Flash Flood Warning 19, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
521 PM MST Thu Aug 31 2023

.UPDATE...Updated 00z Aviation Discussion.

&&

.SYNOPSIS...
Rain chances will be increasing today with some strong to severe
thunderstorms possible across south-central Arizona later this
evening. Isolated to scattered showers and thunderstorms are
likely to persist mainly across south-central Arizona tonight into
Friday morning before more widespread showers and thunderstorms
are expected Friday afternoon and evening. Heavy rainfall with
localized flash flooding will be the primary threat on Friday, but
localized strong winds will still be a concern. A Flood Watch has
been issued for much of the area from this evening through Friday
evening. Near to below normal temperatures are expected starting
Friday with drier conditions settling in by Sunday.

&&

.DISCUSSION...
Now that the heat is abating as the strong high pressure system is
starting to shift east of the region, forecast attention has
turned toward increasing storm chances by later today. Boundary
layer moisture has been on the increase since yesterday and will
continue through Friday afternoon as deep southerly flow sets up
across the region. PWATs have already risen to near 1.7" across
southwest Arizona with guidance indicating PWATs increasing to
1.5-1.7" across all of the southern Arizona deserts this evening,
but DCAPEs remaining very high at 1500-1800 J/kg. Forecast MUCAPE
of 1000-1500 J/kg for late this afternoon and evening across the
lower deserts will support strong to possibly severe storms where
minimal CIN is expected centered over Pima into Maricopa
counties. Morning and early afternoon runs of the HRRR has been
consistently depicting a cluster of storms developing across Pima
County late this afternoon with a light southeasterly steering
flow pushing the storms through the Phoenix area during the latter
half of the evening. It seems very plausible this cluster of
storms will produce strong to possibly localized severe winds,
areas of blowing dust, and localized heavy rainfall. SPC also has
this area in a marginal risk of severe thunderstorms.

As moisture continues to increase into tonight and then on Friday,
PWATs are expected to increase to 1.7-1.9" across the majority of
the lower deserts. This will create an increasing threat for heavy
rainfall and flash flooding, possibly as early as this evening
across south-central Arizona if the cluster of storms is quite
robust. Broad scale ascent will also be increasing starting
tonight in response to a deep upper level trough digging southward
through the Pacific Northwest into California and the Great Basin.
There is also another weak shortwave trough currently over Baja
which is forecast to move northward through Arizona on Friday
which will bring PV energy for further enhancement of ascent over
our area Friday afternoon/evening. Given the more than adequate
moisture levels, good instability, and upper level support
portions of the area on Friday are likely to see strong
thunderstorms producing heavy rainfall. The 12Z HREF shows a
corridor of persistent QPF starting this evening and lasting
through Friday morning focused over Maricopa County. With expected
better instability by Friday afternoon, scattered to numerous
thunderstorms are likely to develop across the area with the HREF
focusing on an swath from Tucson through Phoenix into the Lower
Colorado River Valley. A Flood Watch has been issued starting this
evening across south-central and much of southwest Arizona before
expanding into portions of Gila County for Friday afternoon and
evening.

Aside from the rain chances, temperatures will top out around 105-
110 degrees today across the lower deserts before quickly dropping
off for Friday. For Friday, the NBM has high temperatures struggling
to climb into the triple digits with highs expected to stay in the
90s across the Valley. Overall confidence in temperatures will be
more uncertain Friday due to factors such as cloud cover and rain
with this uncertainty reflected in the greater spread in the
interquartile range of the NBM.

Chances for showers and thunderstorms will linger around for
Saturday before much drier air is ushered into the region as the low
pressure system along the West Coast shifts eastward during the
latter part of the weekend into early next week. This will lead to
rain chances falling to near 0% for the start of next week.
Temperatures then looking to gradually trend back upward next week
as ensembles favor ridging building back in with heights climbing
above 590 dm by the middle part of next week.

&&

.AVIATION...Updated at 0020Z.

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

Thunderstorm activity remains well south of the metro area late
this afternoon, thus the timing on outflow/thunderstorm potential
remains generally 03-07z range from south to north going through
this evening and into the early overnight hours. A weak outflow
out of the easterly direction may precede the strong southerly
outflow, but confidence was too low to include any change groups
with that potential. With the main outflow out of the southerly
direction, strong winds of 30 kts or greater is likely (at least
70%) and reduced visibilities in blowing dust will be the expected
outcomes. TS conditions remains around a 40-50% potential in the
04-07z time range will follow the expected outflow traversing
through the metro with CB's around 10 kft. VCSH conditions will
prevail into early tomorrow morning, with VCTS or TS conditions
not out of the question during this period. Winds will eventually
become generally southeasterly by tomorrow morning before gusty
southerly cross winds developing in the late morning to early
afternoon hours. VCTS conditions will once again be developing by
early to mid afternoon and last into the early evening hours
tomorrow.

Southeast California/Southwest Arizona including KIPL and KBLH:

Storms off the Baja Peninsula remain well south to warrant any
VCSH/VCTS concerns at IPL this evening. Generally southerly flow
will persist across the region. At BLH, winds will remain strong
with gusts in the 20-25 kt range through the overnight period.
VCSH/VCTS conditions anticipated to develop at both terminals late
tomorrow morning with CB's around 10 kft, with higher chances at
BLH around 30-40%. Stronger winds gusting in excess of 30 kts
expected at BLH during this period.

&&

.FIRE WEATHER...
Increasing moisture will translate to cooler temperatures and
quite good chances for rain through Friday. The best chances for
thunderstorms today will be across south-central Arizona with
strong winds a good possibility along with localized heavy
rainfall. Chances for wetting rains will further increase for
Friday with greater chances for seeing flash flooding, primarily
across central Arizona. Outside of thunderstorm winds, breezy
southerly winds are expected through Friday, particularly across
southeast California and southwest Arizona. Min RHs today will be
around 20-30% before increasing into the 35-45% range Friday.
Rain chances linger around Saturday before much drier air is
ushered in, eliminating rain chances by the start of next week.
Min RHs are expected to fall to around 15-25% by early next week.
Below normal temperatures will gradually trend upward again next
week as ridging is expected to build back into the region.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through Friday evening for AZZ530-531-533-534-
     536>544-546-548>551-553>555-559.

     Blowing Dust Advisory until 11 PM MST this evening for AZZ539-
     550-551-553>555-559.

     Flood Watch from Friday afternoon through Friday evening for
     AZZ545-547-552-556>558-561-562.

CA...Flood Watch through Friday evening for CAZ569.

&&

DISCUSSION...Smith/Kuhlman
AVIATION...Young
FIRE WEATHER...Smith/Kuhlman

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>