Flash Flood Warning 337, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
141 PM MST Sat Oct 15 2022

.SYNOPSIS...
Active weather is expected today as low pressure moves eastward
across the forecast area. A Flood Watch remains in effect for the
majority of the forecast area as multiple rounds of showers and
thunderstorms may lead to localized flooding issues today and into
Sunday. There is also the possibility for strong thunderstorms
with gusty winds and hail. After Sunday, drier and warmer
conditions return to the forecast area through the middle of next
week.

&&

.DISCUSSION...
Isolated showers and thunderstorms developed even before sunrise
this morning, mainly along a boundary lifting north and eastward
through central Arizona. The storms were generally organized and
multi-cellular in nature, supported by southwesterly winds atop a
moist southeasterly low-level flow with dewpoints in the 50s and
60s. The 12Z Tucson sounding sampled PWATs of 1.06" (just above
the 90th percentile moving average for this time of year). Further
west, the well-defined closed low is evident on water vapor
imagery just to southwest of San Diego, and will move slowly but
steadily eastward today. The setup remains quite favorable as much
of the forecast area is situated within a diffluent and divergent
flow regime downstream of the upper-low, coupled with the jet-
forced ascent in the left exit region of a subtropical jet streak
over Northern Mexico.

Main concern today is the potential for strong/severe storms
along with the threat of flash flooding. Modest looking cells
already produced hail between a quarter and a half of an inch in
diameter this morning. Meanwhile, gage data from the FCDMC
indicates rainfall rates as high as a half of an inch of rain in
only ten minutes.

Latest mesoanalysis indicates instability already in place across
much of southern Arizona and southeastern California, with values
of MLCAPE between 500 and 1500 J/kg. Further increases in SBCAPE
are anticipated this afternoon with the addition of surface
heating and the eastward encroaching cold air aloft. Latest HREF
suggests SBCAPE could potentially reach as high as 2000 J/kg this
afternoon as far east as Phoenix. South-southeasterly oriented
effective bulk shear will also strengthen to 25-35 kts as the
subtropical jet streak lifts northward into southeastern Arizona.

Hail as large as nickel-sized was reported this morning near
Tucson. Given the combination of increasing CAPE above -10 deg C
and strong deep-layer shear, it is not inconceivable to see
localized hail up to 1.0-1.5 inches in diameter. HREF maximum
updraft helicities also suggest the potential for a weak tornado.
However, strong winds are more likely with roughly a 70 percent
chance of maximum gusts exceeding 35 mph across southeastern
California. Strong/severe gusts cannot be ruled out elsewhere,
particularly in areas that receive more insolation.

Short-term CAMS remain in good agreement that precipitation will
fill in across the forecast area this afternoon and overnight.
Meanwhile, soil moisture anomalies remain in the 95th-98th
percentiles across a large portion of southwestern Arizona and
southeastern California and the threat of flash flooding has been
increased to the moderate category in these areas. Given the
cyclonic flow associated with the low, there is the potential for
multiple rounds of heavy rain, particularly across southwestern
Arizona where pivoting of the clusters will be enhanced as the
mid-level low migrates eastward towards Mexicali.

Further east across Maricopa, Gila and Pinal Counties, flash
flooding will also remain a threat, particularly later today and
tonight as the strongest forcing for ascent shifts eastward ahead
of the low. Latest ECMWF storm total QPF has again trended a bit
higher and this is reflected in the latest WPC forecast,
indicating most likely totals as high as 1.5-2.0 inches across
portions of the southwest deserts and from Phoenix eastward.

Latest GEFS ensemble mean places the center of the aforementioned
low across south-central Arizona by early Sunday. In the wake of
the low, a subsident regime will overspread much of the lower
deserts. NBM PoPs have decreased, now suggesting only a 20 percent
chance for the Phoenix area. However, leftover low-level moisture
and instability associated with the cold air aloft could promote
additional shower and thunderstorm activity during the late
morning and afternoon. The strongest storms will be capable of
producing brief heavy rain and pea-sized hail. Cooler conditions
are also anticipated with high temperatures struggling to even
reach the 80 degree mark in Phoenix. The last time the high
temperature failed to reach the 80 degree mark was more than 6
months ago on April 13.

&&

.AVIATION...Updated 1715Z.

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

IMPACTS:
-Periods of showers and storms through tonight and early Sunday
 -Lightning, small hail, and gusts locally exceeding 35KT.
 -Temporarily reduced VIS/CIG due to heavy rain.
-Erratic wind directions through at least 8Z.

Low pressure is moving over the area. We will see continued
opportunities for showers and thunderstorms. These thunderstorms
have already developed around the metro, and will continue to form
through the rest of the day and through tonight. Prevailing winds
will be E to SE, but that will be interrupted frequently due to
thunderstorm activity producing localized outflows. This has
recently occurred at PHX due to storms to the west, pushing an
outflow of westerly winds to the terminal. Ceilings and
visibilities could drop into MVFR range or worse in short time
intervals in thunderstorms. Expect further TAF amendments as this
activity transpires through the next 18HR.

Tomorrow, this activity will become more isolated. However,
showers will continue to be possible. IWA/SDL/DVT may see a short
period of MVFR ceilings Sunday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

IMPACTS:
-Periods of showers and storms later today into this evening
 -Lightning, small hail, and gusts locally exceeding 35KT.
 -Temporarily reduced VIS/CIG due to heavy rain.
-Erratic wind directions through at least 6Z.

Low pressure is moving over the area. Scattered thunderstorms are
expected to form over the next few hours. Prevailing winds will
be S to SE through early evening, but that will be interrupted
frequently due to thunderstorm activity producing localized
outflows. Visibilities could drop into MVFR range or worse for
short time intervals in thunderstorms. Expect further TAF
amendments as this activity transpires through the next 12HR.

Improving aviation conditions are expected after 8Z through
tomorrow.

&&

.FIRE WEATHER...
Scattered showers and thunderstorms will be the main weather
headlines through the weekend. Increased moisture into the area
has brought up RH's and will keep the minimum RH's elevated
(above 35%) through Sunday. Heavy rains will be capable of
producing flash flooding, as well as some strong wind gusts and
hail. Thunderstorm outflows will produce gusty winds, but general
wind flow expected to remain in the 10-20 mph range. Drying and
warming conditions expected to commence on Monday and last through
the middle of the week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through late tonight for AZZ530-531-533.

     Flood Watch through Sunday morning for AZZ547-552-555>558-
     560>563.

     Flood Watch through Sunday morning for AZZ534-537-538-540>546-
     548>551.

CA...Flood Watch through late tonight for CAZ560>570.

&&

DISCUSSION...Hirsch
AVIATION...Heil
FIRE WEATHER...Young

Special Weather Statement
National Weather Service Phoenix AZ
405 PM MST Sat Oct 15 2022

AZZ530-CAZ564-565-569-152345-
Parker Valley AZ-Chuckwalla Mountains CA-
Imperial County Southeast CA-Palo Verde Valley CA-
405 PM MST Sat Oct 15 2022

...A strong thunderstorm will impact portions of southwestern La Paz,
northwestern Yuma and east central Imperial Counties through 445 PM
MST/445 PM PDT/...

At 404 PM MST/404 PM PDT/, Doppler radar was tracking a strong
thunderstorm 7 miles southwest of Martinez Lake, moving northwest at
20 mph.

HAZARD...Wind gusts up to 50 mph and half inch hail.

SOURCE...Radar indicated.

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

This storm will remain over mainly rural areas of southwestern La
Paz, northwestern Yuma and east central Imperial Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3300 11495 3322 11468 3289 11445 3288 11446
      3280 11459
TIME...MOT...LOC 2304Z 139DEG 17KT 3290 11456

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...50 MPH

Heil

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