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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 182313

Area Forecast Discussion
National Weather Service Phoenix AZ
410 PM MST Sat Sep 18 2021

Updated Aviation


A weak weather system moving through the area will bring another
round of isolated to scattered shower and storms today with
activity focused mainly east of the Colorado River. Most locations
will likely not see measurable rain, but there is still the
potential for localized flash flooding. A few strong storms capable
of producing strong winds, blowing dust, and frequent lightning are
possible. Conditions will rapidly dry out through Sunday and high
temperatures will warm to slightly above normal as high pressure
builds in.


Today`s convective forecast has a high ceiling and low floor. As
for the ceiling, a slow moving upper level low, now approaching
southern Nevada, continues to make the atmosphere more favorable for
showers and thunderstorms east of the Colorado River. Prolonged deep
southerly flow continues to provide moisture as PWATs have more than
doubled in two days surpassing 1.50 inches. Satellite derived CAPE
indicates 300-600 J/kg over the southern half of Arizona, but that
should increase through the day potentially peaking between 1000-
1500 J/kg. Indications of increasing 0-6 km shear give some
potential for a few rotating updrafts and stronger storms.

As for the low floor, there is plenty of uncertainty regarding the
coverage and timing of showers/storms given a very pronounced
inversion between 500-400 mb on the 18/12z KPSR sounding that could
keep a lid on activity today. Modest daytime heating and continued
moisture advection may help destabilization in some spots and allow
isolated instances of deeper convection while most capped areas see
no more than shallow convection/showers. As suggested by very recent
hi-res guidance, deeper convection may instead have to wait until
late this evening/tonight when a minor mid level wave moves through
that could support better forcing capable of surmounting the cap.
Given this, forecast PoPs remain unimpressive, mostly between 25-
35%, as occurrence of either case described above (or both) may not
be enough for widespread storms or showers. The few thunderstorms
currently on radar between the Estrellas and Gila Bend may give us a
first look at how convection may fare today.

Otherwise, HREF ensemble shows showers and thunderstorms mostly
developing west of the Phoenix area early this afternoon (not too
dissimilar to what is occurring now) with the bulk of activity then
shifting east. The main threat with storms will be strong wind gusts
of 40-60 mph and blowing dust. If shear is realized and there are
any rotating updrafts, a minor threat for hail and/or a funnel cloud
report would not surprise. That does not mean tornadoes are a threat
but a relatively commonplace terrestrial based rotating vapor shaft
could not be ruled out either. Trends remain consistent for lower
QPF potential as 20-25 kt steering flow will keep storms moving and
limit accumulations. NBM probability for rainfall above 0.10" is
low, below 15%, but HREF does show potential for higher amounts
nearing 1 inch and rainfall rates of 1.00-1.50 in/hr where strong
storms develop and/or training occurs.

Heading through Sunday the environment will begin rapidly drying
out from west to east with PWATs likely falling below 0.7" by
Monday. However, isolated showers and storms may still develop
over portions of southern Gila Sunday afternoon depending on the
amount of remaining moisture at the time. Otherwise, dry conditions
will extend through at least early next week and high pressure is
expected to set back in over the Southwest. This will result in a
slight warming trend with highs a few degrees above normal. With the
dry conditions, dew points in the 40s, morning low temperatures are
expected to remain in the 70s.


.AVIATION...Updated at 2310Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Showers with embedded thunderstorms causing gusty, erratic shifting
winds and brief periods of reduced visibilities will be a significant
weather hazard through mid evening. While the main swath of activity
has passed, residual SHRA may linger in the metro during the early
evening. Highly variable wind directions will exist across the metro
resulting from the exiting activity, though a general S/SE component
will be favored (except KIWA where SW should be preferred).

Confidence is moderate that renewed storms well southeast of the
metro will send an outflow back into the terminals sometime in the
01Z-04Z time frame. The threat of actual TS at any site (aside from
KIWA) has lowered such that a mention has been removed. Even without
a more robust SE outflow, easterly winds should prevail from mid
evening into Sunday early afternoon. Tranquil weather anticipated
Sunday with a westerly wind shift early/mid afternoon and gusts to
15kt likely.

Southeast California/Southwest Arizona including KIPL and KBLH:
Minimal weather impacts will exist through Sunday evening under
mostly clear skies. SW winds will be preferred into the overnight
with good confidence of prolonged variability Sunday morning. A
general southerly component will be favored Sunday with occasional
gusts 15-20kt possible.


Monday through Friday:
Dry conditions will dominate through most if not the entirety of
next week as high pressure strengthens across the region. High
temperatures through the period will be near to slightly above
normal. Afternoon RH values will mostly be in the 10-20% range
with overnight recoveries to 30-45%. Winds will be fairly light
while favoring local diurnal/terrain influences.


Spotters should follow standard reporting procedures.






Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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