Flash Flood Warning 256, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
1259 PM MST Sat Aug 27 2022

.SYNOPSIS...
The weather pattern will shift into a drier regime through the
weekend with chances for showers and thunderstorms gradually
lowering, then migrating exclusively into higher terrain regions.
Storm activity early next week may be extremely limited before
better moisture possibly begins to return during the latter half of
the week. Temperatures will hover slightly below normal through the
weekend before a warming allows for above normal temperatures to
return next week.

&&

.DISCUSSION...
High pressure remains centered south of the international border,
promoting continued westerly flow aloft over our region. Given the
flow aloft, drier air continues to advect into the region with this
morning's 12Z PHX sounding recording a PWAT value of 1.49".
Nevertheless, sufficient enough moisture and instability is expected
today to lead to some showers and thunderstorms this afternoon and
evening across the region. In fact, current radar showed isolated
showers/thunderstorms already developing across parts of Maricopa
County as well as parts of La Paz County. Areas across the high
terrain will stand the best chances for seeing additional shower
and thunderstorm development through the afternoon and evening.
The westerly steering flow will be unfavorable for thunderstorms
moving into the Valley from the high terrain. However, orographic
lift along lower desert terrain features as well as the potential
influence of a weak disturbance near northern Baja may aid in some
additional showers and thunderstorms developing across the lower
elevations.

The latest HRRR and 12Z HREF favor continued development across
much of the state through the afternoon as diurnal heating leads
to increasing destabilization. Across southwest Arizona, HiRes
guidance shows little in the way of thunderstorms today, aside
from ongoing development along the Kofas. Further west across
southeast California, chances look to remain very low (~5%) for
showers and thunderstorms. There are indications that a few
isolated thunderstorms may develop in northern Baja and
potentially extend up into eastern Imperial County. Modeled
soundings indicate high DCAPE values today climbing upwards of
1,500-2,000 J/kg this afternoon. This will promote the potential
for a few strong/severe wind gusts. The HREF shows upwards of a
50-70% chance for outflow winds to climb in excess of 35 mph
across parts of south-central Arizona. The probability of gusts in
excess of 58 mph is less than 10%.

Continued deep westerly flow will advect additional dry air with
storm chances further waining Sunday, likely confined completely to
the higher terrain. Storm chances decline to near zero areawide
starting Monday due to these factors and enhanced subsidence from
upper level ridging. In fact, many ensemble members show 500 hPa
heights rising upwards of 594-595 dm by Tuesday, near the 95th
percentile for this time of year. Temperatures will rise above
normal with highs potentially exceeding 110F for much of the lower
deserts and potentially upwards of 110-115 across southeast
California. Given the temperatures expected, moderate to locally
high HeatRisk is expected by midweek across the lower deserts and
may lead to excessive heat conditions being met.

By the middle of next week the anomalous high pressure area will
settle over the Great Basin with east to northeast midlevel flow
developing. Meanwhile, lower level water vapor may tick up just
slightly due to lower level southerly flow. This could allow for an
uptick in storm activity once again, but for now ensemble guidance
is mostly minimal with fairly lackluster QPF. Ensemble guidance
continues to show some uncertainty with respect to any easterly
waves, but overall guidance has consolidated around the idea of
keeping any such waves too far to the south to really impact the
weather for the desert southwest. NBM Pops do pick up a bit, peaking
at 5-20% in the valleys (highest values to the east) and 20-30% in
the higher terrain. With limited storm (and cloud) activity, high
temperatures will likely stay elevated at 105-110 for the lower
deserts.

&&

.AVIATION...Updated 1720Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Isolated showers have already developed to the north of the
terminals this morning acting on available instability. The latest
short term guidance is being a bit more aggressive with more
isolated to scattered storms reaching the valleys later this
afternoon into this evening. Have therefore added in a VCTS into
the TAFs for a short period beginning 21-22z, ending around
01-02z. Could be some gusty and erratic winds around storms but
otherwise favor the typical diurnal trends.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will favor a southerly component through the TAF period at
KBLH while favoring southeast during the day at KIPL and southwest
in the evening. Expect breezy conditions in the afternoon and
evening with gusts up to 20-25 kts at both terminals. Isolated
storms are expected this afternoon, mainly east of the CO River
and south of the international border. However, a storm still
cannot be ruled out at either terminal. FEW to SCT daytime clouds
are anticipated with bases aoa 10 kft.

&&

.FIRE WEATHER...
A gradual decline in storm chances will continue, but the high
terrain north and east of Phoenix should remain fairly active
today with a modest (40%) chance of thunderstorms. Gusty winds
will be possible with any storms. Substantial drying will spread
across the area starting Sunday, essentially ending rain chances
across the lower deserts and then likely even over the Arizona
high terrain for early next week. MinRH values will generally
remain in a 30-35% range over lower elevations to 40-50% over the
higher terrain through Sunday, before lowering starting Monday.
Apart from thunderstorms, winds will follow familiar diurnal
patterns. A chance of higher terrain storms will resume by mid
week.

&&

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

DISCUSSION...Smith/Hodges
AVIATION...Frieders
FIRE WEATHER...Hodges

Special Weather Statement
National Weather Service Phoenix AZ
100 PM MST Sat Aug 27 2022

AZZ537-540>542-544-272045-
Buckeye/Avondale AZ-Cave Creek/New River AZ-Deer Valley AZ-
Northwest Valley AZ-North Phoenix/Glendale AZ-
100 PM MST Sat Aug 27 2022

...A strong thunderstorm will impact portions of Maricopa County
through 145 PM MST...

At 100 PM MST, Doppler radar was tracking a strong thunderstorm near
White Tank Mountain Park, or 8 miles south of Wittmann, moving east
at 15 mph.

HAZARD...Wind gusts of 50 to 55 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Phoenix, Glendale, Peoria, Surprise, Avondale, Goodyear, El Mirage,
Tolleson, Youngtown, Sun City West, Sun City, Waddell, Wittmann,
Litchfield Park, Glendale Sports Complex, White Tank Mountain Park,
Luke AFB, Beardsley, Downtown Peoria and Surprise Stadium.

This includes the following highways...
AZ Interstate 10 between mile markers 131 and 135.
US Highway 60 between mile markers 127 and 152.
AZ Route 101 between mile markers 2 and 19.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 3359 11265 3381 11256 3378 11214 3343 11226
TIME...MOT...LOC 2000Z 289DEG 15KT 3366 11253

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...55 MPH

TF

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