Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
435 PM MST Tue Jul 20 2021

.UPDATE...Updated Aviation Discussion...

&&

.SYNOPSIS...
Abundant monsoon moisture will continue to bring daily chances
for at least isolated to scattered showers and thunderstorms
across much of Arizona with a tendency to mostly favor the higher
terrain of eastern Arizona, especially for the first half of the
week. Near normal temperatures are favored through midweek,
followed by below normal by late in the week. More scattered to
possibly widespread storms are expected for the second half of the
week and into the weekend as a more organized weather system is
expected to affect the region. Storms will be capable of producing
both locally heavy rain, strong winds, and blowing dust.

&&

.DISCUSSION...
Showers and thunderstorms scooted across the Phoenix metro and
southwest AZ/southeast CA this morning, dropping 0.10 to 0.25" of
rain over a few fortunate spots, in response to a passing
inverted trough and nearly 1500 j/kg of CAPE. Often, morning
convection will spoil storm chances for the afternoon and the
morning HREF showed a relatively quiet day for our forecast area.
However, latest radar trends indicate that may not be the case
with multiple thunderstorms now developing southwest, east, and
north of the Phoenix metro. Storms forming over the higher terrain
is not surprising but does cause some pause as to whether or not
we can get more storms into the lower desert today than previously
thought. The latest aircraft soundings show less inhibition with
a weaker midlevel cap and much more instability than guidance
suggested...but those soundings may not be adequately sampling the
storm environment either. Lower desert storm chances for this
afternoon are ~10-25%.

Overall, water vapor analysis shows a ridge of high pressure
centered over the Colorado Rockies with a fairly moist air mass
stretching from tropical Mexico through the Intermountain West.
The H5 ridge center will continue to weaken while settling close
to the Four Corners region over the next day or two, providing
stronger southeasterly flow through southern Arizona. This
position will keep the monsoon in gear with chances for
thunderstorms seemingly every day. PV anomalies associated with a
modest midlevel jet streak over western TX and New Mexico are
expected to move through the region later this week, potentially
increasing chances for thunderstorms perhaps as early as Wednesday
evening, but more likely Thursday and/or Friday.

Aside from the aforementioned 1500 j/kg CAPE, the 12z KPSR
sounding also showed a whopping 2.13" in precipitable water,
exceeding the daily maximum for date according to SPC Sounding
Climatology. Daytime mixing should lower PWs somewhat, but not by
all that much, as the latest aircraft soundings out of Sky Harbor
also measured 1.98" PW. These moisture levels are much more
typical of a late August tropical airmass rather than July 20th.
This ultimately means that any thunderstorms that do develop will
have the potential for producing heavy rain that could cause flash
flooding. To illustrate, a thunderstorm that developed rapidly
over the Bradshaw Mountains around noon dumped nearly 3.00" of
rain over one gauge within one hour, and exceeded rainfall rates
of 1.00"/hr over a few other rain gauges! That does not mean every
storm will dump that much rain but it does not take nearly that
much rain to cause flooding in our arid environment. The threat
for flash flooding is anticipated to increase daily through the
end of this week as storm chances and coverage are also forecast
to increase.

Temperatures through Wednesday are expected to be near seasonal
norms to slightly above normal across western deserts before
dropping several degrees as the storm chances increase and the
upper low moves into the area late week. Latest NBM has Wednesday
as the warmest day and Friday and Saturday as the coolest. NBM is
even supporting a 30% chance for sub-90 degree highs Friday and
Saturday in Phoenix. Temperatures should warm back up during early
next week as the pattern continues to shift with ensemble
guidance supporting a transition to increased south to southwest
flow and a gradual drying trend.

&&

.AVIATION...Updated at 2335Z.

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

As expected, storm activity today has been fairly limited with the
current activity well west of Phoenix. For the rest of the
afternoon/evening, expect little if any aviation impacts with
prevailing light westerly winds. Can't completely rule out a minor
outflow during the evening hours, potentially shifting winds, but
chances are pretty low (less than 20%). Tonight, expect a more
typical diurnal shift to easterly winds.

As far as Wednesday is concerned, expecting easterly winds to
persist into the early afternoon hours with a typical diurnal
switch to westerly during the mid/late afternoon hours. Prospects
for convective activity impacting the terminals look better for
Wed evening (30-40%) with an even better chance for easterly
outflows (60-70%). Current timing of a potential wind switch to
easterly Wednesday evening is somewhere in a 01-03Z time period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Generally quiet conditions will prevail with winds favoring the
south at both KIPL and KBLH through Wednesday afternoon. Can't
completely rule out a few isolated showers or storms Wednesday
afternoon, mainly across southwest Arizona, but it is unlikely to
impact the terminals.

&&

.FIRE WEATHER...
Friday through Tuesday:
No major changes to the forecast...Lower than normal afternoon
temperatures and ample monsoonal moisture will persist. Storm
activity is anticipated to spread as far west as southeast
California with most storms having good potential for wetting rain
(0.10" or more). The best potential for more widespread storm
activity will likely be Thursday through Friday and into Saturday
as an upper level disturbance moves through the region. Threats
from storms will continue to involve locally heavy rainfall and
flash flooding along with strong outflow winds and localized
downbursts within storms. Significant flooding may be experienced
on previously burned areas of Gila County, particularly during the
second half of the week. Min RH levels are expected to be in the
25-40% range across south-central Arizona (30-50% higher terrain
of south- central AZ) and 20-30% across southeast California and
southwest Arizona. Max RH levels, outside of storms, will be
45-70% most places (60-90% higher terrain of south-central AZ).
Apart from thunderstorms, winds will favor familiar diurnal
patterns.

&&

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

&&

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

DISCUSSION...AD/Benedict
AVIATION...Kuhlman
FIRE WEATHER...Smith/Kuhlman