Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion...CORRECTED
National Weather Service Phoenix AZ
456 PM MST Sun Aug 7 2022

.UPDATE...
Updated Aviation section.

&&

.SYNOPSIS...
Storm chances will be on the increase beginning today
and remain elevated throughout the upcoming week as moisture levels
rise. With the elevated moisture levels and increased cloud cover,
temperatures will be near to slightly below seasonable norms.

&&

.DISCUSSION...
A pair of compact mesovortices rotated through the area this
morning, one near Phoenix and another just south of the Arizona
border. These brought scattered showers and thunderstorms to
portions of Gila/Maricopa Counties and Yuma/Imperial Counties.
Most of these showers and the associated cloud cover has now
cleared out, but due to somewhat cooler air in these locations it
will be more difficult to achieve convective initiation.
Elsewhere, strong insolation is leading to rapid destabilization
with showers and storms already popping up over the higher terrain
of the eastern third of Arizona. Moisture values are elevated with PWAT
values ranging from close to 2" over southwest Arizona to 1.25"
over Gila County, with most of the moisture concentrated above 700
hPa. Meanwhile, an inverted upper level trough over New Mexico is
nudging into eastern Arizona at this time, which is leading to
increased winds aloft as well as an improved wind shear profile.
MLCAPE has risen to 500-1500 J/Kg across much of southern Arizona
but remains strongly capped over the lower valleys at this time,
which reflect the cooling influence of the morning cloud cover.

HREF model guidance shows that we will continue to destabilize
heading into the afternoon hours and see the wind profile
improve. Scattered storms are already developing along the
Mogollon Rim. The likely storm progression the next few hours will
be a gradual increase in storm activity north and east of
Phoenix. Due to the aforementioned instability issues, these
storms may struggle to propagate into the valleys. By mid to late
afternoon, as the upper level trough continues to move closer to
the area, convection will initiate south and southeast of Phoenix.
Once these storms develop, conditions appear favorable for storm
maintenance heading westward into southern Maricopa and Yuma
Counties. Finally, interactions between these storms may result in
additional activity with a 20-40% chance of showers and storms
over the Phoenix area and points westward towards La Paz County.

The weather hazards today will vary considerably across the CWA. Over
Gila County where the boundary layer is not as dry, rain leading
to flash flooding will be the primary threat with the WPC showing
a slight (15% chance) risk of excessive rainfall. Accordingly, a
Flood Watch is in effect there. Further to the southwest, where
dry air in the lower atmosphere will be more of a concern, wind
and dust will be the primary threats. HREF guidance is showing a
50-70% chance of wind gusts in the lower deserts exceeding 35 kts
out of the east to northeast. The highest risk of strong to severe
winds appears to be along the I-8 corridor, where a Blowing Dust
Advisory is in effect this evening. The risk of flash flooding
across the lower deserts is relatively low, but remains possible.
Assuming that storms develop as expected, outflow winds and dust
could potentially move into portions of the Colorado River Valley
later tonight.

The upper level high will settle to the northeast of the 4-corners
region for much if not all of the next week which will be ideal
for continued deep moisture advection under southeast flow.
Ensemble mean PWAT generally remains 1.75-2" across the lower
deserts. With the high positioned further to the northeast than
typical, upper level winds may also stay somewhat elevated. WPC
clusters reveal some minor uncertainty with respect to the high
pressure location and intensity, which will ultimately dictate
important factors such as the strength and orientation of the
steering flow. Models are also hinting at periodic easterly waves
rotating around the base of the high pressure, which will increase
storm chances when present, but such features are always
difficult to time many days in advance. For now, the NBM maintains
a 25-50% chance of rain each day over the next week in the lower
elevations, with a 40-70% chance over the higher terrain.
Temperatures will remain slightly below normal for the entire
week.

&&

.AVIATION...Updated 2353Z.

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

Thunderstorms to the north and east of the metro area are expected
to generate outflows that will overspread the Valley floor this
evening. It is less likely to have outflows from southeast AZ.
There is considerable uncertainty in just how the outflows will
play out. There could be something of a one-two punch with a
north-northwesterly outflow quickly followed by an east-northeast
outflow - or vice versa. Also uncertainty in timing of the
associated wind shifts with best estimate of 02Z-04Z. At this
time, it appears that storms will struggle to survive over the
Valley floor. Of the TAF sites, better chance of TSRA at KDVT
(and possibly KIWA) than the other sites since it is closer to
where more robust storm activity is underway. Apart from
thunderstorms, if any, anticipate ceilings to remain Above FL100.
Prior to arrival of outflows, westerly surface winds will prevail.
After outflows dissipate later tonight, anticipate considerable
variability with an eventual trend toward nocturnal downvalley
patterns. Westerly winds redevelop by late morning/early afternoon
Monday. Depending on how storm activity plays out today, TAF sites
will have potential for thunderstorm impacts late Monday
afternoon/evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

KBLH: Southeast surface winds will prevail the rest of the
afternoon and into early evening. Expect thunderstorm outflow to
affect the airfield originating from storms over the higher
terrain southern Yavapai and southern Mohave Counties. Anticipate
arrival roughly between 02Z-04Z. Anticipate decreased visibilities
in blowing dust with the arrival and passage of the gust front.
New storms development is anticipated near the airfield. At this
time, confidence in TSRA right over the airfield is too low to
reflect in the TAF. But there could be follow-on windshifts.
Otherwise, anticipate increasing cloudiness followed by decreasing
cloudiness late tonight along with redevelopment of
south/southeasterly surface winds.

KIPL: Southeasterly surface winds will prevail through the
evening. Expect outflow from the northeast late tonight but not
quite strong enough to drop visibilities below 6SM. There will be
an increase in mainly high clouds as well. At this time,
showers/thunderstorms are anticipated to remain far enough away
from the airfield to preclude mention of TSRA or VCTS.
Anticipate a slow return of southeasterly winds overnight that
will continue through the day Monday.

&&

.FIRE WEATHER...
Active monsoon storm conditions are expected during the next
several days as moisture increases across the area. With the
elevated moisture levels in place, MinRHs will range between
25-40% with excellent overnight recoveries. Therefore, the fire
danger threat is expected to remain low for the foreseeable
future. Winds for the most part will remain light, with the
exception of thunderstorm outflows generating stronger gusts.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Blowing Dust Advisory until 11 PM MST this evening for AZZ539-
     550-551-553-554-559.

     Flood Watch through late tonight for AZZ545-547-552-556>558-
     560>563.

CA...None.
&&

DISCUSSION...Hodges
AVIATION...AJ
FIRE WEATHER...Lojero

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