105 degrees in the Phoenix area. Cooling will occur by Friday
and last through the weekend with forecast highs from the upper
90s in the Phoenix area to 100-105 degrees over the western
deserts. Temperatures should return to near normal readings for
the western deserts during the first half of next week, while the
south-central Arizona deserts will likely fall just short of
normal readings.


.AVIATION...Updated at 1130Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Typical diurnal wind patterns expected throughout the majority of
the TAF period with speeds generally below 10 kt. Isolated
thunderstorms will develop north and east of Phoenix and may send
an outflow boundary towards the Phoenix area during the early
evening. Confidence is low in timing and direction of any
potential wind shift. Otherwise, clouds will mostly remain SCT-
BKN with decks mostly staying aoa 15 kft.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL will favor an east/southeast component through most
of the TAF period, while winds at KBLH favor the south. Expect
extended periods of light and variable winds at the terminals
through the next 24 hours. Isolated showers and thunderstorms may
develop near the lower Colorado River Valley and approach KBLH
during late afternoon and evening. Added VCSH in the 22-01Z period
to account for this possibility. A sudden wind shift accompanying
an outflow boundary is also a possibility during this time. Clouds
will mostly remain FEW-SCT with decks mostly aoa 25 kft.


Friday through Tuesday:
Unsettled conditions are expected late week and likely through the
weekend with a couple of weather disturbances moving through the
region. This should bring daily shower and storm chances across
the entire area Friday and Saturday before decreasing chances from
west to east Sunday into Monday. Main threats will be strong
thunderstorm winds, heavy rainfall and localized flooding.
Temperatures will range from near to slightly below normals the
bulk of the period. Minimum afternoon humidity values will range
from 25-45% through the weekend before falling back to 15-30%
early next week. Overnight recovery will be mostly good to
excellent in a 40-80% range. Apart from thunderstorms, winds will
favor typical daily upslope/drainage patterns.


Spotters should follow standard reporting procedures.




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